cmd/go: go list -e does not assign missing import errors to correct package

[rsc]

See TODO in testdata/script/mod_lists_bad_import.txt.

[rsc]

It's complicated. The current behavior might even be almost correct. Leaving for Go 1.12.

[jayconrod]

@bcmills, @matloob, and I discussed this last week in the context of #36188. There's currently some ambiguity about whether import errors should be attached to the importing package or the imported package in go list -e -json output.

Here's what we came up with:

• If the package being imported cannot be built, for example, because it doesn't exist or has no .go files, the error should be attached to the imported package. The error may still have a position in a file in the importing package. If multiple packages import an unbuildable package with the same path, multiple packages with errors should be listed.
• If the package can be built but the importing package is not allowed to import it, for example because it's an internal package, a main package, or has a different canonical path, the error should be attached to the importing package, and the imported package should be listed normally without error. If the importing package has multiple import errors, we'll only include the first one in go list -e -json output.

[bcmills]

If the package being imported cannot be built, … the error may still have a position in a file in the importing package. If multiple packages import an unbuildable package with the same path, multiple packages with errors should be listed.

I don't think that's quite right: if the error for a given package is not dependent on the importer, the Error field of the go list -e -json entry for that package should not itself include information about importers. (However, the information in the DepsErrors field of each of those importers should populate the Pos field with the position of the corresponding import line.)

[bcmills]

If the package can be built but the importing package is not allowed to import it, … the error should be attached to the importing package, and the imported package should be listed normally without error.

The details of this part are a bit subtle. I think in that case the error should be encoded in the Error field for the importing package (rather than the DepsErrors field), because the error did not strictly occur in the course of “loading” the dependency.

[bcmills]

@matloob, is this fixed by your recent import-stack CL?

I think the relevant TODO is here:

go/src/cmd/go/testdata/script/mod_list_bad_import.txt

Lines 55 to 58 in 6f52790

	# TODO: go list creates a dummy package with the import-not-found
	# but really the Error belongs on example.com/direct, and this package
	# should not be printed.
	# ! stdout example.com/notfound

[matloob]

I just checked, and this isn't fixed at head. I'll take a more in-depth look tomorrow.

[gopherbot]

Change https://golang.org/cl/235925 mentions this issue: cmd/go: assign missing import errors to importing package

[ianlancetaylor]

There is a CL out for this issue but nobody has reviewed it. Is this still expected for 1.15, or should we push it to 1.16? Thanks.

[bcmills]

Michael tagged the CL as wait-release shortly after it was mailed, so looks like 1.16.

[gopherbot]

Change https://golang.org/cl/251445 mentions this issue: cmd/go/internal/modload: rework import resolution

[gopherbot]

Change https://golang.org/cl/258219 mentions this issue: cmd/go: test more commands in mod_build_info_error

[ianlancetaylor]

Moving to Backlog.