From f095481862ccd175d297da67d0e313d135bd75c3 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Mon, 27 May 2024 13:37:49 -0400
Subject: eval: Print more information on filesystem loops

---
 src/bftw.c |  2 ++
 src/bftw.h |  2 ++
 src/eval.c | 16 +++++++++++++---
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/src/bftw.c b/src/bftw.c
index c80ac74..b3fd62a 100644
--- a/src/bftw.c
+++ b/src/bftw.c
@@ -1676,6 +1676,7 @@ static void bftw_init_ftwbuf(struct bftw_state *state, enum bftw_visit visit) {
 	ftwbuf->visit = visit;
 	ftwbuf->type = BFS_UNKNOWN;
 	ftwbuf->error = state->direrror;
+	ftwbuf->loopoff = 0;
 	ftwbuf->at_fd = AT_FDCWD;
 	ftwbuf->at_path = ftwbuf->path;
 	bftw_stat_init(&ftwbuf->stat_bufs, &state->stat_buf, &state->lstat_buf);
@@ -1733,6 +1734,7 @@ static void bftw_init_ftwbuf(struct bftw_state *state, enum bftw_visit visit) {
 			if (ancestor->dev == statbuf->dev && ancestor->ino == statbuf->ino) {
 				ftwbuf->type = BFS_ERROR;
 				ftwbuf->error = ELOOP;
+				ftwbuf->loopoff = ancestor->nameoff + ancestor->namelen;
 				return;
 			}
 		}
diff --git a/src/bftw.h b/src/bftw.h
index 8656ca7..a2a201c 100644
--- a/src/bftw.h
+++ b/src/bftw.h
@@ -56,6 +56,8 @@ struct BFTW {
 	enum bfs_type type;
 	/** The errno that occurred, if type == BFS_ERROR. */
 	int error;
+	/** For filesystem loops, the length of the loop prefix. */
+	size_t loopoff;
 
 	/** A parent file descriptor for the *at() family of calls. */
 	int at_fd;
diff --git a/src/eval.c b/src/eval.c
index 8490bd4..36b2f11 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1433,10 +1433,20 @@ static enum bftw_action eval_callback(const struct BFTW *ftwbuf, void *ptr) {
 	}
 
 	if (ftwbuf->type == BFS_ERROR) {
-		if (!eval_should_ignore(&state, ftwbuf->error)) {
-			eval_error(&state, "%s.\n", xstrerror(ftwbuf->error));
-		}
 		state.action = BFTW_PRUNE;
+
+		if (ftwbuf->error == ELOOP && ftwbuf->loopoff > 0) {
+			char *loop = strndup(ftwbuf->path, ftwbuf->loopoff);
+			if (loop) {
+				eval_error(&state, "Filesystem loop back to ${di}%pq${rs}\n", loop);
+				free(loop);
+				goto done;
+			}
+		} else if (eval_should_ignore(&state, ftwbuf->error)) {
+			goto done;
+		}
+
+		eval_error(&state, "%s.\n", xstrerror(ftwbuf->error));
 		goto done;
 	}
 
-- 
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