desc/protoparse: fix recently identified regressions from v1.14 (#594)

* ParseFilesButDoNotLink should not use ImportPaths field
* remove constraint that files explicitly named by caller must be provided as source instead of as descriptors

desc/protoparse/parser.go

@@ -241,6 +241,7 @@ func (r noCloneParseResult) Clone() parser.Result {
 // ErrorReporter always returns nil, the parse fails with ErrInvalidSource.
 func (p Parser) ParseFilesButDoNotLink(filenames ...string) ([]*descriptorpb.FileDescriptorProto, error) {
 	rep := newReporter(p.ErrorReporter, p.WarningReporter)
+	p.ImportPaths = nil // not used for this "do not link" operation.
 	res, _ := p.getResolver(filenames)
 	results, err := parseToProtos(res, filenames, reporter.NewHandler(rep), p.ValidateUnlinkedFiles)
 	if err != nil {
@@ -555,16 +556,9 @@ func (p Parser) getResolver(filenames []string) (protocompile.Resolver, *ast2.So
 		}))
 	}
 	backupResolver := protocompile.WithStandardImports(importResolver)
-	mustBeSource := make(map[string]struct{}, len(filenames))
-	for _, name := range filenames {
-		mustBeSource[name] = struct{}{}
-	}
 	return protocompile.CompositeResolver{
 		sourceResolver,
 		protocompile.ResolverFunc(func(path string) (protocompile.SearchResult, error) {
-			if _, ok := mustBeSource[path]; ok {
-				return protocompile.SearchResult{}, os.ErrNotExist
-			}
 			return backupResolver.FindFileByPath(path)
 		}),
 	}, &srcSpan

desc/protoparse/parser_test.go

@@ -525,3 +525,22 @@ func TestParseInferImportPaths_FixesNestedPaths(t *testing.T) {
 		})
 	}
 }
+
+func TestParseFilesButDoNotLink_DoesNotUseImportPaths(t *testing.T) {
+	tempdir, err := os.MkdirTemp("", "protoparse")
+	testutil.Ok(t, err)
+	defer func() {
+		_ = os.RemoveAll(tempdir)
+	}()
+	err = os.WriteFile(filepath.Join(tempdir, "extra.proto"), []byte("package extra;"), 0644)
+	testutil.Ok(t, err)
+	mainPath := filepath.Join(tempdir, "main.proto")
+	err = os.WriteFile(mainPath, []byte("package main; import \"extra.proto\";"), 0644)
+	testutil.Ok(t, err)
+	p := Parser{
+		ImportPaths: []string{tempdir},
+	}
+	fds, err := p.ParseFilesButDoNotLink(mainPath)
+	testutil.Ok(t, err)
+	testutil.Eq(t, 1, len(fds))
+}