zdb/ztest: send dbgmsg output to stderr

And, make the output fd an arg to zfs_dbgmsg_print(). This is a change
in behaviour, but keeps it consistent with where crash traces go, and
it's easy to argue this is what we want anyway; this is information
about the task, not the actual output of the task.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16181

cmd/zdb/zdb.c

@@ -837,8 +837,8 @@ dump_debug_buffer(void)
 	 * We use write() instead of printf() so that this function
 	 * is safe to call from a signal handler.
 	 */
-	ret = write(STDOUT_FILENO, "\n", 1);
-	zfs_dbgmsg_print("zdb");
+	ret = write(STDERR_FILENO, "\n", 1);
+	zfs_dbgmsg_print(STDERR_FILENO, "zdb");
 }
 
 static void sig_handler(int signo)

cmd/ztest.c

@@ -615,8 +615,8 @@ dump_debug_buffer(void)
 	 * We use write() instead of printf() so that this function
 	 * is safe to call from a signal handler.
 	 */
-	ret = write(STDOUT_FILENO, "\n", 1);
-	zfs_dbgmsg_print("ztest");
+	ret = write(STDERR_FILENO, "\n", 1);
+	zfs_dbgmsg_print(STDERR_FILENO, "ztest");
 }
 
 static void sig_handler(int signo)

include/sys/zfs_debug.h

@@ -104,7 +104,7 @@ extern void zfs_dbgmsg_fini(void);
 
 #ifndef _KERNEL
 extern int dprintf_find_string(const char *string);
-extern void zfs_dbgmsg_print(const char *tag);
+extern void zfs_dbgmsg_print(int fd, const char *tag);
 #endif
 
 #ifdef	__cplusplus

module/os/freebsd/zfs/zfs_debug.c

@@ -232,30 +232,29 @@ __dprintf(boolean_t dprint, const char *file, const char *func,
 #else
 
 void
-zfs_dbgmsg_print(const char *tag)
+zfs_dbgmsg_print(int fd, const char *tag)
 {
 	ssize_t ret __attribute__((unused));
 
-	mutex_enter(&zfs_dbgmsgs_lock);
-
 	/*
 	 * We use write() in this function instead of printf()
 	 * so it is safe to call from a signal handler.
 	 */
-	ret = write(STDOUT_FILENO, "ZFS_DBGMSG(", 11);
-	ret = write(STDOUT_FILENO, tag, strlen(tag));
-	ret = write(STDOUT_FILENO, ") START:\n", 9);
+	ret = write(fd, "ZFS_DBGMSG(", 11);
+	ret = write(fd, tag, strlen(tag));
+	ret = write(fd, ") START:\n", 9);
+
+	mutex_enter(&zfs_dbgmsgs_lock);
 
-	for (zfs_dbgmsg_t zdm = list_head(&zfs_dbgmsgs); zdm != NULL;
+	for (zfs_dbgmsg_t *zdm = list_head(&zfs_dbgmsgs); zdm != NULL;
 	    zdm = list_next(&zfs_dbgmsgs, zdm))
-		ret = write(STDOUT_FILENO, zdm->zdm_msg,
-		    strlen(zdm->zdm_msg));
-		ret = write(STDOUT_FILENO, "\n", 1);
+		ret = write(fd, zdm->zdm_msg, strlen(zdm->zdm_msg));
+		ret = write(fd, "\n", 1);
 	}
 
-	ret = write(STDOUT_FILENO, "ZFS_DBGMSG(", 11);
-	ret = write(STDOUT_FILENO, tag, strlen(tag));
-	ret = write(STDOUT_FILENO, ") END\n", 6);
+	ret = write(fd, "ZFS_DBGMSG(", 11);
+	ret = write(fd, tag, strlen(tag));
+	ret = write(fd, ") END\n", 6);
 
 	mutex_exit(&zfs_dbgmsgs_lock);
 }

module/os/linux/zfs/zfs_debug.c

@@ -221,7 +221,7 @@ __dprintf(boolean_t dprint, const char *file, const char *func,
 #else
 
 void
-zfs_dbgmsg_print(const char *tag)
+zfs_dbgmsg_print(int fd, const char *tag)
 {
 	ssize_t ret __attribute__((unused));
 
@@ -231,20 +231,19 @@ zfs_dbgmsg_print(const char *tag)
 	 * We use write() in this function instead of printf()
 	 * so it is safe to call from a signal handler.
 	 */
-	ret = write(STDOUT_FILENO, "ZFS_DBGMSG(", 11);
-	ret = write(STDOUT_FILENO, tag, strlen(tag));
-	ret = write(STDOUT_FILENO, ") START:\n", 9);
+	ret = write(fd, "ZFS_DBGMSG(", 11);
+	ret = write(fd, tag, strlen(tag));
+	ret = write(fd, ") START:\n", 9);
 
 	for (zfs_dbgmsg_t *zdm = list_head(&zfs_dbgmsgs.pl_list); zdm != NULL;
 	    zdm = list_next(&zfs_dbgmsgs.pl_list, zdm)) {
-		ret = write(STDOUT_FILENO, zdm->zdm_msg,
-		    strlen(zdm->zdm_msg));
-		ret = write(STDOUT_FILENO, "\n", 1);
+		ret = write(fd, zdm->zdm_msg, strlen(zdm->zdm_msg));
+		ret = write(fd, "\n", 1);
 	}
 
-	ret = write(STDOUT_FILENO, "ZFS_DBGMSG(", 11);
-	ret = write(STDOUT_FILENO, tag, strlen(tag));
-	ret = write(STDOUT_FILENO, ") END\n", 6);
+	ret = write(fd, "ZFS_DBGMSG(", 11);
+	ret = write(fd, tag, strlen(tag));
+	ret = write(fd, ") END\n", 6);
 
 	mutex_exit(&zfs_dbgmsgs.pl_lock);
 }