cmd/go/internal/modfetch: treat a missing go.mod file as a “not exist” error

If we have found a repository at the requested version but it does not
contain a go.mod file in an appropriate subdirectory, then the module
with the given path does not exist at that version. Therefore, we
should report it with an error equivalent to os.ErrNotExist so that
modload.Query will continue to check other possible module paths.

Updates #27173

Change-Id: Ica73f4bb97f58e611a7f7d38183ee52fef5ee69a
Reviewed-on: https://go-review.googlesource.com/c/go/+/183618
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modfetch/coderepo.go

@@ -255,7 +255,13 @@ func (r *codeRepo) convert(info *codehost.RevInfo, statVers string) (*RevInfo, e
 			if err != nil {
 				// TODO: It would be nice to return an error like "not a module".
 				// Right now we return "missing go.mod", which is a little confusing.
-				return nil, err
+				return nil, &module.ModuleError{
+					Path: r.modPath,
+					Err: &module.InvalidVersionError{
+						Version: info2.Version,
+						Err:     notExistError(err.Error()),
+					},
+				}
 			}
 		}
 

src/cmd/go/internal/modfetch/coderepo_test.go

@@ -638,7 +638,7 @@ var latestTests = []struct {
 	{
 		vcs:  "git",
 		path: "github.com/rsc/vgotest1/subdir",
-		err:  "missing github.com/rsc/vgotest1/subdir/go.mod at revision a08abb797a67",
+		err:  "github.com/rsc/vgotest1/subdir@v0.0.0-20180219223237-a08abb797a67: invalid version: missing github.com/rsc/vgotest1/subdir/go.mod at revision a08abb797a67",
 	},
 	{
 		vcs:     "mod",

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

@@ -29,10 +29,17 @@ cp go.mod.orig go.mod
 go mod edit -require golang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c
 cd outside
 ! go list -m golang.org/x/text
-stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c: missing golang.org/x/text/unicode/go.mod at revision 14c0d48ead0c'
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c: invalid version: missing golang.org/x/text/unicode/go.mod at revision 14c0d48ead0c'
 cd ..
 ! go list -m golang.org/x/text
-stderr 'golang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c: missing golang.org/x/text/unicode/go.mod at revision 14c0d48ead0c'
+stderr 'golang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c: invalid version: missing golang.org/x/text/unicode/go.mod at revision 14c0d48ead0c'
+
+# However, arguments to 'go get' can name packages above the root.
+cp go.mod.orig go.mod
+go get -d golang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c
+go list -m golang.org/x/text/...
+stdout 'golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c'
+! stdout 'golang.org/x/text/unicode'
 
 # A major version that does not match the module path is invalid.
 cp go.mod.orig go.mod