Allow zdb -r to understand object ids as well

[allanjude]

Sponsored-by: Klara, Inc.
Sponsored-By: Wasabi Technology, Inc.

Motivation and Context

Previously I created: zdb -r pool/dataset path/to/file /tmp/recover.dat

But it turns out, if there is a problem with a directory, you might want to recover a file by its inode number instead

Description

If the argument is a number, thread it as the object-id instead of a filename

How Has This Been Tested?

Manually

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[robn]

There's a couple of problems here. zdb_numeric() and strtoull(base=0) permit different things, for example, zdb_numeric allows abc, but strtoull() does not, and will return 0.

The other more obvious problem is that this prevents a file name that zdb_numeric() returns true for, eg, there's no way to actually dump a file called abc.

Probably it needs a switch or magic value to say "interpret as number, not name". Maybe it's -N to match the "dataset is an objset id" case? Or maybe it can be a leading / on the filename, since that can't appear relative to the dataset? This seems to work reasonably well:

diff --git cmd/zdb/zdb.c cmd/zdb/zdb.c
index 3b8b67bce..9018eea58 100644
--- cmd/zdb/zdb.c
+++ cmd/zdb/zdb.c
@@ -9372,8 +9372,12 @@ main(int argc, char **argv)
 		if (argc != 3)
 			usage();
 		dump_opt['v'] = verbose;
-		if (zdb_numeric(argv[1])) {
-			object = strtoull(argv[1], NULL, 0);
+		if (argv[1][0] == '/') {
+			errno = 0;
+			object = strtoull(&(argv[1])[1], NULL, 0);
+			error = errno;
+			if (error == 0 && object == 0)
+				error = EINVAL;
 		} else {
 			error = dump_path(argv[0], argv[1], &object);
 		}

Absent that, maybe trying a number if dump_path() returns ENOENT? It has the reverse problem in that you can't dump by object number if you have a file named that, so maybe it's not useful.