(This is a new issue / regression - did not fail in this way as late as one or two weeks ago)

Describe the problem

1. I started with a working, building repository.

2. I ran dev generate bazel.

3. The command failed to complete due to some missing UI tool.

4. As a result, all the previously-good generated files were removed, causing the following git diff:

 modified   pkg/gen/docs.bzl…
 modified   pkg/gen/excluded.bzl…
 modified   pkg/gen/execgen.bzl…
 modified   pkg/gen/gomock.bzl…
 modified   pkg/gen/misc.bzl…
 modified   pkg/gen/optgen.bzl…
 modified   pkg/gen/protobuf.bzl…
 modified   pkg/gen/stringer.bzl…

(all these files had empty contents)

It's important that generated files remain in-place even when a generate command fails.

The “traditional” algorithm for file generation is this:

file:
   generate >file.tmp || rm -f file.tmp
   mv -f file.tmp file

This way if the generate file fails, the tmp file gets deleted and the final file does not get altered.

It would be good if dev generate would use the same idea for all generated files.