fix(ingestion): brain/raw/ readers go through an os.Root; symlinks are inert (#153) (#156)

Co-authored-by: Mathias <mthbqv@gmail.com>

ingestion/internal/api/rawroot.go

@@ -0,0 +1,67 @@
+package api
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/extract"
+)
+
+// errNotANote is returned for a raw/ entry that is not a regular file — in
+// particular a symlink, whose target may be anywhere (hyperguild#153).
+var errNotANote = errors.New("not a regular note file in raw/ (symlinks are refused)")
+
+// errRawNotDir means brain/raw is not a real directory (e.g. a symlink), so
+// containment inside it would mean nothing (hyperguild#153 item 4).
+var errRawNotDir = errors.New("brain/raw is not a real directory")
+
+// OpenRawRoot opens brain/raw as an os.Root, refusing a raw/ that is itself a
+// symlink. Reads through the root cannot leave it, even if an entry is
+// swapped for a symlink between a check and the read. It returns the root
+// and raw/'s path. A missing raw/ returns an error wrapping os.ErrNotExist.
+func OpenRawRoot(brainDir string) (*os.Root, string, error) {
+	p := filepath.Join(brainDir, "raw")
+	fi, err := os.Lstat(p)
+	if err != nil {
+		return nil, "", err
+	}
+	if fi.Mode()&os.ModeSymlink != 0 || !fi.IsDir() {
+		return nil, "", errRawNotDir
+	}
+	r, err := os.OpenRoot(p)
+	if err != nil {
+		return nil, "", fmt.Errorf("open raw root: %w", err)
+	}
+	return r, p, nil
+}
+
+// readRawNote reads one raw/ entry by basename through the root, refusing
+// anything that is not a regular file.
+//
+// Accepted residual: a HARD link to an outside file is a regular file and is
+// read. Syncthing syncs contents, not links, so a peer cannot create one;
+// fs.protected_hardlinks=1 and the pod's secrets live 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 assumption changes.
+func readRawNote(r *os.Root, base string) ([]byte, os.FileInfo, error) {
+	fi, err := r.Lstat(base)
+	if err != nil {
+		return nil, nil, err
+	}
+	if !fi.Mode().IsRegular() {
+		return nil, nil, errNotANote
+	}
+	f, _, err := extract.OpenRegular(r, base)
+	if err != nil {
+		return nil, nil, err
+	}
+	defer func() { _ = f.Close() }()
+	b, err := io.ReadAll(f)
+	if err != nil {
+		return nil, nil, err
+	}
+	return b, fi, nil
+}