'zfs receive' fails with "dataset is busy"

Receiving an incremental stream after an interrupted "zfs receive -s"
fails with the message "dataset is busy": this is because we still have
the hidden clone ../%recv from the resumable receive.

Improve the error message suggesting the existence of a partially
complete resumable stream from "zfs receive -s" which can be either
aborted ("zfs receive -A") or resumed ("zfs send -t").

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes openzfs#7129 
Closes openzfs#7154

cmd/zfs/zfs_main.c

@@ -6072,7 +6072,7 @@ share_mount_one(zfs_handle_t *zhp, int op, int flags, char *protocol,
 
 		(void) fprintf(stderr, gettext("cannot %s '%s': "
 		    "Contains partially-completed state from "
-		    "\"zfs receive -r\", which can be resumed with "
+		    "\"zfs receive -s\", which can be resumed with "
 		    "\"zfs send -t\"\n"),
 		    cmdname, zfs_get_name(zhp));
 		return (1);

lib/libzfs/libzfs_sendrecv.c

@@ -3254,6 +3254,7 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 	zfs_type_t type;
 	boolean_t toplevel = B_FALSE;
 	boolean_t zoned = B_FALSE;
+	boolean_t hastoken = B_FALSE;
 
 	begin_time = time(NULL);
 	bzero(origin, MAXNAMELEN);
@@ -3535,6 +3536,11 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 		/* we want to know if we're zoned when validating -o|-x props */
 		zoned = zfs_prop_get_int(zhp, ZFS_PROP_ZONED);
 
+		/* may need this info later, get it now we have zhp around */
+		if (zfs_prop_get(zhp, ZFS_PROP_RECEIVE_RESUME_TOKEN, NULL, 0,
+		    NULL, NULL, 0, B_TRUE) == 0)
+			hastoken = B_TRUE;
+
 		/* gather existing properties on destination */
 		origprops = fnvlist_alloc();
 		fnvlist_merge(origprops, zhp->zfs_props);
@@ -3741,9 +3747,19 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 			break;
 		case EDQUOT:
 			zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
-			    "destination %s space quota exceeded"), name);
+			    "destination %s space quota exceeded."), name);
 			(void) zfs_error(hdl, EZFS_NOSPC, errbuf);
 			break;
+		case EBUSY:
+			if (hastoken) {
+				zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
+				    "destination %s contains "
+				    "partially-complete state from "
+				    "\"zfs receive -s\"."), name);
+				(void) zfs_error(hdl, EZFS_BUSY, errbuf);
+				break;
+			}
+			/* fallthru */
 		default:
 			(void) zfs_standard_error(hdl, ioctl_errno, errbuf);
 		}