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/brainfs/brainfs.go

@@ -114,7 +114,7 @@ func (b *Brain) ReadRegular(rel string) ([]byte, error) {
 	if !fi.Mode().IsRegular() {
 		return nil, fmt.Errorf("%s: %w", rel, ErrNotRegular)
 	}
-	return b.readOpened(rel)
+	return b.readOpenedMax(rel, -1, fi)
 }
 
 // readOpened opens rel non-blocking and reads it only if the OPENED handle is
@@ -128,7 +128,7 @@ 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) }
+func (b *Brain) readOpened(rel string) ([]byte, error) { return b.readOpenedMax(rel, -1, nil) }
 
 // ReadRegularMax is ReadRegular with a size limit, checked on the opened
 // handle before reading (max < 0 means no limit).
@@ -140,10 +140,14 @@ 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)
+	return b.readOpenedMax(rel, max, fi)
 }
 
-func (b *Brain) readOpenedMax(rel string, max int64) ([]byte, error) {
+// readOpenedMax opens rel and reads it if the opened handle is a regular file
+// 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
@@ -156,6 +160,9 @@ func (b *Brain) readOpenedMax(rel string, max int64) ([]byte, error) {
 	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)
 	}