cmd/go/internal/vcweb: set GIT_PROTOCOL in the git CGI handler

This works around a bug in 'git http-backend' that was fixed in
Git 2.34.0,¹ and will hopefully allow the tests in
cmd/go/internal/modfetch/codehost to pass reliably using older
Git releases (I tested with 2.30.2).

¹git/git@ff6a37c

Fixes golang#56881.

Change-Id: Icd2e4d252d5f712685d146f34e11922dd0c41ff0
Reviewed-on: https://go-review.googlesource.com/c/go/+/549795
Reviewed-by: Michael Matloob <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>

src/cmd/go/internal/modfetch/codehost/git_test.go

@@ -280,9 +280,6 @@ func TestLatest(t *testing.T) {
 				t.Fatal(err)
 			}
 			if !reflect.DeepEqual(info, tt.info) {
-				if !reflect.DeepEqual(info.Tags, tt.info.Tags) {
-					testenv.SkipFlaky(t, 56881)
-				}
 				t.Errorf("Latest: incorrect info\nhave %+v (origin %+v)\nwant %+v (origin %+v)", info, info.Origin, tt.info, tt.info.Origin)
 			}
 		}
@@ -661,9 +658,6 @@ func TestStat(t *testing.T) {
 			}
 			info.Origin = nil // TestLatest and ../../../testdata/script/reuse_git.txt test Origin well enough
 			if !reflect.DeepEqual(info, tt.info) {
-				if !reflect.DeepEqual(info.Tags, tt.info.Tags) {
-					testenv.SkipFlaky(t, 56881)
-				}
 				t.Errorf("Stat: incorrect info\nhave %+v\nwant %+v", *info, *tt.info)
 			}
 		}

src/cmd/go/internal/vcweb/git.go

@@ -37,16 +37,35 @@ func (h *gitHandler) Handler(dir string, env []string, logger *log.Logger) (http
 		return nil, ServerNotInstalledError{name: "git"}
 	}
 
-	handler := &cgi.Handler{
-		Path:   h.gitPath,
-		Logger: logger,
-		Args:   []string{"http-backend"},
-		Dir:    dir,
-		Env: append(slices.Clip(env),
-			"GIT_PROJECT_ROOT="+dir,
-			"GIT_HTTP_EXPORT_ALL=1",
-		),
-	}
+	baseEnv := append(slices.Clip(env),
+		"GIT_PROJECT_ROOT="+dir,
+		"GIT_HTTP_EXPORT_ALL=1",
+	)
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		// The Git client sends the requested Git protocol version as a
+		// "Git-Protocol" HTTP request header, which the CGI host then converts
+		// to an environment variable (HTTP_GIT_PROTOCOL).
+		//
+		// However, versions of Git older that 2.34.0 don't recognize the
+		// HTTP_GIT_PROTOCOL variable, and instead need that value to be set in the
+		// GIT_PROTOCOL variable. We do so here so that vcweb can work reliably
+		// with older Git releases. (As of the time of writing, the Go project's
+		// builders were on Git version 2.30.2.)
+		env := slices.Clip(baseEnv)
+		if p := req.Header.Get("Git-Protocol"); p != "" {
+			env = append(env, "GIT_PROTOCOL="+p)
+		}
+
+		h := &cgi.Handler{
+			Path:   h.gitPath,
+			Logger: logger,
+			Args:   []string{"http-backend"},
+			Dir:    dir,
+			Env:    env,
+		}
+		h.ServeHTTP(w, req)
+	})
 
 	return handler, nil
 }