Enable directmountstrict mode for fuse

[sfc-gh-guwang]

When fuse uses directmount mode, and direct mount fails, it will fall back to use 'fusermount'. BB binaries(bb-worker) doesn't have fusermount, and will always output the error message 'Fatal error: rpc error: code = Unknown desc = Failed to expose build directory mount: Failed to create FUSE server: exec: "/bin/fusermount": stat /bin/fusermount: no such file or directory'

This error message is confusing and the real error has been swallowed.

This PR enables Buildbarn to pass along DirectMountStrict options that will not do fallbacks.

[EdSchouten]

I'm always a big fan of making illegal states unrepresentable. There are only three behaviors:

• Always use fusermount
• Attempt to mount directly, falling back to fusermount
• Only attempt to mount directly

Yet two booleans give us four different options. Could you replace direct_mount and direct_mount_strict by an enum? Maybe something like this:

enum MountMethod {
  // Call into the fusermount utility to create the FUSE mount.
  FUSERMOUNT = 0;

  // Create the FUSE mount using the mount() system call. This can be
  // used in environments where the fusermount utility is not available,
  // such as the bb_worker container images.
  DIRECT = 1;

  // First attempt to create the FUSE mount using the mount() system
  // call. Upon failure, fall back to the fusermount utility.
  DIRECT_AND_FUSERMOUNT = 2;
}

[sfc-gh-guwang]

Updated!

[EdSchouten]

This looks good. I have one final minor comment. Thanks for working on this!

[EdSchouten]

Please make the default fail:

return status.Error(codes.InvalidArgument, “Invalid mount method”)

And add an explicit case for FUSERMOUNT.

[EdSchouten]

Also make sure to remove the +x bit from that .pb.go file to keep CI happy.