[BUG] Do not allow deletion of internal references

- Currently it's possible to modify remote references such as
`refs/pull/<idx>/head` and `refs/heads/<branch>`.
- Disallow that the pull request reference is deleted, as this should
not be at the control of the user. Doing so would result in
inconsistencies within Forgejo and lead to internal server errors when
trying access the pull request, this action should be reserved for
Forgejo.
- Do this by utilizing the `update` hook, which process each reference
individually and therefore allow to only skip deleting internal
references and still allow other modifications that is being done in
the same push.
- Ref: https://codeberg.org/Codeberg/Community/issues/1517

cmd/hook.go

@@ -293,11 +293,34 @@ Forgejo or set your environment appropriately.`, "")
 	return nil
 }
 
+// runHookUpdate process the update hook: https://git-scm.com/docs/githooks#update
 func runHookUpdate(c *cli.Context) error {
-	// Update is empty and is kept only for backwards compatibility
+	// Now if we're an internal don't do anything else
+	if isInternal, _ := strconv.ParseBool(os.Getenv(repo_module.EnvIsInternal)); isInternal {
 		return nil
 	}
 
+	ctx, cancel := installSignals()
+	defer cancel()
+
+	// The last three arguments given to the hook are in order: reference name, old commit ID and new commit ID.
+	args := os.Args[len(os.Args)-3:]
+	refFullName := git.RefName(args[0])
+	newCommitID := args[2]
+
+	// Only process pull references.
+	if !refFullName.IsPull() {
+		return nil
+	}
+
+	// Deletion of the ref means that the new commit ID is only composed of '0'.
+	if strings.ContainsFunc(newCommitID, func(e rune) bool { return e != '0' }) {
+		return nil
+	}
+
+	return fail(ctx, fmt.Sprintf("The deletion of %s is skipped as it's an internal reference.", refFullName), "")
+}
+
 func runHookPostReceive(c *cli.Context) error {
 	ctx, cancel := installSignals()
 	defer cancel()

tests/integration/git_test.go

@@ -84,6 +84,7 @@ func testGit(t *testing.T, u *url.URL) {
 		mediaTest(t, &httpContext, little, big, littleLFS, bigLFS)
 
 		t.Run("CreateAgitFlowPull", doCreateAgitFlowPull(dstPath, &httpContext, "master", "test/head"))
+		t.Run("InternalReferences", doInternalReferences(&httpContext, dstPath))
 		t.Run("BranchProtectMerge", doBranchProtectPRMerge(&httpContext, dstPath))
 		t.Run("AutoMerge", doAutoPRMerge(&httpContext, dstPath))
 		t.Run("CreatePRAndSetManuallyMerged", doCreatePRAndSetManuallyMerged(httpContext, httpContext, dstPath, "master", "test-manually-merge"))
@@ -125,6 +126,7 @@ func testGit(t *testing.T, u *url.URL) {
 			mediaTest(t, &sshContext, little, big, littleLFS, bigLFS)
 
 			t.Run("CreateAgitFlowPull", doCreateAgitFlowPull(dstPath, &sshContext, "master", "test/head2"))
+			t.Run("InternalReferences", doInternalReferences(&sshContext, dstPath))
 			t.Run("BranchProtectMerge", doBranchProtectPRMerge(&sshContext, dstPath))
 			t.Run("MergeFork", func(t *testing.T) {
 				defer tests.PrintCurrentTest(t)()
@@ -734,6 +736,20 @@ func doAutoPRMerge(baseCtx *APITestContext, dstPath string) func(t *testing.T) {
 	}
 }
 
+func doInternalReferences(ctx *APITestContext, dstPath string) func(t *testing.T) {
+	return func(t *testing.T) {
+		defer tests.PrintCurrentTest(t)()
+
+		repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{OwnerName: ctx.Username, Name: ctx.Reponame})
+		pr1 := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{HeadRepoID: repo.ID})
+
+		_, stdErr, gitErr := git.NewCommand(git.DefaultContext, "push", "origin").AddDynamicArguments(fmt.Sprintf(":refs/pull/%d/head", pr1.Index)).RunStdString(&git.RunOpts{Dir: dstPath})
+		assert.Error(t, gitErr)
+		assert.Contains(t, stdErr, fmt.Sprintf("remote: Forgejo: The deletion of refs/pull/%d/head is skipped as it's an internal reference.", pr1.Index))
+		assert.Contains(t, stdErr, fmt.Sprintf("[remote rejected] refs/pull/%d/head (hook declined)", pr1.Index))
+	}
+}
+
 func doCreateAgitFlowPull(dstPath string, ctx *APITestContext, baseBranch, headBranch string) func(t *testing.T) {
 	return func(t *testing.T) {
 		defer tests.PrintCurrentTest(t)()