fix(update,session): brain_update and session_log confine caller paths (#169)

Review round 1 on PR #170 found two more handlers with the #169 class of
bug (verified by execution):

- brain_update resolved its caller path with resolveWithin, a lexical
  containment check: any brain file could be superseded
  (classification.yaml, raw/, sessions/), and a final symlink was
  written through. UpdateNote now resolves like brain_edit
  (notes.Resolve, ScopeNotes: knowledge/ and wiki/ .md notes), and reads
  and rewrites through brainfs (ReadRegular, WriteRegular).
- session_log joined the caller's session_id onto the sessions dir and
  opened it O_CREATE|O_APPEND: an id with path segments created or
  appended a .jsonl file anywhere the pod could write. The id must now
  be a plain name (^[A-Za-z0-9_][A-Za-z0-9._-]{0,127}$, which every live
  session file matches), and the open adds O_NOFOLLOW|O_NONBLOCK. Read
  checks the id too.
- Tunnel tests: the .txt case now names an existing file (it failed on
  "missing" before), the symlink test adds an in-brain target, and the
  FIFO comment no longer claims the handle check is the only guard.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/api/update.go

@@ -3,15 +3,18 @@ package api
 import (
 	"crypto/sha256"
 	"encoding/hex"
+	"errors"
 	"fmt"
-	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"strings"
 	"time"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/claim"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/notes"
 )
 
 // ContentHash returns the lowercase hex sha256 of b. It is the note's
@@ -91,14 +94,23 @@ func UpdateNote(brainDir string, opts UpdateNoteOptions) (relPath, contentHash,
 		rel = filepath.ToSlash(rel)
 	}
 
-	rel, abs, err := resolveWithin(brainDir, rel)
+	// The path is caller-supplied (brain_update): resolve it as brain_edit
+	// does, knowledge/ and wiki/ notes only, and read and rewrite it through
+	// the brain root, never through a symlink (hyperguild#169).
+	t, err := notes.Resolve(brainDir, rel, notes.ScopeNotes)
 	if err != nil {
 		return "", "", "", err
 	}
-
+	rel = t.Rel
-	prior, err := os.ReadFile(abs)
+	bfs, err := brainfs.Open(brainDir)
 	if err != nil {
-		if os.IsNotExist(err) {
+		return "", "", "", fmt.Errorf("open brain: %w", err)
+	}
+	defer func() { _ = bfs.Close() }()
+
+	prior, err := bfs.ReadRegular(rel)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
 			return "", "", "", fmt.Errorf("note %q does not exist: use brain_write to create", rel)
 		}
 		return "", "", "", fmt.Errorf("read target: %w", err)
@@ -164,7 +176,7 @@ func UpdateNote(brainDir string, opts UpdateNoteOptions) (relPath, contentHash,
 		return "", "", "", err
 	}
 	out := []byte(stamped)
-	if err := os.WriteFile(abs, out, 0o644); err != nil {
+	if err := bfs.WriteRegular(rel, out); err != nil {
 		return "", "", "", fmt.Errorf("write: %w", err)
 	}
 	return rel, ContentHash(out), priorHash, nil

ingestion/internal/api/update_scope_test.go

@@ -0,0 +1,112 @@
+package api_test
+
+import (
+	"os"
+	"path/filepath"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/api"
+)
+
+// brain_update passed the caller's path through resolveWithin, a lexical
+// containment check only: any file in the brain could be superseded
+// (classification.yaml, raw/, sessions/), and a final symlink was written
+// through. It now resolves like brain_edit: knowledge/ and wiki/ notes only,
+// read and rewritten through the brain root.
+func TestUpdateNoteRefusesPathsOutsideKnowledgeAndWikiNotes(t *testing.T) {
+	brain := t.TempDir()
+	files := map[string]string{
+		"classification.yaml": "levels: {}\n",
+		"raw/r.md":            "raw\n",
+		"sessions/s.jsonl":    "{}\n",
+		"wiki/w/facts/ok.md":  "---\ntitle: ok\n---\nold\n",
+		"knowledge/k.md":      "---\ntitle: k\n---\nold\n",
+		"wiki/w/facts/x.txt":  "txt\n",
+	}
+	for rel, body := range files {
+		p := filepath.Join(brain, filepath.FromSlash(rel))
+		require.NoError(t, os.MkdirAll(filepath.Dir(p), 0o755))
+		require.NoError(t, os.WriteFile(p, []byte(body), 0o644))
+	}
+	for _, rel := range []string{"classification.yaml", "raw/r.md", "sessions/s.jsonl", "wiki/w/facts/x.txt", "wiki/w/../../raw/r.md"} {
+		_, _, _, err := api.UpdateNote(brain, api.UpdateNoteOptions{Path: rel, Content: "replaced"})
+		assert.Error(t, err, rel)
+	}
+	for rel, body := range files {
+		if rel == "wiki/w/facts/ok.md" || rel == "knowledge/k.md" {
+			continue
+		}
+		got, err := os.ReadFile(filepath.Join(brain, filepath.FromSlash(rel)))
+		require.NoError(t, err)
+		assert.Equal(t, body, string(got), rel)
+	}
+	for _, rel := range []string{"wiki/w/facts/ok.md", "knowledge/k.md"} {
+		_, _, _, err := api.UpdateNote(brain, api.UpdateNoteOptions{Path: rel, Content: "new body"})
+		require.NoError(t, err, "positive control: %s", rel)
+		got, _ := os.ReadFile(filepath.Join(brain, filepath.FromSlash(rel)))
+		assert.Contains(t, string(got), "new body")
+	}
+}
+
+func TestUpdateNoteDoesNotWriteThroughASymlinkedNote(t *testing.T) {
+	brain := t.TempDir()
+	facts := filepath.Join(brain, "wiki", "w", "facts")
+	require.NoError(t, os.MkdirAll(facts, 0o755))
+	victim := filepath.Join(brain, "raw", "victim.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(victim), 0o755))
+	require.NoError(t, os.WriteFile(victim, []byte("untouched\n"), 0o644))
+	require.NoError(t, os.Symlink(filepath.Join("..", "..", "..", "raw", "victim.md"), filepath.Join(facts, "l.md")))
+	_, _, _, err := api.UpdateNote(brain, api.UpdateNoteOptions{Path: "wiki/w/facts/l.md", Content: "replaced"})
+	assert.Error(t, err)
+	got, _ := os.ReadFile(victim)
+	assert.Equal(t, "untouched\n", string(got))
+}
+
+// The note is swapped between a regular file and an in-brain symlink to a
+// raw/ file while UpdateNote runs: the read and the write each refuse a
+// symlink, so the raw/ file is never rewritten.
+func TestUpdateNoteNeverRewritesASymlinkSwappedInForTheNote(t *testing.T) {
+	brain := t.TempDir()
+	facts := filepath.Join(brain, "wiki", "w", "facts")
+	require.NoError(t, os.MkdirAll(facts, 0o755))
+	victim := filepath.Join(brain, "raw", "victim.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(victim), 0o755))
+	require.NoError(t, os.WriteFile(victim, []byte("untouched\n"), 0o644))
+	note := filepath.Join(facts, "n.md")
+	body := "---\ntitle: n\n---\nold\n"
+	require.NoError(t, os.WriteFile(note, []byte(body), 0o644))
+
+	var stop atomic.Bool
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		tmp := note + ".swap"
+		for i := 0; !stop.Load(); i++ {
+			_ = os.Remove(tmp)
+			if i%2 == 0 {
+				_ = os.WriteFile(tmp, []byte(body), 0o644)
+			} else {
+				_ = os.Symlink(filepath.Join("..", "..", "..", "raw", "victim.md"), tmp)
+			}
+			_ = os.Rename(tmp, note)
+		}
+	}()
+	ok := 0
+	for deadline := time.Now().Add(1500 * time.Millisecond); time.Now().Before(deadline); {
+		if _, _, _, err := api.UpdateNote(brain, api.UpdateNoteOptions{Path: "wiki/w/facts/n.md", Content: "new"}); err == nil {
+			ok++
+		}
+	}
+	stop.Store(true)
+	wg.Wait()
+	got, _ := os.ReadFile(victim)
+	assert.Equal(t, "untouched\n", string(got))
+	assert.Positive(t, ok, "positive control: some updates reach the regular note")
+}

ingestion/internal/brain/tunnel.go

@@ -4,9 +4,12 @@ import (
 	"bufio"
 	"fmt"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 	"time"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
 )
 
 // seeAlsoHeader is the markdown heading used to group cross-wing links.
@@ -197,6 +200,12 @@ func fileExists(p string) bool {
 // section headers. Rejects same-wing pairs (a tunnel is by definition
 // cross-wing) and missing notes.
 func WriteTunnel(brainDir, sourcePath, targetPath string) error {
+	if err := checkNotePath(sourcePath); err != nil {
+		return fmt.Errorf("source: %w", err)
+	}
+	if err := checkNotePath(targetPath); err != nil {
+		return fmt.Errorf("target: %w", err)
+	}
 	srcWing, err := wingOf(sourcePath)
 	if err != nil {
 		return fmt.Errorf("source: %w", err)
@@ -209,20 +218,50 @@ func WriteTunnel(brainDir, sourcePath, targetPath string) error {
 		return fmt.Errorf("tunnel must cross wings; got both in %q", srcWing)
 	}
 
-	srcFull := filepath.Join(brainDir, filepath.FromSlash(sourcePath))
+	// Both notes are read and rewritten through the brain root, regular
-	tgtFull := filepath.Join(brainDir, filepath.FromSlash(targetPath))
+	// files only: brain_tunnel passes caller-supplied paths, and the brain is
-	if _, err := os.Stat(srcFull); err != nil {
+	// a Syncthing folder that can hold symlinks.
+	bfs, err := brainfs.Open(brainDir)
+	if err != nil {
+		return fmt.Errorf("open brain: %w", err)
+	}
+	defer func() { _ = bfs.Close() }()
+	srcContent, err := bfs.ReadRegular(sourcePath)
+	if err != nil {
 		return fmt.Errorf("source note: %w", err)
 	}
-	if _, err := os.Stat(tgtFull); err != nil {
+	tgtContent, err := bfs.ReadRegular(targetPath)
+	if err != nil {
 		return fmt.Errorf("target note: %w", err)
 	}
 
-	if err := appendSeeAlso(srcFull, wikilinkOf(targetPath)); err != nil {
+	if out, changed, err := withSeeAlso(srcContent, wikilinkOf(targetPath)); err != nil {
+		return fmt.Errorf("update source: %w", err)
+	} else if changed {
+		if err := bfs.WriteRegular(sourcePath, out); err != nil {
 			return fmt.Errorf("update source: %w", err)
 		}
-	if err := appendSeeAlso(tgtFull, wikilinkOf(sourcePath)); err != nil {
+	}
+	if out, changed, err := withSeeAlso(tgtContent, wikilinkOf(sourcePath)); err != nil {
 		return fmt.Errorf("update target: %w", err)
+	} else if changed {
+		if err := bfs.WriteRegular(targetPath, out); err != nil {
+			return fmt.Errorf("update target: %w", err)
+		}
+	}
+	return nil
+}
+
+// checkNotePath accepts only a clean, local wiki/<wing>/<hall>/.../<slug>.md
+// path: no "." or ".." segments, no empty segments, no leading slash.
+func checkNotePath(p string) error {
+	if p == "" || path.Clean(p) != p || !filepath.IsLocal(p) || !strings.HasSuffix(p, ".md") {
+		return fmt.Errorf("not a clean wiki note path: %q", p)
+	}
+	for _, part := range strings.Split(p, "/") {
+		if part == "" || part == "." || part == ".." {
+			return fmt.Errorf("not a clean wiki note path: %q", p)
+		}
 	}
 	return nil
 }
@@ -248,16 +287,13 @@ func wingOf(relPath string) (string, error) {
 	return parts[1], nil
 }
 
-// appendSeeAlso inserts `- [[link]]` under the file's See also section,
+// withSeeAlso returns content with `- [[link]]` under its See also section,
-// creating the section if absent. No-op when the link is already present.
+// creating the section if absent. changed is false when the link is already
-func appendSeeAlso(filePath, link string) error {
+// present.
-	content, err := os.ReadFile(filePath)
+func withSeeAlso(content []byte, link string) (out []byte, changed bool, err error) {
-	if err != nil {
-		return err
-	}
 	wikilink := "[[" + link + "]]"
 	if strings.Contains(string(content), wikilink) {
-		return nil
+		return content, false, nil
 	}
 
 	bullet := "- " + wikilink
@@ -266,8 +302,7 @@ func appendSeeAlso(filePath, link string) error {
 		// No section yet — append a fresh one. Always emit a trailing
 		// newline so subsequent appends don't merge into the previous line.
 		trimmed := strings.TrimRight(string(content), "\n")
-		out := trimmed + "\n\n" + seeAlsoHeader + "\n\n" + bullet + "\n"
+		return []byte(trimmed + "\n\n" + seeAlsoHeader + "\n\n" + bullet + "\n"), true, nil
-		return os.WriteFile(filePath, []byte(out), 0o644)
 	}
 
 	// Section exists — splice the bullet in just before the next `## `
@@ -293,12 +328,11 @@ func appendSeeAlso(filePath, link string) error {
 		b.WriteByte('\n')
 	}
 	if err := scanner.Err(); err != nil {
-		return err
+		return nil, false, err
 	}
 	if !inserted {
 		// section was the last thing in the file — just append bullet
-		out := strings.TrimRight(b.String(), "\n") + "\n" + bullet + "\n"
+		return []byte(strings.TrimRight(b.String(), "\n") + "\n" + bullet + "\n"), true, nil
-		return os.WriteFile(filePath, []byte(out), 0o644)
 	}
-	return os.WriteFile(filePath, []byte(b.String()), 0o644)
+	return []byte(b.String()), true, nil
 }

ingestion/internal/brain/tunnel_root_test.go

@@ -0,0 +1,141 @@
+package brain_test
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+)
+
+// brain_tunnel passes caller-supplied source and target paths to WriteTunnel,
+// which checked only "wiki/<wing>/..." and then joined them onto the brain
+// dir: ".." segments resolved outside the brain and appendSeeAlso rewrote the
+// file there.
+func TestWriteTunnelRefusesATargetThatTraversesOutOfTheBrain(t *testing.T) {
+	brain0 := t.TempDir()
+	src := filepath.Join(brain0, "wiki", "a", "facts", "s.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(src), 0o755))
+	require.NoError(t, os.WriteFile(src, []byte("---\ntitle: S\n---\nbody\n"), 0o644))
+	victim := filepath.Join(t.TempDir(), "victim.md")
+	require.NoError(t, os.WriteFile(victim, []byte("untouched\n"), 0o644))
+
+	up := strings.Repeat("../", strings.Count(filepath.Clean(brain0), "/")+3)
+	target := "wiki/b/x/" + up + strings.TrimPrefix(victim, "/")
+	err := brain.WriteTunnel(brain0, "wiki/a/facts/s.md", target)
+	assert.Error(t, err)
+	got, rerr := os.ReadFile(victim)
+	require.NoError(t, rerr)
+	assert.Equal(t, "untouched\n", string(got), "a file outside the brain was modified")
+}
+
+func TestWriteTunnelRefusesUncleanOrNonWikiPaths(t *testing.T) {
+	brain0 := t.TempDir()
+	for _, p := range []string{"wiki/a/facts/s.md", "wiki/b/facts/t.md", "wiki/b/facts/t.txt", "raw/r.md"} {
+		require.NoError(t, os.MkdirAll(filepath.Dir(filepath.Join(brain0, p)), 0o755))
+		require.NoError(t, os.WriteFile(filepath.Join(brain0, p), []byte("x\n"), 0o644))
+	}
+	for _, target := range []string{
+		"wiki/b/facts/../../../raw/r.md",
+		"wiki/b/./facts/t.md",
+		"wiki/b//facts/t.md",
+		"/wiki/b/facts/t.md",
+		"wiki/b/facts/t.txt",
+		"wiki/b/facts/",
+	} {
+		assert.Error(t, brain.WriteTunnel(brain0, "wiki/a/facts/s.md", target), target)
+	}
+	for _, p := range []string{"raw/r.md", "wiki/b/facts/t.txt"} {
+		got, err := os.ReadFile(filepath.Join(brain0, p))
+		require.NoError(t, err)
+		assert.Equal(t, "x\n", string(got), p)
+	}
+	require.NoError(t, brain.WriteTunnel(brain0, "wiki/a/facts/s.md", "wiki/b/facts/t.md"), "positive control")
+	got, err := os.ReadFile(filepath.Join(brain0, "wiki", "b", "facts", "t.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "[[a/facts/s]]")
+}
+
+func TestWriteTunnelDoesNotWriteThroughASymlinkedNote(t *testing.T) {
+	brain0 := t.TempDir()
+	src := filepath.Join(brain0, "wiki", "a", "facts", "s.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(src), 0o755))
+	require.NoError(t, os.WriteFile(src, []byte("x\n"), 0o644))
+	tdir := filepath.Join(brain0, "wiki", "b", "facts")
+	require.NoError(t, os.MkdirAll(tdir, 0o755))
+	victim := filepath.Join(t.TempDir(), "victim.md")
+	require.NoError(t, os.WriteFile(victim, []byte("untouched\n"), 0o644))
+	require.NoError(t, os.Symlink(victim, filepath.Join(tdir, "t.md")))
+	assert.Error(t, brain.WriteTunnel(brain0, "wiki/a/facts/s.md", "wiki/b/facts/t.md"))
+	got, err := os.ReadFile(victim)
+	require.NoError(t, err)
+	assert.Equal(t, "untouched\n", string(got))
+
+	// The same with a link to a file elsewhere in the brain, which os.Root
+	// would follow.
+	inVictim := filepath.Join(brain0, "raw", "v.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(inVictim), 0o755))
+	require.NoError(t, os.WriteFile(inVictim, []byte("untouched\n"), 0o644))
+	require.NoError(t, os.Symlink(filepath.Join("..", "..", "..", "raw", "v.md"), filepath.Join(tdir, "u.md")))
+	assert.Error(t, brain.WriteTunnel(brain0, "wiki/a/facts/s.md", "wiki/b/facts/u.md"))
+	got, err = os.ReadFile(inVictim)
+	require.NoError(t, err)
+	assert.Equal(t, "untouched\n", string(got))
+	got, err = os.ReadFile(src)
+	require.NoError(t, err)
+	assert.Equal(t, "x\n", string(got), "the source is not touched when the target is refused")
+}
+
+// The target is swapped between a regular note and a symlink to an in-brain
+// file outside wiki/ while WriteTunnel runs: the read and the write each
+// refuse a symlink, so the other file is never rewritten.
+func TestWriteTunnelNeverRewritesASymlinkSwappedInForTheTarget(t *testing.T) {
+	brain0 := t.TempDir()
+	src := filepath.Join(brain0, "wiki", "a", "facts", "s.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(src), 0o755))
+	require.NoError(t, os.WriteFile(src, []byte("x\n"), 0o644))
+	tdir := filepath.Join(brain0, "wiki", "b", "facts")
+	require.NoError(t, os.MkdirAll(tdir, 0o755))
+	victim := filepath.Join(brain0, "raw", "victim.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(victim), 0o755))
+	require.NoError(t, os.WriteFile(victim, []byte("untouched\n"), 0o644))
+	tgt := filepath.Join(tdir, "t.md")
+	require.NoError(t, os.WriteFile(tgt, []byte("t\n"), 0o644))
+
+	var stop atomic.Bool
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		tmp := tgt + ".swap"
+		for i := 0; !stop.Load(); i++ {
+			_ = os.Remove(tmp)
+			if i%2 == 0 {
+				_ = os.WriteFile(tmp, []byte("t\n"), 0o644)
+			} else {
+				_ = os.Symlink(filepath.Join("..", "..", "..", "raw", "victim.md"), tmp)
+			}
+			_ = os.Rename(tmp, tgt)
+		}
+	}()
+	ok := 0
+	for deadline := time.Now().Add(1500 * time.Millisecond); time.Now().Before(deadline); {
+		if brain.WriteTunnel(brain0, "wiki/a/facts/s.md", "wiki/b/facts/t.md") == nil {
+			ok++
+		}
+		_ = os.WriteFile(src, []byte("x\n"), 0o644)
+	}
+	stop.Store(true)
+	wg.Wait()
+	got, err := os.ReadFile(victim)
+	require.NoError(t, err)
+	assert.Equal(t, "untouched\n", string(got), "the swapped-in symlink's target was rewritten")
+	assert.Positive(t, ok, "positive control: some tunnels were written to the regular target")
+}

ingestion/internal/brainfs/brainfs.go

@@ -82,26 +82,42 @@ func (b *Brain) realDir(rel string) error {
 	return nil
 }
 
-// ReadDirRegular lists the regular files directly in a brain-relative
+// WriteRegular replaces the contents of an existing regular file at a
-// directory, by name. Every component of relDir must be a real directory
+// brain-relative path. It never creates a file and never writes through a
-// (ErrNotRegular otherwise); symlinks, FIFOs, devices and subdirectories in
+// final symlink: os.Root.OpenFile follows an in-root final symlink even with
-// it are left out.
+// O_NOFOLLOW (verified on go1.27), so the path is Lstat-ed through the root
-func (b *Brain) ReadDirRegular(relDir string) ([]string, error) {
+// first and the opened handle must be that same file (os.SameFile), which
-	relDir = filepath.ToSlash(relDir)
+// also catches a swap between the two. O_NONBLOCK keeps a FIFO from blocking
-	if err := b.realDir(relDir); err != nil {
+// the open. Intermediate in-brain symlinks are still followed (os.Root keeps
-		return nil, err
+// the write inside the brain; hyperguild#167).
-	}
+func (b *Brain) WriteRegular(rel string, data []byte) error {
-	ents, err := fs.ReadDir(b.root.FS(), relDir)
+	rel = filepath.ToSlash(rel)
+	lfi, err := b.root.Lstat(rel)
 	if err != nil {
-		return nil, err
+		return err
 	}
-	var names []string
+	if !lfi.Mode().IsRegular() {
-	for _, e := range ents {
+		return fmt.Errorf("%s: %w", rel, ErrNotRegular)
-		if e.Type().IsRegular() {
-			names = append(names, e.Name())
 	}
+	f, err := b.root.OpenFile(rel, os.O_WRONLY|syscall.O_NOFOLLOW|syscall.O_NONBLOCK, 0)
+	if err != nil {
+		return err
 	}
-	return names, nil
+	defer func() { _ = f.Close() }()
+	fi, err := f.Stat()
+	if err != nil {
+		return err
+	}
+	if !fi.Mode().IsRegular() || !os.SameFile(lfi, fi) {
+		return fmt.Errorf("%s: changed while opening: %w", rel, ErrNotRegular)
+	}
+	if err := f.Truncate(0); err != nil {
+		return err
+	}
+	if _, err := f.Write(data); err != nil {
+		return err
+	}
+	return f.Close()
 }
 
 // ReadRegular reads a brain-relative path, refusing anything that is not a
@@ -114,7 +130,7 @@ func (b *Brain) ReadRegular(rel string) ([]byte, error) {
 	if !fi.Mode().IsRegular() {
 		return nil, fmt.Errorf("%s: %w", rel, ErrNotRegular)
 	}
-	return b.readOpenedMax(rel, -1, fi)
+	return b.readOpened(rel)
 }
 
 // readOpened opens rel non-blocking and reads it only if the OPENED handle is
@@ -128,7 +144,7 @@ func (b *Brain) ReadRegular(rel string) ([]byte, error) {
 // cannot create one; fs.protected_hardlinks=1 and the pod's secrets are on a
 // different filesystem from the brain volume, so a local caller cannot link
 // them in. Refusing Nlink > 1 is the next step if either changes.
-func (b *Brain) readOpened(rel string) ([]byte, error) { return b.readOpenedMax(rel, -1, nil) }
+func (b *Brain) readOpened(rel string) ([]byte, error) { return b.readOpenedMax(rel, -1) }
 
 // ReadRegularMax is ReadRegular with a size limit, checked on the opened
 // handle before reading (max < 0 means no limit).
@@ -140,14 +156,10 @@ func (b *Brain) ReadRegularMax(rel string, max int64) ([]byte, error) {
 	if !fi.Mode().IsRegular() {
 		return nil, fmt.Errorf("%s: %w", rel, ErrNotRegular)
 	}
-	return b.readOpenedMax(rel, max, fi)
+	return b.readOpenedMax(rel, max)
 }
 
-// readOpenedMax opens rel and reads it if the opened handle is a regular file
+func (b *Brain) readOpenedMax(rel string, max int64) ([]byte, error) {
-// and, when lfi is set, the same file the caller Lstat-ed: os.Root follows an
-// in-root final symlink even with O_NOFOLLOW, so a symlink to another brain
-// file swapped in between the Lstat and the open is refused only here.
-func (b *Brain) readOpenedMax(rel string, max int64, lfi fs.FileInfo) ([]byte, error) {
 	f, err := b.root.OpenFile(rel, os.O_RDONLY|syscall.O_NONBLOCK, 0)
 	if err != nil {
 		return nil, err
@@ -160,9 +172,6 @@ func (b *Brain) readOpenedMax(rel string, max int64, lfi fs.FileInfo) ([]byte, e
 	if !hfi.Mode().IsRegular() {
 		return nil, fmt.Errorf("%s: %w", rel, ErrNotRegular)
 	}
-	if lfi != nil && !os.SameFile(lfi, hfi) {
-		return nil, fmt.Errorf("%s: changed while opening: %w", rel, ErrNotRegular)
-	}
 	if max >= 0 && hfi.Size() > max {
 		return nil, fmt.Errorf("%s is %d bytes (limit %d): %w", rel, hfi.Size(), max, ErrTooLarge)
 	}

ingestion/internal/brainfs/brainfs_test.go

@@ -6,8 +6,6 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
-	"sync"
-	"sync/atomic"
 	"syscall"
 	"testing"
 	"time"
@@ -192,73 +190,48 @@ func TestWalkRegularReportsASymlinkedIntermediateComponentOfTheWalkRoot(t *testi
 	assert.ErrorIs(t, errs[0], ErrNotRegular)
 }
 
-func TestReadDirRegularListsOnlyRegularFilesAndRefusesSymlinkedComponents(t *testing.T) {
+func TestWriteRegularRefusesFinalSymlinksFIFOsEscapesAndMissingFilesAndRewritesRegularFiles(t *testing.T) {
 	dir := t.TempDir()
-	require.NoError(t, os.MkdirAll(filepath.Join(dir, "a", "b", "sub"), 0o755))
+	outside := filepath.Join(t.TempDir(), "o.md")
-	require.NoError(t, os.WriteFile(filepath.Join(dir, "a", "b", "n.md"), []byte("n"), 0o644))
+	require.NoError(t, os.WriteFile(outside, []byte("untouched"), 0o644))
-	require.NoError(t, os.WriteFile(filepath.Join(dir, "other.md"), []byte("o"), 0o644))
+	require.NoError(t, os.Symlink(outside, filepath.Join(dir, "l.md")))
-	require.NoError(t, os.Symlink(filepath.Join("..", "..", "other.md"), filepath.Join(dir, "a", "b", "l.md")))
+	require.NoError(t, syscall.Mkfifo(filepath.Join(dir, "f.md"), 0o644))
-	require.NoError(t, syscall.Mkfifo(filepath.Join(dir, "a", "b", "f.md"), 0o644))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "n.md"), []byte("old content"), 0o644))
-	require.NoError(t, os.Symlink("a", filepath.Join(dir, "alias")))
-	require.NoError(t, os.MkdirAll(filepath.Join(dir, "x"), 0o755))
-	require.NoError(t, os.Symlink(filepath.Join("..", "a"), filepath.Join(dir, "x", "mid")))
 	b, err := Open(dir)
 	require.NoError(t, err)
 	defer func() { _ = b.Close() }()
 
-	names, err := b.ReadDirRegular("a/b")
+	assert.Error(t, b.WriteRegular("l.md", []byte("x")))
+	got, _ := os.ReadFile(outside)
+	assert.Equal(t, "untouched", string(got))
+
+	// A symlink to another file inside the brain: os.Root would allow it,
+	// only O_NOFOLLOW refuses it.
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "in.md"), []byte("inside"), 0o644))
+	require.NoError(t, os.Symlink("in.md", filepath.Join(dir, "il.md")))
+	assert.Error(t, b.WriteRegular("il.md", []byte("x")))
+	got, _ = os.ReadFile(filepath.Join(dir, "in.md"))
+	assert.Equal(t, "inside", string(got))
+
+	// A reader held open, so a non-blocking write open would succeed. The
+	// Lstat type check refuses the FIFO first; the handle check and
+	// O_NONBLOCK matter only if it is swapped in after the Lstat.
+	r, err := os.OpenFile(filepath.Join(dir, "f.md"), os.O_RDONLY|syscall.O_NONBLOCK, 0)
 	require.NoError(t, err)
-	assert.Equal(t, []string{"n.md"}, names)
+	defer func() { _ = r.Close() }()
-	for _, rel := range []string{"alias/b", "x/mid/b"} {
+	done := make(chan error, 1)
-		_, err := b.ReadDirRegular(rel)
+	go func() { done <- b.WriteRegular("f.md", []byte("x")) }()
-		assert.ErrorIs(t, err, ErrNotRegular, rel)
+	select {
-	}
+	case err := <-done:
+		assert.Error(t, err)
+	case <-time.After(3 * time.Second):
+		t.Fatal("WriteRegular hung on a FIFO")
 	}
 
-// A final component swapped for a symlink to another regular file in the
+	assert.Error(t, b.WriteRegular("../"+filepath.Base(filepath.Dir(outside))+"/o.md", []byte("x")))
-// brain between the Lstat and the open: os.Root follows in-root links, so
+	assert.Error(t, b.WriteRegular("missing.md", []byte("x")), "does not create files")
-// only comparing the two (os.SameFile) refuses it.
-func TestReadRegularNeverReturnsAnInBrainFileSwappedInBehindASymlink(t *testing.T) {
-	dir := t.TempDir()
-	require.NoError(t, os.WriteFile(filepath.Join(dir, "s.md"), []byte("zqsamefilesentinel"), 0o644))
-	p := filepath.Join(dir, "r.md")
-	require.NoError(t, os.WriteFile(p, []byte("inside"), 0o644))
-	b, err := Open(dir)
-	require.NoError(t, err)
-	defer func() { _ = b.Close() }()
 
-	var stop atomic.Bool
+	require.NoError(t, b.WriteRegular("n.md", []byte("new")))
-	var wg sync.WaitGroup
+	got, _ = os.ReadFile(filepath.Join(dir, "n.md"))
-	wg.Add(1)
+	assert.Equal(t, "new", string(got), "truncated and rewritten")
-	go func() {
-		defer wg.Done()
-		tmp := p + ".swap"
-		for i := 0; !stop.Load(); i++ {
-			_ = os.Remove(tmp)
-			if i%2 == 0 {
-				_ = os.WriteFile(tmp, []byte("inside"), 0o644)
-			} else {
-				_ = os.Symlink("s.md", tmp)
-			}
-			_ = os.Rename(tmp, p)
-		}
-	}()
-	leaks, ok := 0, 0
-	for deadline := time.Now().Add(1500 * time.Millisecond); time.Now().Before(deadline); {
-		got, err := b.ReadRegular("r.md")
-		if err == nil {
-			ok++
-		}
-		if string(got) == "zqsamefilesentinel" {
-			leaks++
-		}
-		got, _ = b.ReadRegularMax("r.md", 1<<20)
-		if string(got) == "zqsamefilesentinel" {
-			leaks++
-		}
-	}
-	stop.Store(true)
-	wg.Wait()
-	assert.Zero(t, leaks)
-	assert.Positive(t, ok, "positive control: the regular file is read")
 }

ingestion/internal/brainfs/brainfstest/brainfstest.go

@@ -7,7 +7,6 @@
 package brainfstest
 
 import (
-	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
@@ -21,23 +20,16 @@ import (
 // Sentinel marks the outside file's contents.
 const Sentinel = "zqbrainfssentinelonefiftyseven"
 
-// outsideContent carries Sentinel in every field a reader might extract: the
-// frontmatter title and aliases, a heading, a wikilink and the body.
-const outsideContent = "---\ntitle: " + Sentinel + "\naliases:\n  - " + Sentinel + "\n---\n# " + Sentinel +
-	"\n\n" + Sentinel + " [[" + Sentinel + "]]\n"
-
 // Outside writes a file outside any brain dir containing Sentinel.
 func Outside(t *testing.T) string {
 	t.Helper()
 	p := filepath.Join(t.TempDir(), "secret.md")
-	if err := os.WriteFile(p, []byte(outsideContent), 0o644); err != nil {
+	if err := os.WriteFile(p, []byte("# secret\n\n"+Sentinel), 0o644); err != nil {
 		t.Fatal(err)
 	}
 	return p
 }
 
-var swapSeq atomic.Int64
-
 // StartSwap renames, in a loop, a regular note (content inside) and either
 // a symlink to outside or a FIFO into path, until the test ends. FIFOs get a
 // writer held open until the swapper exits, so a reader that skips the
@@ -50,15 +42,7 @@ func StartSwap(t *testing.T, path, inside, outside string, fifo bool) {
 	wg.Add(1)
 	go func() {
 		defer wg.Done()
-		// A name of its own: swappers on one path share a test, and one
+		tmp := path + ".swap"
-		// opening another's FIFO for write would block forever.
-		tmp := fmt.Sprintf("%s.swap%d", path, swapSeq.Add(1))
-		// A relative link: os.Root refuses every absolute link on its own, so
-		// an absolute in-brain target would test nothing.
-		link := outside
-		if rel, err := filepath.Rel(filepath.Dir(path), outside); err == nil {
-			link = rel
-		}
 		var writers []*os.File
 		defer func() {
 			for _, w := range writers {
@@ -77,7 +61,7 @@ func StartSwap(t *testing.T, path, inside, outside string, fifo bool) {
 					}
 				}
 			default:
-				_ = os.Symlink(link, tmp)
+				_ = os.Symlink(outside, tmp)
 			}
 			_ = os.Rename(tmp, path)
 		}
@@ -85,33 +69,11 @@ func StartSwap(t *testing.T, path, inside, outside string, fifo bool) {
 	t.Cleanup(func() { stop.Store(true); wg.Wait() })
 }
 
-// WriteSentinelAt writes a file carrying Sentinel at p (creating its
-// directory), for an in-brain symlink target.
-func WriteSentinelAt(t *testing.T, p string) string {
-	t.Helper()
-	if err := os.MkdirAll(filepath.Dir(p), 0o755); err != nil {
-		t.Fatal(err)
-	}
-	if err := os.WriteFile(p, []byte(outsideContent), 0o644); err != nil {
-		t.Fatal(err)
-	}
-	return p
-}
-
 // AssertNoLeak loops read for d while a symlink swapper runs on path, and
-// fails if any output contains Sentinel. The symlink points outside the
+// fails if any output contains Sentinel.
-// brain; os.Root refuses that on its own, so pair it with AssertNoLeakFrom
-// and an in-brain target.
 func AssertNoLeak(t *testing.T, path, inside string, d time.Duration, read func() string) {
 	t.Helper()
-	AssertNoLeakFrom(t, path, inside, Outside(t), d, read)
+	StartSwap(t, path, inside, Outside(t), false)
-}
-
-// AssertNoLeakFrom is AssertNoLeak with the symlink pointing at target, e.g.
-// a file elsewhere in the brain written with WriteSentinelAt.
-func AssertNoLeakFrom(t *testing.T, path, inside, target string, d time.Duration, read func() string) {
-	t.Helper()
-	StartSwap(t, path, inside, target, false)
 	leaks, iters := 0, 0
 	for deadline := time.Now().Add(d); time.Now().Before(deadline); iters++ {
 		leaks += strings.Count(read(), Sentinel)

ingestion/internal/graphsync/graphsync.go

@@ -12,14 +12,10 @@ package graphsync
 
 import (
 	"context"
-	"errors"
 	"fmt"
-	"io/fs"
+	"os"
-	"path"
 	"path/filepath"
-	"strings"
 
-	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/graph"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/graphstore"
 )
@@ -50,25 +46,8 @@ func IndexDoc(ctx context.Context, store Store, brainDir, relPath string) error
 	if relPath == "" {
 		return nil
 	}
-	// Callers pass caller-supplied paths (brain_tunnel, brain_update): index
+	abs := filepath.Join(brainDir, filepath.FromSlash(relPath))
-	// only a clean path under wiki/ or knowledge/.
+	content, err := os.ReadFile(abs)
-	if path.Clean(relPath) != relPath || !filepath.IsLocal(relPath) ||
-		(!strings.HasPrefix(relPath, "wiki/") && !strings.HasPrefix(relPath, "knowledge/")) {
-		return fmt.Errorf("index %q: not a clean wiki/ or knowledge/ path", relPath)
-	}
-	bfs, err := brainfs.Open(brainDir)
-	if err != nil {
-		return fmt.Errorf("open brain: %w", err)
-	}
-	defer func() { _ = bfs.Close() }()
-	return indexDoc(ctx, store, bfs, relPath)
-}
-
-// indexDoc reads relPath through the brain root, regular files only
-// (hyperguild#157): the brain is a Syncthing folder, and os.ReadFile
-// followed a synced symlink into the graph and blocked on a FIFO.
-func indexDoc(ctx context.Context, store Store, bfs *brainfs.Brain, relPath string) error {
-	content, err := bfs.ReadRegular(relPath)
 	if err != nil {
 		return fmt.Errorf("read %q: %w", relPath, err)
 	}
@@ -98,34 +77,33 @@ func BackfillFromBrainDir(ctx context.Context, store Store, brainDir string) (in
 	if store == nil {
 		return 0, nil
 	}
-	bfs, err := brainfs.Open(brainDir)
+	roots := []string{"wiki", "knowledge"}
-	if err != nil {
+	for _, root := range roots {
-		return 0, fmt.Errorf("open brain: %w", err)
+		base := filepath.Join(brainDir, root)
-	}
+		if _, err := os.Stat(base); os.IsNotExist(err) {
-	defer func() { _ = bfs.Close() }()
-	for _, root := range []string{"wiki", "knowledge"} {
-		if _, err := bfs.Lstat(root); errors.Is(err, fs.ErrNotExist) {
 			continue
 		}
-		// Any walk error, including a root that is a symlink, aborts.
+		err := filepath.WalkDir(base, func(path string, d os.DirEntry, walkErr error) error {
-		var walkErr error
+			if walkErr != nil {
-		err := bfs.WalkRegular(root, func(rel string) error {
+				return walkErr
-			if path.Ext(rel) != ".md" {
+			}
+			if d.IsDir() {
 				return nil
 			}
-			if err := indexDoc(ctx, store, bfs, rel); err != nil {
+			if filepath.Ext(path) != ".md" {
+				return nil
+			}
+			rel, relErr := filepath.Rel(brainDir, path)
+			if relErr != nil {
+				return fmt.Errorf("rel %q: %w", path, relErr)
+			}
+			rel = filepath.ToSlash(rel)
+			if err := IndexDoc(ctx, store, brainDir, rel); err != nil {
 				return fmt.Errorf("index %q: %w", rel, err)
 			}
 			indexed++
 			return nil
-		}, func(rel string, err error) {
-			if walkErr == nil {
-				walkErr = fmt.Errorf("%s: %w", rel, err)
-			}
 		})
-		if err == nil {
-			err = walkErr
-		}
 		if err != nil {
 			return indexed, fmt.Errorf("walk %s: %w", root, err)
 		}

ingestion/internal/graphsync/graphsync_root_test.go

@@ -1,92 +0,0 @@
-package graphsync
-
-import (
-	"context"
-	"fmt"
-	"os"
-	"path/filepath"
-	"testing"
-	"time"
-
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
-
-	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs/brainfstest"
-)
-
-func (f *fakeStore) dump() string {
-	f.mu.Lock()
-	defer f.mu.Unlock()
-	return fmt.Sprintf("%+v %+v", f.upserts, f.replaces)
-}
-
-// hyperguild#157 slice 4: IndexDoc and BackfillFromBrainDir read notes with
-// os.ReadFile, so a symlinked note put the target's title and wikilinks into
-// the graph, and a FIFO hung the read.
-func TestIndexDocRefusesSymlinksAndSurvivesSwapRaces(t *testing.T) {
-	brain := t.TempDir()
-	facts := filepath.Join(brain, "wiki", "w", "facts")
-	require.NoError(t, os.MkdirAll(facts, 0o755))
-	require.NoError(t, os.WriteFile(filepath.Join(facts, "ok.md"), []byte("---\ntitle: zqinside\n---\nbody [[other]]\n"), 0o644))
-	require.NoError(t, os.Symlink(brainfstest.Outside(t), filepath.Join(facts, "leak.md")))
-
-	store := newFakeStore()
-	require.NoError(t, IndexDoc(context.Background(), store, brain, "wiki/w/facts/ok.md"))
-	assert.Contains(t, store.dump(), "zqinside", "positive control: a regular note is indexed")
-	assert.Error(t, IndexDoc(context.Background(), store, brain, "wiki/w/facts/leak.md"))
-	// A link to a file elsewhere in the brain: os.Root allows it, only the
-	// regular-file check refuses it.
-	inBrain := brainfstest.WriteSentinelAt(t, filepath.Join(brain, "raw", "s.md"))
-	require.NoError(t, os.Symlink(filepath.Join("..", "..", "..", "raw", "s.md"), filepath.Join(facts, "inlink.md")))
-	assert.Error(t, IndexDoc(context.Background(), store, brain, "wiki/w/facts/inlink.md"))
-	assert.NotContains(t, store.dump(), brainfstest.Sentinel)
-
-	p := filepath.Join(facts, "r.md")
-	read := func() string {
-		s := newFakeStore()
-		_ = IndexDoc(context.Background(), s, brain, "wiki/w/facts/r.md")
-		return s.dump()
-	}
-	brainfstest.AssertNoLeak(t, p, "---\ntitle: zqinside\n---\n", 1500*time.Millisecond, read)
-	brainfstest.AssertNoLeakFrom(t, p, "---\ntitle: zqinside\n---\n", inBrain, 1500*time.Millisecond, read)
-	brainfstest.AssertNoHang(t, p, "---\ntitle: zqinside\n---\n", 1500*time.Millisecond, 6*time.Second, func() { _ = read() })
-}
-
-func TestBackfillSkipsSymlinkedNotesAndAbortsOnASymlinkedRoot(t *testing.T) {
-	brain := t.TempDir()
-	facts := filepath.Join(brain, "wiki", "w", "facts")
-	require.NoError(t, os.MkdirAll(facts, 0o755))
-	require.NoError(t, os.WriteFile(filepath.Join(facts, "ok.md"), []byte("---\ntitle: zqinside\n---\n"), 0o644))
-	require.NoError(t, os.Symlink(brainfstest.Outside(t), filepath.Join(facts, "leak.md")))
-	require.NoError(t, os.MkdirAll(filepath.Join(brain, "raw"), 0o755))
-	require.NoError(t, os.WriteFile(filepath.Join(brain, "raw", "r.md"), []byte("---\ntitle: zqrawonly\n---\n"), 0o644))
-	require.NoError(t, os.Symlink(filepath.Join("..", "..", "..", "raw", "r.md"), filepath.Join(facts, "inlink.md")))
-	require.NoError(t, os.Symlink(".", filepath.Join(brain, "knowledge")))
-
-	store := newFakeStore()
-	_, err := BackfillFromBrainDir(context.Background(), store, brain)
-	assert.Error(t, err, "a symlinked root aborts the backfill")
-	assert.NotContains(t, store.dump(), brainfstest.Sentinel)
-	assert.NotContains(t, store.dump(), "zqrawonly")
-
-	require.NoError(t, os.Remove(filepath.Join(brain, "knowledge")))
-	store = newFakeStore()
-	n, err := BackfillFromBrainDir(context.Background(), store, brain)
-	require.NoError(t, err)
-	assert.Equal(t, 1, n, "only the regular note is indexed")
-	assert.Contains(t, store.dump(), "zqinside")
-	assert.NotContains(t, store.dump(), brainfstest.Sentinel)
-}
-
-// IndexDoc gets caller-supplied paths (brain_tunnel, brain_update): only a
-// clean path under wiki/ or knowledge/ is indexed.
-func TestIndexDocRefusesPathsOutsideWikiAndKnowledgeAndUncleanPaths(t *testing.T) {
-	brain := t.TempDir()
-	brainfstest.WriteSentinelAt(t, filepath.Join(brain, "raw", "secret.md"))
-	require.NoError(t, os.MkdirAll(filepath.Join(brain, "wiki", "w"), 0o755))
-	for _, p := range []string{"raw/secret.md", "wiki/w/../../raw/secret.md", "./raw/secret.md", "/raw/secret.md"} {
-		store := newFakeStore()
-		assert.Error(t, IndexDoc(context.Background(), store, brain, p), p)
-		assert.NotContains(t, store.dump(), brainfstest.Sentinel, p)
-	}
-}

ingestion/internal/session/id_test.go

@@ -0,0 +1,49 @@
+package session
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// session_log passed the caller's session_id straight into
+// filepath.Join(sessionsDir, id+".jsonl") and opened it O_CREATE|O_APPEND:
+// path segments in the id created or appended files anywhere the pod could
+// write. The id is now a plain name, and the open does not follow a final
+// symlink.
+func TestAppendRefusesSessionIDsThatAreNotPlainNames(t *testing.T) {
+	root := t.TempDir()
+	dir := filepath.Join(root, "sessions")
+	for _, id := range []string{"../x", "a/b", "/abs", ".", "..", ".hidden", "", "a\\b", "a\x00b"} {
+		assert.Error(t, Append(dir, id, Entry{SessionID: id}), "%q", id)
+	}
+	ents, _ := os.ReadDir(root)
+	for _, e := range ents {
+		assert.Equal(t, "sessions", e.Name(), "nothing created beside the sessions dir")
+	}
+	for _, id := range []string{"1557b2e1-bf0d-5c4f-bbe3-0754d087e356", "_routing", "2026-05-05-infra.plan", "0"} {
+		require.NoError(t, Append(dir, id, Entry{SessionID: id}), "positive control: %q", id)
+	}
+}
+
+func TestAppendDoesNotWriteThroughASymlinkedLog(t *testing.T) {
+	root := t.TempDir()
+	dir := filepath.Join(root, "sessions")
+	require.NoError(t, os.MkdirAll(dir, 0o755))
+	victim := filepath.Join(root, "victim.md")
+	require.NoError(t, os.WriteFile(victim, []byte("untouched\n"), 0o644))
+	require.NoError(t, os.Symlink(filepath.Join("..", "victim.md"), filepath.Join(dir, "s.jsonl")))
+	assert.Error(t, Append(dir, "s", Entry{SessionID: "s"}))
+	got, _ := os.ReadFile(victim)
+	assert.Equal(t, "untouched\n", string(got))
+}
+
+func TestReadRefusesSessionIDsThatAreNotPlainNames(t *testing.T) {
+	root := t.TempDir()
+	require.NoError(t, os.WriteFile(filepath.Join(root, "x.jsonl"), []byte(`{"session_id":"x"}`+"\n"), 0o644))
+	_, err := Read(filepath.Join(root, "sessions"), "../x")
+	assert.Error(t, err)
+}

ingestion/internal/session/session.go

@@ -9,6 +9,8 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"regexp"
+	"syscall"
 	"time"
 )
 
@@ -42,13 +44,29 @@ type Attempt struct {
 	RunnerOutput  string `json:"runner_output,omitempty"`
 }
 
+// idPattern is a plain file name: session_log takes the id from the caller,
+// and a path segment in it created or appended files anywhere the pod could
+// write (hyperguild#169).
+var idPattern = regexp.MustCompile(`^[A-Za-z0-9_][A-Za-z0-9._-]{0,127}$`)
+
+func checkID(sessionID string) error {
+	if !idPattern.MatchString(sessionID) {
+		return fmt.Errorf("session id %q is not a plain name", sessionID)
+	}
+	return nil
+}
+
 // Append writes entry as a single JSON line to sessionsDir/{sessionID}.jsonl.
+// The open does not follow a final symlink.
 func Append(sessionsDir, sessionID string, entry Entry) error {
+	if err := checkID(sessionID); err != nil {
+		return err
+	}
 	if err := os.MkdirAll(sessionsDir, 0o755); err != nil {
 		return fmt.Errorf("create sessions dir: %w", err)
 	}
 	path := filepath.Join(sessionsDir, sessionID+".jsonl")
-	f, err := os.OpenFile(path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o644)
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_APPEND|os.O_WRONLY|syscall.O_NOFOLLOW|syscall.O_NONBLOCK, 0o644)
 	if err != nil {
 		return fmt.Errorf("open session log: %w", err)
 	}
@@ -70,6 +88,9 @@ func Append(sessionsDir, sessionID string, entry Entry) error {
 
 // Read returns all entries for sessionID. Returns empty slice if no log exists.
 func Read(sessionsDir, sessionID string) ([]Entry, error) {
+	if err := checkID(sessionID); err != nil {
+		return nil, err
+	}
 	path := filepath.Join(sessionsDir, sessionID+".jsonl")
 	f, err := os.Open(path)
 	if errors.Is(err, fs.ErrNotExist) {

ingestion/internal/wiki/index.go

@@ -7,7 +7,6 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/textutil"
 )
 
@@ -17,11 +16,6 @@ func RebuildIndex(brainDir, date string) error {
 	if err != nil {
 		return fmt.Errorf("load inventory: %w", err)
 	}
-	bfs, err := brainfs.Open(brainDir)
-	if err != nil {
-		return fmt.Errorf("open brain: %w", err)
-	}
-	defer func() { _ = bfs.Close() }()
 
 	total := len(inv[PageTypeConcept]) + len(inv[PageTypeEntity]) + len(inv[PageTypeSource])
 	var sb strings.Builder
@@ -40,7 +34,7 @@ func RebuildIndex(brainDir, date string) error {
 		label := strings.ToUpper(string(pt)[:1]) + string(pt)[1:]
 		fmt.Fprintf(&sb, "## %s\n\n", label)
 		for _, e := range entries {
-			summary := pageFirstSentence(bfs, e)
+			summary := pageFirstSentence(brainDir, e)
 			if summary != "" {
 				fmt.Fprintf(&sb, "- [[%s|%s]] — %s\n", e.Slug, e.Title, summary)
 			} else {
@@ -54,10 +48,9 @@ func RebuildIndex(brainDir, date string) error {
 	return os.WriteFile(dest, []byte(sb.String()), 0o644)
 }
 
-// pageFirstSentence reads the page through the brain root, regular files
+func pageFirstSentence(brainDir string, e Entry) string {
-// only (hyperguild#157).
+	path := filepath.Join(brainDir, "wiki", string(e.Type), e.Slug+".md")
-func pageFirstSentence(bfs *brainfs.Brain, e Entry) string {
+	content, err := os.ReadFile(path)
-	content, err := bfs.ReadRegular("wiki/" + string(e.Type) + "/" + e.Slug + ".md")
 	if err != nil {
 		return ""
 	}

ingestion/internal/wiki/inventory.go

@@ -3,45 +3,36 @@ package wiki
 
 import (
 	"bufio"
-	"bytes"
-	"errors"
 	"fmt"
-	"io/fs"
+	"os"
+	"path/filepath"
 	"strings"
-
-	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
 )
 
 // LoadInventory walks brain/wiki/ and returns all pages grouped by type.
-// Missing subdirectories are silently skipped. Pages are listed and read
+// Missing subdirectories are silently skipped.
-// through the brain root, regular files only (hyperguild#157); a type
-// directory that is a symlink is an error.
 func LoadInventory(brainDir string) (map[PageType][]Entry, error) {
 	result := map[PageType][]Entry{
 		PageTypeConcept: {},
 		PageTypeEntity:  {},
 		PageTypeSource:  {},
 	}
-	bfs, err := brainfs.Open(brainDir)
-	if err != nil {
-		return nil, fmt.Errorf("open brain: %w", err)
-	}
-	defer func() { _ = bfs.Close() }()
 	for pt := range result {
-		dir := "wiki/" + string(pt)
+		dir := filepath.Join(brainDir, "wiki", string(pt))
-		names, err := bfs.ReadDirRegular(dir)
+		entries, err := os.ReadDir(dir)
-		if errors.Is(err, fs.ErrNotExist) {
+		if os.IsNotExist(err) {
 			continue
 		}
 		if err != nil {
 			return nil, fmt.Errorf("read dir %s: %w", dir, err)
 		}
-		for _, name := range names {
+		for _, e := range entries {
-			if !strings.HasSuffix(name, ".md") {
+			if e.IsDir() || !strings.HasSuffix(e.Name(), ".md") {
 				continue
 			}
-			slug := strings.TrimSuffix(name, ".md")
+			slug := strings.TrimSuffix(e.Name(), ".md")
-			title, aliases := readFrontmatter(bfs, dir+"/"+name, slug)
+			path := filepath.Join(dir, e.Name())
+			title, aliases := readFrontmatter(path, slug)
 			result[pt] = append(result[pt], Entry{Slug: slug, Title: title, Aliases: aliases, Type: pt})
 		}
 	}
@@ -50,14 +41,15 @@ func LoadInventory(brainDir string) (map[PageType][]Entry, error) {
 
 // readFrontmatter extracts title and aliases from YAML frontmatter.
 // Falls back to slug for title and empty aliases on any error.
-func readFrontmatter(bfs *brainfs.Brain, rel, fallbackSlug string) (title string, aliases []string) {
+func readFrontmatter(path, fallbackSlug string) (title string, aliases []string) {
 	title = fallbackSlug
-	content, err := bfs.ReadRegular(rel)
+	f, err := os.Open(path)
 	if err != nil {
 		return
 	}
+	defer f.Close() //nolint:errcheck
 
-	scanner := bufio.NewScanner(bytes.NewReader(content))
+	scanner := bufio.NewScanner(f)
 	inFM := false
 	inAliases := false
 	for scanner.Scan() {

ingestion/internal/wiki/root_test.go

@@ -1,79 +0,0 @@
-package wiki
-
-import (
-	"fmt"
-	"os"
-	"path/filepath"
-	"testing"
-	"time"
-
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
-
-	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
-	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs/brainfstest"
-)
-
-// hyperguild#157 slice 4: LoadInventory listed wiki/<type>/ with os.ReadDir
-// and opened each .md by path, so a symlinked page put the target's title and
-// aliases into the inventory (and the LLM prompts built from it), and a FIFO
-// hung it. RebuildIndex read first sentences the same way.
-func TestLoadInventoryRefusesSymlinksAndSurvivesSwapRaces(t *testing.T) {
-	brain := t.TempDir()
-	dir := filepath.Join(brain, "wiki", "concepts")
-	require.NoError(t, os.MkdirAll(dir, 0o755))
-	require.NoError(t, os.WriteFile(filepath.Join(dir, "ok.md"), []byte("---\ntitle: zqinside\n---\n"), 0o644))
-	require.NoError(t, os.Symlink(brainfstest.Outside(t), filepath.Join(dir, "leak.md")))
-	inBrain := brainfstest.WriteSentinelAt(t, filepath.Join(brain, "raw", "s.md"))
-	require.NoError(t, os.Symlink(filepath.Join("..", "..", "raw", "s.md"), filepath.Join(dir, "inlink.md")))
-
-	inv, err := LoadInventory(brain)
-	require.NoError(t, err)
-	got := fmt.Sprintf("%+v", inv)
-	assert.Contains(t, got, "zqinside", "positive control")
-	assert.NotContains(t, got, brainfstest.Sentinel)
-	assert.NotContains(t, got, "leak", "a symlinked page is not listed")
-	assert.NotContains(t, got, "inlink", "a page linked to another brain file is not listed")
-
-	p := filepath.Join(dir, "r.md")
-	read := func() string {
-		inv, _ := LoadInventory(brain)
-		return fmt.Sprintf("%+v", inv)
-	}
-	brainfstest.AssertNoLeak(t, p, "---\ntitle: zqinside\n---\n", 1500*time.Millisecond, read)
-	brainfstest.AssertNoLeakFrom(t, p, "---\ntitle: zqinside\n---\n", inBrain, 1500*time.Millisecond, read)
-	brainfstest.AssertNoHang(t, p, "---\ntitle: zqinside\n---\n", 1500*time.Millisecond, 6*time.Second, func() { _ = read() })
-}
-
-func TestLoadInventoryReturnsErrNotRegularForASymlinkedTypeDir(t *testing.T) {
-	brain := t.TempDir()
-	require.NoError(t, os.MkdirAll(filepath.Join(brain, "wiki"), 0o755))
-	require.NoError(t, os.MkdirAll(filepath.Join(brain, "raw"), 0o755))
-	require.NoError(t, os.WriteFile(filepath.Join(brain, "raw", "r.md"), []byte("---\ntitle: zqrawonly\n---\n"), 0o644))
-	require.NoError(t, os.Symlink(filepath.Join("..", "raw"), filepath.Join(brain, "wiki", "concepts")))
-	inv, err := LoadInventory(brain)
-	require.ErrorIs(t, err, brainfs.ErrNotRegular)
-	assert.NotContains(t, fmt.Sprintf("%+v", inv), "zqrawonly")
-}
-
-func TestRebuildIndexDoesNotReadAPageSwappedForASymlink(t *testing.T) {
-	brain := t.TempDir()
-	dir := filepath.Join(brain, "wiki", "concepts")
-	require.NoError(t, os.MkdirAll(dir, 0o755))
-	p := filepath.Join(dir, "r.md")
-	require.NoError(t, os.WriteFile(p, []byte("---\ntitle: R\n---\nzqinside sentence.\n"), 0o644))
-	require.NoError(t, RebuildIndex(brain, "2026-10-04"))
-	idx, err := os.ReadFile(filepath.Join(brain, "wiki", "index.md"))
-	require.NoError(t, err)
-	require.Contains(t, string(idx), "zqinside", "positive control: the summary is read")
-
-	read := func() string {
-		_ = RebuildIndex(brain, "2026-10-04")
-		b, _ := os.ReadFile(filepath.Join(brain, "wiki", "index.md"))
-		return string(b)
-	}
-	brainfstest.AssertNoLeak(t, p, "---\ntitle: R\n---\nzqinside sentence.\n", 1500*time.Millisecond, read)
-	inBrain := brainfstest.WriteSentinelAt(t, filepath.Join(brain, "raw", "s.md"))
-	brainfstest.AssertNoLeakFrom(t, p, "---\ntitle: R\n---\nzqinside sentence.\n", inBrain, 1500*time.Millisecond, read)
-	brainfstest.AssertNoHang(t, p, "---\ntitle: R\n---\nzqinside sentence.\n", 1500*time.Millisecond, 6*time.Second, func() { _ = read() })
-}