fix(brainfs): refuse an in-brain file swapped in behind a symlink; in-brain test targets (#157)

Review round 1 on PR #168:

- ReadRegular/ReadRegularMax Lstat the path, then open it again by path;
  os.Root follows an in-root final symlink even with O_NOFOLLOW, so a
  symlink to another brain file swapped in between was read (8k-109k
  raw/ leaks in RebuildIndex, LoadInventory, IndexDoc). The opened handle
  must now be the same file as the Lstat (os.SameFile). This covers every
  ReadRegular caller, slices 1-3 included.
- Every symlink test pointed outside the brain, which os.Root refuses on
  its own. Each now has an in-brain case too: a static in-brain link per
  reader, AssertNoLeakFrom with an in-brain target, and brainfs unit tests
  for ReadDirRegular and the in-brain swap.
- brainfstest: swap links are relative (os.Root refuses every absolute
  link, so an absolute in-brain target tested nothing), and each swapper
  has its own temp name (two swappers on one path deadlocked on each
  other's FIFO).
- IndexDoc indexes only a clean wiki/ or knowledge/ path (callers pass
  caller-supplied paths).
- Rule 9: TestLoadInventoryReturnsErrNotRegularForASymlinkedTypeDir now
  asserts the error; the backfill test says it aborts on a symlinked root.

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

ingestion/internal/graphsync/graphsync_root_test.go

@@ -34,6 +34,11 @@ func TestIndexDocRefusesSymlinksAndSurvivesSwapRaces(t *testing.T) {
 	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")
@@ -43,10 +48,11 @@ func TestIndexDocRefusesSymlinksAndSurvivesSwapRaces(t *testing.T) {
 		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 TestBackfillSkipsSymlinkedNotesAndASymlinkedRoot(t *testing.T) {
+func TestBackfillSkipsSymlinkedNotesAndAbortsOnASymlinkedRoot(t *testing.T) {
 	brain := t.TempDir()
 	facts := filepath.Join(brain, "wiki", "w", "facts")
 	require.NoError(t, os.MkdirAll(facts, 0o755))
@@ -54,6 +60,7 @@ func TestBackfillSkipsSymlinkedNotesAndASymlinkedRoot(t *testing.T) {
 	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()
@@ -70,3 +77,16 @@ func TestBackfillSkipsSymlinkedNotesAndASymlinkedRoot(t *testing.T) {
 	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)
+	}
+}