ZTS: Add test for snapshot automount race

Add snapshot_019_pos to verify parallel snapshot automount operations
don't cause AVL tree panic. Regression test for commit 4ce030e025.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #18035

tests/runfiles/common.run

@@ -1020,7 +1020,7 @@ tests = ['clone_001_pos', 'rollback_001_pos', 'rollback_002_pos',
     'snapshot_006_pos', 'snapshot_007_pos', 'snapshot_008_pos',
     'snapshot_009_pos', 'snapshot_010_pos', 'snapshot_011_pos',
     'snapshot_012_pos', 'snapshot_013_pos', 'snapshot_014_pos',
-    'snapshot_017_pos', 'snapshot_018_pos']
+    'snapshot_017_pos', 'snapshot_018_pos', 'snapshot_019_pos']
 tags = ['functional', 'snapshot']
 
 [tests/functional/snapused]

tests/runfiles/sanity.run

@@ -580,7 +580,7 @@ tests = ['clone_001_pos', 'rollback_001_pos', 'rollback_002_pos',
     'snapshot_007_pos', 'snapshot_008_pos', 'snapshot_009_pos',
     'snapshot_010_pos', 'snapshot_011_pos', 'snapshot_012_pos',
     'snapshot_013_pos', 'snapshot_014_pos', 'snapshot_017_pos',
-    'snapshot_018_pos']
+    'snapshot_018_pos', 'snapshot_019_pos']
 tags = ['functional', 'snapshot']
 
 [tests/functional/snapused]

tests/zfs-tests/tests/Makefile.am

@@ -2122,6 +2122,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/snapshot/snapshot_016_pos.ksh \
 	functional/snapshot/snapshot_017_pos.ksh \
 	functional/snapshot/snapshot_018_pos.ksh \
+	functional/snapshot/snapshot_019_pos.ksh \
 	functional/snapused/cleanup.ksh \
 	functional/snapused/setup.ksh \
 	functional/snapused/snapused_001_pos.ksh \

tests/zfs-tests/tests/functional/snapshot/snapshot_019_pos.ksh

@@ -0,0 +1,82 @@
+#!/bin/ksh -p
+# SPDX-License-Identifier: CDDL-1.0
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or https://opensource.org/licenses/CDDL-1.0.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright 2025 iXsystems, Inc.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/snapshot/snapshot.cfg
+
+#
+# DESCRIPTION:
+# Verify that parallel snapshot automount operations don't cause AVL tree
+# panic due to duplicate mount attempts.
+#
+# STRATEGY:
+# 1. Create a filesystem with snapdir=visible
+# 2. Create a snapshot
+# 3. Trigger parallel ls operations on the snapshot directory
+# 4. Verify no kernel panic occurred and snapshot is accessible
+#
+
+function cleanup
+{
+	destroy_pool $TESTPOOL
+}
+
+verify_runnable "both"
+
+log_assert "Verify parallel snapshot automount doesn't cause AVL tree panic"
+
+log_onexit cleanup
+
+# Create pool and filesystem
+create_pool $TESTPOOL $DISKS
+log_must zfs create -o snapdir=visible -o mountpoint=$TESTDIR $TESTPOOL/$TESTFS
+
+# Create a snapshot
+log_must zfs snapshot $SNAPFS
+
+# Trigger parallel automount operations to reproduce the race condition.
+# Multiple concurrent ls operations will attempt to automount the same
+# unmounted snapshot, which previously could cause duplicate mount helpers
+# and AVL tree panic.
+snapdir_path="$TESTDIR/.zfs/snapshot/$TESTSNAP"
+for i in {1..100}
+do
+	ls $snapdir_path >/dev/null 2>&1 &
+done
+
+# Wait for all background processes to complete
+wait
+
+# Verify the snapshot is accessible and properly mounted after parallel access
+log_must ls $snapdir_path
+
+# Verify we can unmount the filesystem cleanly. This confirms no processes
+# are stuck in a syscall and all automated snapshots were unmounted properly.
+# If the AVL panic occurred, unmount would fail.
+log_must zfs unmount $TESTPOOL/$TESTFS
+
+log_pass "Parallel snapshot automount completed without AVL tree panic"