Add 'zfs wait' command

Add a mechanism to wait for delete queue to drain.

When doing redacted send/recv, many workflows involve deleting files 
that contain sensitive data. Because of the way zfs handles file 
deletions, snapshots taken quickly after a rm operation can sometimes 
still contain the file in question, especially if the file is very 
large. This can result in issues for redacted send/recv users who 
expect the deleted files to be redacted in the send streams, and not 
appear in their clones.

This change duplicates much of the zpool wait related logic into a 
zfs wait command, which can be used to wait until the internal
deleteq has been drained.  Additional wait activities may be added 
in the future. 

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Gallagher <john.gallagher@delphix.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #9707

module/zfs/zfs_ioctl.c

@@ -4072,6 +4072,83 @@ zfs_ioc_wait(const char *name, nvlist_t *innvl, nvlist_t *outnvl)
 	return (error);
 }
 
+/*
+ * This ioctl waits for activity of a particular type to complete. If there is
+ * no activity of that type in progress, it returns immediately, and the
+ * returned value "waited" is false. If there is activity in progress, and no
+ * tag is passed in, the ioctl blocks until all activity of that type is
+ * complete, and then returns with "waited" set to true.
+ *
+ * If a thread waiting in the ioctl receives a signal, the call will return
+ * immediately, and the return value will be EINTR.
+ *
+ * innvl: {
+ *     "wait_activity" -> int32_t
+ * }
+ *
+ * outnvl: "waited" -> boolean_t
+ */
+static const zfs_ioc_key_t zfs_keys_fs_wait[] = {
+	{ZFS_WAIT_ACTIVITY,	DATA_TYPE_INT32,		0},
+};
+
+static int
+zfs_ioc_wait_fs(const char *name, nvlist_t *innvl, nvlist_t *outnvl)
+{
+	int32_t activity;
+	boolean_t waited = B_FALSE;
+	int error;
+	dsl_pool_t *dp;
+	dsl_dir_t *dd;
+	dsl_dataset_t *ds;
+
+	if (nvlist_lookup_int32(innvl, ZFS_WAIT_ACTIVITY, &activity) != 0)
+		return (SET_ERROR(EINVAL));
+
+	if (activity >= ZFS_WAIT_NUM_ACTIVITIES || activity < 0)
+		return (SET_ERROR(EINVAL));
+
+	if ((error = dsl_pool_hold(name, FTAG, &dp)) != 0)
+		return (error);
+
+	if ((error = dsl_dataset_hold(dp, name, FTAG, &ds)) != 0) {
+		dsl_pool_rele(dp, FTAG);
+		return (error);
+	}
+
+	dd = ds->ds_dir;
+	mutex_enter(&dd->dd_activity_lock);
+	dd->dd_activity_waiters++;
+
+	/*
+	 * We get a long-hold here so that the dsl_dataset_t and dsl_dir_t
+	 * aren't evicted while we're waiting. Normally this is prevented by
+	 * holding the pool, but we can't do that while we're waiting since
+	 * that would prevent TXGs from syncing out. Some of the functionality
+	 * of long-holds (e.g. preventing deletion) is unnecessary for this
+	 * case, since we would cancel the waiters before proceeding with a
+	 * deletion. An alternative mechanism for keeping the dataset around
+	 * could be developed but this is simpler.
+	 */
+	dsl_dataset_long_hold(ds, FTAG);
+	dsl_pool_rele(dp, FTAG);
+
+	error = dsl_dir_wait(dd, ds, activity, &waited);
+
+	dsl_dataset_long_rele(ds, FTAG);
+	dd->dd_activity_waiters--;
+	if (dd->dd_activity_waiters == 0)
+		cv_signal(&dd->dd_activity_cv);
+	mutex_exit(&dd->dd_activity_lock);
+
+	dsl_dataset_rele(ds, FTAG);
+
+	if (error == 0)
+		fnvlist_add_boolean_value(outnvl, ZFS_WAIT_WAITED, waited);
+
+	return (error);
+}
+
 /*
  * fsname is name of dataset to rollback (to most recent snapshot)
  *
@@ -6915,6 +6992,11 @@ zfs_ioctl_init(void)
 	    POOL_CHECK_SUSPENDED | POOL_CHECK_READONLY, B_FALSE, B_FALSE,
 	    zfs_keys_pool_wait, ARRAY_SIZE(zfs_keys_pool_wait));
 
+	zfs_ioctl_register("wait_fs", ZFS_IOC_WAIT_FS,
+	    zfs_ioc_wait_fs, zfs_secpolicy_none, DATASET_NAME,
+	    POOL_CHECK_SUSPENDED | POOL_CHECK_READONLY, B_FALSE, B_FALSE,
+	    zfs_keys_fs_wait, ARRAY_SIZE(zfs_keys_fs_wait));
+
 	/* IOCTLS that use the legacy function signature */
 
 	zfs_ioctl_register_legacy(ZFS_IOC_POOL_FREEZE, zfs_ioc_pool_freeze,