Only rewrite output files if changes are needed.

[sodul]

Actual behavior
We have a fairly large project and running mockgen takes several minutes. During that time all of the generated mock files get deleted and rewritten. This causes 2 issues:

• if we run the mocks in parallel mockgen can try to read the files while they are being rewritten, causing mockgen to fail as it was trying to read the file. We worked around this by targeting directories in parallel but run mockgen sequentially within a directory.
• our IDEs see the files being modified and re-index the codebase over and over, making the IDE mostly unusable while mockgen is running.

Expected behavior
If the output file already exist and has the same content mockgen should not rewrite it and leave it untouched.

To Reproduce Steps to reproduce the behavior

Just run mockgen as usual.

Additional Information

• gomock 1.6.0
• golang 1.17.3

[codyoss]

This seems like a reasonable feature request to me. I would be happy to accept a PR for such a change.

[sodul]

@codyoss gentle ping. There are 2 potential PRs, small ones, that can address this issue.

FYI in our situation we have dozens, if not hundreds of generate mock files. So this change really help with the machines load since it reduce the re-indexing from IDEs, and reduces the background antivirus workload as well.

[sodul]

@codyoss @sanposhiho gentle ping. I'm not sure who have merge permission rights on this repository.