Bazel gRPC API does not report missing files

[illicitonion]

If the input directory is not found, the Operation response I get from the scheduler is:

name: "312a8f54-91a1-4ec9-598f-f435b2cdfd0a" metadata {type_url: "type.googleapis.com/google.devtools.remoteexecution.v1test.ExecuteOperationMetadata" value: "\010\004\022\000"} done: true response {type_url: "type.googleapis.com/google.devtools.remoteexecution.v1test.ExecuteResponse" value: "\n\000\032\002\010\001"}

which nests the ExecuteResponse: result {} status {code: 1}

Instead, it should populate a FailedPrecondition status as per the suggestion in the proto file: https://github.com/googleapis/googleapis/blob/eb2730ad8d88eb8e70ac647d1e91a3b41e4ca81c/google/devtools/remoteexecution/v1test/remote_execution.proto#L99

Otherwise clients can't know what to do to rectify the situation.

Ideally the command_digest missing would have the same effect.

[dgassaway]

Note that as-is, the fix that is in progress for this will only report one of two things: the Command is missing or the InputRoot is missing. If Command is missing, we short circuit. If InputRoot is missing, we will not tell you in any higher resolution (we can't because fs_util doesn't tell us). This seems acceptable for now based on existing limitations and overall responsibility of the client in Bazel.

[illicitonion]

Sounds good :) It may be nice at some point in the future to give higher resolution, but we can treat that as a future feature request.