solver: Resolve infinite loop when record is ErrNotFound

[soluwalana]

This MR resolves the following situation:

• e.record is not Nil
• res, err := cm.backend.Load(key, v.ID) -> err == ErrNotFound
• Loop continues without modifying i, addRecord or exportRecord

Also adds a test to the exporter. The added test will hit the infinite loop without the changes in exporter.go.

[soluwalana]

This should might resolve #6131

[soluwalana]

@thaJeztah are these test failures real or flakes?

[thaJeztah]

Oh; honestly, don't know! I'm not a core maintainer of BuildKit (but have permission in the org, so thought I'd give CI a nudge on your PR to let it run 🤗)

[soluwalana]

@thaJeztah Looks like they are flakes, but tests passed now

[soluwalana]

@tonistiigi, may we have a review on this. We found this issue on our ci build system at nvidia and are running it in a patched version of 0.23.

[tonistiigi]

Looks good, but we don't allow such tests with mocks as they would likely just break in any internal code change. If you can't figure out the conditions for an integration test, then just remove the new test you added.

[soluwalana]

@tonistiigi got it, removing the test.

[tonistiigi]

Squash the commits as well.

[soluwalana]

@tonistiigi I uploaded a better test that didn't rely on the mocks. Still correctly tests the ErrNotFound infinite loop. Will Squash the commits now.

[soluwalana]

@tonistiigi / @thaJeztah could one of you restart the tests that couldn't acquire resources?

[MalteMagnussen]

When can we expect this to be released as a bug fix patch? :)