fix(pull): show changed files of pull requests without merge base

When head and base have no merge base, GetCompareInfo leaves
CompareBase empty and viewPullFiles looked up an empty commit ID,
so the files tab returned 404 (400 on older releases). Diff against
the empty tree instead, and let gitdiff accept the empty tree as a
diff base rather than loading it as a commit.

Author: 4RH1T3CT0R7

routers/web/repo/pull.go

@@ -742,6 +742,9 @@ func viewPullFiles(ctx *context.Context, beforeCommitID, afterCommitID string) {
 			return
 		}
 		beforeCommitID = beforeCommit.ID.String()
+	} else if beforeCommitID == "" && prCompareInfo.CompareBase == "" {
+		// no merge base (unrelated histories): the pull request brings in the whole head tree
+		beforeCommitID = afterCommit.ID.Type().EmptyTree().String()
 	} else {
 		beforeCommitID = util.IfZero(beforeCommitID, prCompareInfo.CompareBase)
 		beforeCommit = indexCommit(prCompareInfo.Commits, beforeCommitID)
@@ -753,10 +756,10 @@ func viewPullFiles(ctx *context.Context, beforeCommitID, afterCommitID string) {
 				return
 			}
 		}
-	}
-	if beforeCommit == nil {
-		ctx.NotFound(nil)
-		return
+		if beforeCommit == nil {
+			ctx.NotFound(nil)
+			return
+		}
 	}
 
 	ctx.Data["CompareInfo"] = prCompareInfo

services/gitdiff/git_diff_tree.go

@@ -112,6 +112,10 @@ func validateGitDiffTreeArguments(ctx context.Context, gitRepo *git.Repository,
 		return useMergeBase, baseCommit.ID.String(), headCommitID, nil
 	}
 
+	if baseSha == headCommit.ID.Type().EmptyTree().String() {
+		return false, baseSha, headCommitID, nil // the empty tree is not a commit, but a valid diff base
+	}
+
 	// try and get the base commit
 	baseCommit, err := gitRepo.GetCommit(ctx, baseSha)
 	// propagate the error if we couldn't get the base commit

services/gitdiff/gitdiff.go

@@ -1309,7 +1309,7 @@ func guessBeforeCommitForDiff(ctx context.Context, gitRepo *git.Repository, befo
 	commitObjectFormat := afterCommit.ID.Type()
 	isBeforeCommitIDEmpty := beforeCommitID == "" || beforeCommitID == commitObjectFormat.EmptyObjectID().String()
 
-	if isBeforeCommitIDEmpty && afterCommit.ParentCount() == 0 {
+	if (isBeforeCommitIDEmpty && afterCommit.ParentCount() == 0) || beforeCommitID == commitObjectFormat.EmptyTree().String() {
 		actualBeforeCommitID = commitObjectFormat.EmptyTree()
 	} else {
 		if isBeforeCommitIDEmpty {

tests/integration/pull_diff_test.go

@@ -4,13 +4,22 @@
 package integration
 
 import (
+	"fmt"
 	"net/http"
+	"strings"
 	"testing"
 
+	auth_model "gitea.dev/models/auth"
+	repo_model "gitea.dev/models/repo"
+	"gitea.dev/models/unittest"
+	user_model "gitea.dev/models/user"
+	"gitea.dev/modules/git/gitcmd"
+	api "gitea.dev/modules/structs"
 	"gitea.dev/tests"
 
 	"github.com/PuerkitoBio/goquery"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestPullDiff(t *testing.T) {
@@ -65,3 +74,36 @@ func testPullDiffAssertPage(t *testing.T, prDiffURL string, reviewBtnDisabled bo
 	// Ensure the review button is enabled for full PR reviews
 	assert.Equal(t, reviewBtnDisabled, doc.Find(".js-btn-review").HasClass("disabled"))
 }
+
+func TestPullDiffNoCommonMergeBase(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{Name: "user2"})
+	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{OwnerID: user2.ID, Name: "repo1"})
+	_, _, err := gitcmd.NewCommand("fast-import").WithRepo(repo1).WithStdinBytes([]byte(strings.TrimSpace(`
+commit refs/heads/unrelated-history
+committer User <user@example.com> 1714310400 +0000
+data 13
+Second commit
+M 100644 inline file2.txt
+data 12
+Hello from 2
+`))).RunStdString(t.Context())
+	require.NoError(t, err)
+
+	// the compare page refuses branches without a merge base, but the API (and history rewrites) still produce such pull requests
+	session := loginUser(t, "user2")
+	token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeWriteRepository)
+	req := NewRequestWithJSON(t, "POST", "/api/v1/repos/user2/repo1/pulls", &api.CreatePullRequestOption{
+		Head:  "unrelated-history",
+		Base:  "master",
+		Title: "unrelated histories",
+	}).AddTokenAuth(token)
+	pr := DecodeJSON(t, MakeRequest(t, req, http.StatusCreated), &api.PullRequest{})
+
+	req = NewRequest(t, "GET", fmt.Sprintf("/user2/repo1/pulls/%d/files", pr.Index))
+	resp := session.MakeRequest(t, req, http.StatusOK)
+	doc := NewHTMLParser(t, resp.Body)
+	assert.Equal(t, 1, doc.Find(".diff-file-box").Length())
+	assert.Equal(t, "file2.txt", doc.Find(".diff-file-box").AttrOr("data-new-filename", ""))
+}