fix(repo): load repositories should fail on duplicates (#1937)

We've recently had issues because we have multiple `go_repository` rules
with the same name, at different versions.

If we run `go mod tidy` and `gazelle update-repos`, the rule that gets
updated isn't always the one that ends up being used in the build.

To solve this, we are planning to add a check in CI to make sure no
duplicate go_repository rules are loaded.

This change is needed in order to use `gazelle/repo` to enable this
feature. This can also be potentially used to help aid others in
migrating to Bzlmod or simply cleaning up their repos.

cmd/generate_repo_config/main_test.go

@@ -29,6 +29,7 @@ func TestGenerateRepoConfig(t *testing.T) {
 		giveWorkspace    string
 		giveReposContent string
 		wantContent      string
+		wantFail         bool
 	}{
 		{
 			name: "no duplicates",
@@ -101,19 +102,7 @@ def go_repositories():
         tag = "1.2",
     )
 `,
-			wantContent: `
-# Code generated by generate_repo_config.go; DO NOT EDIT.
-
-go_repository(
-    name = "com_github_pkg_errors",
-    importpath = "github.com2/pkg/errors",
-)
-
-go_repository(
-    name = "org_golang_x_net",
-    importpath = "golang.org2/x/net",
-)
-`,
+			wantFail: true,
 		},
 	}
 
@@ -135,6 +124,13 @@ go_repository(
 			tmp := t.TempDir()
 
 			got, err := generateRepoConfig(filepath.Join(tmp, "WORKSPACE"), filepath.Join(dir, "WORKSPACE"))
+			if tt.wantFail {
+				if err == nil {
+					t.Fatal("wanted error but got nil")
+				}
+				return
+			}
+
 			if err != nil {
 				t.Fatal(err)
 			}

repo/repo.go

@@ -83,16 +83,19 @@ func IsFromDirective(repo *rule.Rule) bool {
 // with the following prioritization:
 //   - rules that were provided as directives have precedence
 //   - rules that were provided last
-func (l *loader) add(file *rule.File, repo *rule.Rule) {
+func (l *loader) add(file *rule.File, repo *rule.Rule) error {
 	name := repo.Name()
 	if name == "" {
-		return
+		return nil
 	}
-
+	
 	if i, ok := l.repoIndexMap[repo.Name()]; ok {
+		if !IsFromDirective(l.repos[i]) && !IsFromDirective(repo) {
+			return fmt.Errorf("found duplicate repo %q", repo.Name())
+		}
 		if IsFromDirective(l.repos[i]) && !IsFromDirective(repo) {
 			// We always prefer directives over non-directives
-			return
+			return nil
 		}
 		// Update existing rule to new rule
 		l.repos[i] = repo
@@ -101,6 +104,7 @@ func (l *loader) add(file *rule.File, repo *rule.Rule) {
 		l.repoIndexMap[name] = len(l.repos) - 1
 	}
 	l.repoFileMap[name] = file
+	return nil
 }
 
 // visit returns true exactly once for each file,function key, and false for all future instances
@@ -113,17 +117,20 @@ func (l *loader) visit(file, function string) bool {
 }
 
 // ListRepositories extracts metadata about repositories declared in a
-// file.
+// file. If there are duplicate rules that are both not defined from a 
+// directive, it will fail.
 func ListRepositories(workspace *rule.File) (repos []*rule.Rule, repoFileMap map[string]*rule.File, err error) {
 	l := &loader{
 		repoRoot:     filepath.Dir(workspace.Path),
 		repoIndexMap: make(map[string]int),
 		repoFileMap:  make(map[string]*rule.File),
 		visited:      make(map[macroKey]struct{}),
 	}
-
+	
 	for _, repo := range workspace.Rules {
-		l.add(workspace, repo)
+		if err := l.add(workspace, repo); err != nil {
+			return nil, nil, err
+		}
 	}
 	if err := l.loadExtraRepos(workspace); err != nil {
 		return nil, nil, err
@@ -164,7 +171,9 @@ func (l *loader) loadRepositoriesFromMacro(macro *RepoMacro) error {
 	for _, rule := range macroFile.Rules {
 		// (Incorrectly) assume that anything with a name attribute is a rule, not a macro to recurse into
 		if rule.Name() != "" {
-			l.add(macroFile, rule)
+			if err := l.add(macroFile, rule); err != nil {
+				return err
+			}
 			continue
 		}
 		if !macro.Leveled {