avoid file system watches on read-only mounts

[andrewrk]

Follow-up from #20580.

name_to_handle_at gives us a mount ID here:

zig/lib/std/Build/Watch.zig

Line 101 in 8ab70f8

try posix.name_to_handle_at(path.root_dir.handle.fd, adjusted_path, stack_ptr, &mount_id, std.os.linux.AT.HANDLE_FID);

If this mount is read-only then the watch system should not waste any resources placing a mark on any directories within because they will never change!

Related:

• file system watching on linux fails to avoid placing marks from different mounts into the same fanotify descriptor #20670

[rohlem]

No insight on the particulars of read-only mounts, but I assume they could be un-mounted and then re-mounted with different data while the program is running?
That could either be considered an error and explicitly unsupported, or supported by a "mount watch" mechanism spanning all files within (maybe by only watching the first file on it, if that tells us enough to detect re-mounting?).

[andrewrk]

ehh if that's possible and it doesn't even change the mount id then this should be closed