[PATCH] RFC: autofs-5.1.9 - flag removed entries as stale

[David Disseldorp <ddiss@suse.de>]

This effectively reverts commit 21ce28d ("autofs-5.1.4 - mark removed
cache entry negative"), which causes the kernel to stall in autofs_wait
for the following workload:

  cat > /etc/auto.direct <<EOF
  echo "/nfs/share  $mount_args ${NFS_SERVER}:/${NFS_SHARE}"
  EOF

  setsid --fork automount --debug --foreground &> /automount.log
  sleep 1

  touch /test.run
  setsid --fork /bin/bash -c \
    "while [[ -f /test.run ]]; do df -ia >> /test.log; sleep 1; done"
  echo "df loop logging to /test.log"

  sleep 2
  echo "changing and reloading auto.direct"
  echo > /etc/auto.direct
  killall -HUP automount

  sleep 2
  echo "unmounting..."
  umount /nfs/share || echo "umount failed"

The current behaviour sees us hit:
  handle_packet_missing_direct:1352: can't find map entry for ()
...which doesn't respond to the kernel, triggering the stall.

This approach adds a new MOUNT_FLAG_STALE flag to track removed map
entries. While keeping enough state around to respond for the
handle_packet_missing_direct case.

RFC:
- needs further testing (e.g. indirect maps)
- I'm not familiar with the codebase so this may be the wrong approach
- we may need a background job to purge MOUNT_FLAG_STALE entries?

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 daemon/direct.c     |  8 ++++++--
 daemon/indirect.c   |  8 ++++++--
 daemon/lookup.c     | 11 ++++-------
 include/automount.h |  3 +++
 4 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/daemon/direct.c b/daemon/direct.c
index 42baac8..5e78c40 100644
--- a/daemon/direct.c
+++ b/daemon/direct.c
@@ -1389,8 +1389,12 @@ int handle_packet_missing_direct(struct autofs_point *ap, autofs_packet_missing_
 		return 0;
 	}
 
-	/* Check if we recorded a mount fail for this key */
-	if (me->status >= monotonic_time(NULL)) {
+	/*
+	 * Check if we recorded a mount fail for this key, or the entry has
+	 * been removed.
+	 */
+	if (me->status >= monotonic_time(NULL) ||
+	    me->flags & MOUNT_FLAG_STALE) {
 		ops->send_fail(ap->logopt,
 			       ioctlfd, pkt->wait_queue_token, -ENOENT);
 		ops->close(ap->logopt, ioctlfd);
diff --git a/daemon/indirect.c b/daemon/indirect.c
index 7d4aad7..934bb74 100644
--- a/daemon/indirect.c
+++ b/daemon/indirect.c
@@ -798,8 +798,12 @@ int handle_packet_missing_indirect(struct autofs_point *ap, autofs_packet_missin
 
 	me = lookup_source_mapent(ap, pkt->name, LKP_DISTINCT);
 	if (me) {
-		/* Check if we recorded a mount fail for this key */
-		if (me->status >= monotonic_time(NULL)) {
+		/*
+		 * Check if we recorded a mount fail for this key, or the entry
+		 * has been removed.
+		 */
+		if (me->status >= monotonic_time(NULL) ||
+		    me->flags & MOUNT_FLAG_STALE) {
 			ops->send_fail(ap->logopt, ap->ioctlfd,
 				       pkt->wait_queue_token, -ENOENT);
 			cache_unlock(me->mc);
diff --git a/daemon/lookup.c b/daemon/lookup.c
index dc77948..ad0b460 100644
--- a/daemon/lookup.c
+++ b/daemon/lookup.c
@@ -1416,15 +1416,12 @@ void lookup_prune_one_cache(struct autofs_point *ap, struct mapent_cache *mc, ti
 		if (valid && valid->mc == mc) {
 			 /*
 			  * We've found a map entry that has been removed from
-			  * the current cache so it isn't really valid. Set the
-			  * mapent negative to prevent further mount requests
+			  * the current cache so it isn't really valid. Flag the
+			  * mapent stale to prevent further mount requests
 			  * using the cache entry.
 			  */
-			debug(ap->logopt, "removed map entry detected, mark negative");
-			if (valid->mapent) {
-				free(valid->mapent);
-				valid->mapent = NULL;
-			}
+			debug(ap->logopt, "removed map entry detected, mark stale");
+			valid->flags |= MOUNT_FLAG_STALE;
 			cache_unlock(valid->mc);
 			valid = NULL;
 		}
diff --git a/include/automount.h b/include/automount.h
index 9548db8..007d020 100644
--- a/include/automount.h
+++ b/include/automount.h
@@ -548,6 +548,9 @@ struct kernel_mod_version {
 /* Indicator for applications to ignore the mount entry */
 #define MOUNT_FLAG_IGNORE		0x1000
 
+/* map has been removed, but we can't clean up yet */
+#define MOUNT_FLAG_STALE		0x2000
+
 struct autofs_point {
 	pthread_t thid;
 	char *path;			/* Mount point name */
-- 
2.43.0