plan 8: apply refuses a source swapped for a symlink or another file

1 files changed, 16 insertions, 4 deletions

diff --git a/internal/apply/apply.go b/internal/apply/apply.go
index f177a17..713913e 100644
--- a/internal/apply/apply.go
+++ b/internal/apply/apply.go
@@ -77,17 +77,29 @@ func Chain(c plan.Chain) []StepResult {
 }
 
 // checkUnchanged is the guard that matters most: before every step, the
-// source is stat'd and compared against the size and mtime the plan
-// recorded for the whole file. A file rewritten or replaced between
-// planning and applying must never be acted on.
+// source is Lstat'd and compared against what the plan recorded for the
+// whole file. A file rewritten or replaced between planning and applying
+// must never be acted on (spec §15.1): its size and mtime must match, it
+// must still be a regular file - not a symlink put in its place - and, at
+// its planned path, it must be the same inode. The inode is not compared
+// once an earlier step has moved the file: a move across filesystems
+// copies it to a new inode, and the chain is still following its own file.
 func checkUnchanged(src string, f scan.File) error {
-	fi, err := os.Stat(src)
+	fi, err := os.Lstat(src)
 	if err != nil {
 		return fmt.Errorf("changed since plan: %w", err)
 	}
+	if !fi.Mode().IsRegular() {
+		return errors.New("changed since plan: no longer a regular file")
+	}
 	if fi.Size() != f.Size || !fi.ModTime().Equal(f.ModTime) {
 		return errors.New("changed since plan")
 	}
+	if src == f.Path && f.Ino != 0 {
+		if now := scan.NewFile(src, f.Rel, fi); now.Dev != f.Dev || now.Ino != f.Ino {
+			return errors.New("changed since plan: another file is in its place")
+		}
+	}
 	return nil
 }