[PATCH v2] libfrog: make cmn_err() emit each message atomically to avoid torn output

[Avinesh Kumar <avinesh.kumar@suse.com>]

From: Avinesh Kumar <avinesh.kumar@suse.com>

fstests xfs/033 fails sporadically with a spurious blank line in the
xfs_repair output:

- output mismatch (see /opt/xfstests/results//xfs/033.out.bad)
    --- tests/xfs/033.out	2026-06-24 15:52:51.000000000 -0400
    +++ /opt/xfstests/results//xfs/033.out.bad	2026-07-14 18:54:46.582495041 -0400
    @@ -103,6 +103,7 @@
     Phase 3 - for each AG...
             - scan and clear agi unlinked lists...
             - process known inodes and perform inode discovery...
    +
     bad magic number 0xffff on inode INO
     bad version number 0xffffffff on inode INO
     inode identifier 18446744073709551615 mismatch on inode INO

Root cause is in cmn_err() (libfrog/util.c), which emits a message
and its newline as two separate writes to unbuffered stderr.
xfs_repair's threads all share stderr. If one is preempted between the
two writes, another thread's line lands in between:

(snips from `cat -A 033.raw`) -
Phase 3 - for each AG...$
        - scan and clear agi unlinked lists...$
        - process known inodes and perform inode discovery...$
Metadata corruption detected at 0x445dd3, xfs_inode block 0x80/0x4000        - agno = 0$
$
bad CRC for inode 128$
bad magic number 0x0 on inode 128$

which should be like -

Phase 3 - for each AG...$
        - scan and clear agi unlinked lists...$
        - process known inodes and perform inode discovery...$
Metadata corruption detected at 0x445dd3, xfs_inode block 0x80/0x4000$
        - agno = 0$
bad CRC for inode 130$
bad magic number 0x0 on inode 130$

_filter_repair() strips the noise line it glued onto but not the lone
newline, which then fails the golden diff.

Make the two writes atomic.

do_error() in xfs_repair.c also has same issue, fix that too.

Signed-off-by: Avinesh Kumar <avinesh.kumar@suse.com>
---
v2: fix the do_error() in xfs_repair.c too.

 libfrog/util.c      | 2 ++
 repair/xfs_repair.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/libfrog/util.c b/libfrog/util.c
index 5bae5bab..3b6df917 100644
--- a/libfrog/util.c
+++ b/libfrog/util.c
@@ -116,8 +116,10 @@ cmn_err(int level, char *fmt, ...)
 	va_list	ap;
 
 	va_start(ap, fmt);
+	flockfile(stderr);
 	vfprintf(stderr, fmt, ap);
 	fputs("\n", stderr);
+	funlockfile(stderr);
 	va_end(ap);
 }
 
diff --git a/repair/xfs_repair.c b/repair/xfs_repair.c
index 6b97a806..dd758ebc 100644
--- a/repair/xfs_repair.c
+++ b/repair/xfs_repair.c
@@ -458,10 +458,12 @@ do_error(char const *msg, ...)
 {
 	va_list args;
 
+	flockfile(stderr);
 	fprintf(stderr, _("\nfatal error -- "));
 
 	va_start(args, msg);
 	vfprintf(stderr, msg, args);
+	funlockfile(stderr);
 	if (dumpcore)
 		abort();
 	exit(1);
-- 
2.55.0