Save errno across LWLockRelease() calls

Fixup for "Drop slot's LWLock before returning from SaveSlotToPath()"
Reported-by: Michael Paquier <michael@paquier.xyz>

src/backend/replication/slot.c

@@ -1259,9 +1259,13 @@ SaveSlotToPath(ReplicationSlot *slot, const char *dir, int elevel)
 		/*
 		 * If not an ERROR, then release the lock before returning.  In case
 		 * of an ERROR, the error recovery path automatically releases the
-		 * lock, but no harm in explicitly releasing even in that case.
+		 * lock, but no harm in explicitly releasing even in that case.  Note
+		 * that LWLockRelease() could affect errno.
 		 */
+		int			save_errno = errno;
+
 		LWLockRelease(&slot->io_in_progress_lock);
+		errno = save_errno;
 		ereport(elevel,
 				(errcode_for_file_access(),
 				 errmsg("could not create file \"%s\": %m",
@@ -1325,7 +1329,10 @@ SaveSlotToPath(ReplicationSlot *slot, const char *dir, int elevel)
 
 	if (CloseTransientFile(fd) != 0)
 	{
+		int			save_errno = errno;
+
 		LWLockRelease(&slot->io_in_progress_lock);
+		errno = save_errno;
 		ereport(elevel,
 				(errcode_for_file_access(),
 				 errmsg("could not close file \"%s\": %m",
@@ -1336,7 +1343,10 @@ SaveSlotToPath(ReplicationSlot *slot, const char *dir, int elevel)
 	/* rename to permanent file, fsync file and directory */
 	if (rename(tmppath, path) != 0)
 	{
+		int			save_errno = errno;
+
 		LWLockRelease(&slot->io_in_progress_lock);
+		errno = save_errno;
 		ereport(elevel,
 				(errcode_for_file_access(),
 				 errmsg("could not rename file \"%s\" to \"%s\": %m",