Actually, the pull_request bit seems likely unrelated, despite those all having it.

These mutations are emitted if any of the issueDiffMethods funcs returns true. The ones I'm suspicious of are:

func (d githubIssueDiffer) diffBody(m *maintpb.GithubIssueMutation) bool {
        if d.a != nil && d.a.Body == d.b.GetBody() {                                                                                                                                                                   return false
        } 
        m.Body = d.b.GetBody()
        return true 
} 
 
func (d githubIssueDiffer) diffTitle(m *maintpb.GithubIssueMutation) bool {
        if d.a != nil && d.a.Title == d.b.GetTitle() {
                return false
        }
        m.Title = d.b.GetTitle()
        return true
}       

Because GetBody or GetTitle can return the empty string and return true and then generate what we see above.

The other methods all look like they can only return true if something got modified.

Confirmed. It's from diffBody.

It happens most often on PRs because we often fix people's PR's for them, removing redundant bodies from their commit messages.

The diffBody func can't express the transition from an issue having a body to not having a body.

For bools we have the BoolChange proto type:

// BoolChange represents a change to a boolean value.                                                                                                                                                                                    message BoolChange { 
  bool val = 1;                                                                                                                                                                                                                          }  

We should do the same for strings.

Change https://golang.org/cl/209967 mentions this issue: maintner: don't generate no-op github issue mutations on empty bodies