ZTS: test response of various sync methods under different failmodes

These are all the same shape: set up the pool to suspend on first write,
then perform some write+sync operation. The pool should suspend, and the
sync operation should respond according to the failmode= property.

We test fsync(), msync() and two forms of write() (open with O_SYNC, and
async with sync=always), which all take slightly different paths to
zil_commit() and back.

A helper function is included to do the write+sync sequence with mmap()
and msync(), since I didn't find a convenient tool to do that.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17398

tests/zfs-tests/cmd/.gitignore

@@ -28,6 +28,7 @@
 /mmap_seek
 /mmap_sync
 /mmapwrite
+/mmap_write_sync
 /nvlist_to_lua
 /randfree_file
 /randwritecomp

tests/zfs-tests/cmd/Makefile.am

@@ -74,7 +74,7 @@ scripts_zfs_tests_bin_PROGRAMS += %D%/mkbusy %D%/mkfile %D%/mkfiles %D%/mktree
 
 scripts_zfs_tests_bin_PROGRAMS += \
 	%D%/mmap_exec %D%/mmap_ftruncate %D%/mmap_seek \
-	%D%/mmap_sync %D%/mmapwrite %D%/readmmap
+	%D%/mmap_sync %D%/mmapwrite %D%/readmmap %D%/mmap_write_sync
 %C%_mmapwrite_LDADD = -lpthread
 
 if WANT_MMAP_LIBAIO

tests/zfs-tests/cmd/mmap_write_sync.c

@@ -0,0 +1,84 @@
+// 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 (c) 2025, Klara, Inc.
+ */
+
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+
+#define	PAGES	(8)
+
+int
+main(int argc, char **argv)
+{
+	if (argc != 2) {
+		fprintf(stderr, "usage: %s <filename>\n", argv[0]);
+		exit(1);
+	}
+
+	long page_size = sysconf(_SC_PAGESIZE);
+	if (page_size < 0) {
+		perror("sysconf");
+		exit(2);
+	}
+	size_t map_size = page_size * PAGES;
+
+	int fd = open(argv[1], O_CREAT|O_RDWR, S_IRWXU|S_IRWXG|S_IRWXO);
+	if (fd < 0) {
+		perror("open");
+		exit(2);
+	}
+
+	if (ftruncate(fd, map_size) < 0) {
+		perror("ftruncate");
+		close(fd);
+		exit(2);
+	}
+
+	uint64_t *p =
+	    mmap(NULL, map_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
+	if (p == MAP_FAILED) {
+		perror("mmap");
+		close(fd);
+		exit(2);
+	}
+
+	for (int i = 0; i < (map_size / sizeof (uint64_t)); i++)
+		p[i] = 0x0123456789abcdef;
+
+	if (msync(p, map_size, MS_SYNC) < 0) {
+		perror("msync");
+		munmap(p, map_size);
+		close(fd);
+		exit(3);
+	}
+
+	munmap(p, map_size);
+	close(fd);
+	exit(0);
+}