btrfs-progs: add option for recursive subvol snapshots

Adds an option -R to btrfs subvolume snapshot, corresponding to the flag
BTRFS_UTIL_CREATE_SNAPSHOT_RECURSIVE.

This is another resubmission of a missed patch of Omar's from 2018:
https://lore.kernel.org/all/e42cdc5d5287269faf4d09e8c9786d0b3adeb658.1516991902.git.osandov@fb.com/

Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Co-authored-by: Omar Sandoval <osandov@fb.com>

Author: maharmstone

Documentation/btrfs-subvolume.rst

@@ -252,17 +252,23 @@ show [options] <path>
         -u|--uuid UUID
                 show details about subvolume with the given *UUID*, looked up in *path*
 
-snapshot [-r] [-i <qgroupid>] <source> <dest>|[<dest>/]<name>
+snapshot [-r|-R] [-i <qgroupid>] <source> <dest>|[<dest>/]<name>
         Create a snapshot of the subvolume *source* with the
         name *name* in the *dest* directory.
 
         If only *dest* is given, the subvolume will be named the basename of *source*.
         If *source* is not a subvolume, btrfs returns an error.
 
+        If you wish to recursively create a readonly snapshot, you can run
+        :command:`btrfs property set <path> ro true` on each subvolume after this command completes.
+
         ``Options``
 
         -r
                 Make the new snapshot read only.
+        -R|--recursive
+                Recursively snapshot subvolumes beneath the source. This option cannot be
+                combined with -r.
         -i <qgroupid>
                 Add the newly created subvolume to a qgroup. This option can be given multiple
                 times.

cmds/subvolume.c

@@ -616,7 +616,7 @@ static int cmd_subvolume_delete(const struct cmd_struct *cmd, int argc, char **a
 static DEFINE_COMMAND_WITH_FLAGS(subvolume_delete, "delete", CMD_DRY_RUN);
 
 static const char * const cmd_subvolume_snapshot_usage[] = {
-	"btrfs subvolume snapshot [-r] [-i <qgroupid>] <subvolume> { <subdir>/<name> | <subdir> }",
+	"btrfs subvolume snapshot [-r|-R] [-i <qgroupid>] <subvolume> { <subdir>/<name> | <subdir> }",
 	"",
 	"Create a snapshot of a <subvolume>. Call it <name> and place it in the <subdir>.",
 	"(<subvolume> will look like a new sub-directory, but is actually a btrfs subvolume",
@@ -625,6 +625,7 @@ static const char * const cmd_subvolume_snapshot_usage[] = {
 	"When only <subdir> is given, the subvolume will be named the basename of <subvolume>.",
 	"",
 	OPTLINE("-r", "make the new snapshot readonly"),
+	OPTLINE("-R|--recursive", "recursively snapshot subvolumes beneath the source; this option cannot be combined with -r"),
 	OPTLINE("-i <qgroupid>", "Add the new snapshot to a qgroup (a quota group). This option can be given multiple times."),
 	HELPINFO_INSERT_GLOBALS,
 	HELPINFO_INSERT_QUIET,
@@ -642,7 +643,7 @@ static int cmd_subvolume_snapshot(const struct cmd_struct *cmd, int argc, char *
 
 	optind = 0;
 	while (1) {
-		int c = getopt(argc, argv, "i:r");
+		int c = getopt(argc, argv, "i:rR");
 		if (c < 0)
 			break;
 
@@ -657,11 +658,21 @@ static int cmd_subvolume_snapshot(const struct cmd_struct *cmd, int argc, char *
 		case 'r':
 			flags |= BTRFS_UTIL_CREATE_SNAPSHOT_READ_ONLY;
 			break;
+		case 'R':
+			flags |= BTRFS_UTIL_CREATE_SNAPSHOT_RECURSIVE;
+			break;
 		default:
 			usage_unknown_option(cmd, argv);
 		}
 	}
 
+	if ((flags & BTRFS_UTIL_CREATE_SNAPSHOT_READ_ONLY) &&
+	    (flags & BTRFS_UTIL_CREATE_SNAPSHOT_RECURSIVE)) {
+		error("-r and -R cannot be combined");
+		retval = 1;
+		goto out;
+	}
+
 	if (check_argc_exact(argc - optind, 2)) {
 		retval = 1;
 		goto out;