fix: don't prepend server URL with https:// if it's an empty string (#1083)

An attempt to address #1074 in a way that makes everyone happy

Right now, any server URL that doesnt start with `http://` or `https://` automatically gets `https://` prepended to it. When no instance is set while running `forgejo-runner exec` this results in the `{FORGEJO,GITHUB}_SERVER_URL` environment variables being set to simply "https://". Any action using the GitHub actions toolkit chokes on this as an invalid URL and fails the run with a nondescript error. This PR instead leaves empty strings alone, which the actions toolkit seems much happier with

Alternate fixes:
* default the `--forgejo-instance` flag to a valid but unresolved url like 'http://example.com' so the actions don't crash
* don't set the environment variables at all when the server URL is an empty string. This causes the actions toolkit to assume GitHub.com as a fallback

<!--start release-notes-assistant-->
<!--URL:https://code.forgejo.org/forgejo/runner-->
- bug fixes
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/1083): <!--number 1083 --><!--line 0 --><!--description Zml4OiBkb24ndCBwcmVwZW5kIHNlcnZlciBVUkwgd2l0aCBodHRwczovLyBpZiBpdCdzIGFuIGVtcHR5IHN0cmluZw==-->fix: don't prepend server URL with https:// if it's an empty string<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/1083
Reviewed-by: earl-warren <earl-warren@noreply.code.forgejo.org>
Co-authored-by: Andrew Cassidy <drewcassidy@me.com>
Co-committed-by: Andrew Cassidy <drewcassidy@me.com>

act/runner/run_context.go

@@ -1208,7 +1208,7 @@ func (rc *RunContext) getGithubContext(ctx context.Context) *model.GithubContext
 			ghc.RetentionDays = preset.RetentionDays
 
 			instance := rc.Config.GitHubInstance
-			if !strings.HasPrefix(instance, "http://") &&
+			if instance != "" && !strings.HasPrefix(instance, "http://") &&
 				!strings.HasPrefix(instance, "https://") {
 				instance = "https://" + instance
 			}
@@ -1251,7 +1251,7 @@ func (rc *RunContext) getGithubContext(ctx context.Context) *model.GithubContext
 
 	{ // Adapt to Gitea
 		instance := rc.Config.GitHubInstance
-		if !strings.HasPrefix(instance, "http://") &&
+		if instance != "" && !strings.HasPrefix(instance, "http://") &&
 			!strings.HasPrefix(instance, "https://") {
 			instance = "https://" + instance
 		}
@@ -1353,16 +1353,6 @@ func (rc *RunContext) withGithubEnv(ctx context.Context, github *model.GithubCon
 	set("SERVER_URL", github.ServerURL)
 	set("API_URL", github.APIURL)
 
-	{ // Adapt to Forgejo
-		instance := rc.Config.GitHubInstance
-		if !strings.HasPrefix(instance, "http://") &&
-			!strings.HasPrefix(instance, "https://") {
-			instance = "https://" + instance
-		}
-		set("SERVER_URL", instance)
-		set("API_URL", instance+"/api/v1")
-	}
-
 	if rc.Config.ArtifactServerPath != "" {
 		setActionRuntimeVars(rc, env)
 	}

act/runner/run_context_test.go

@@ -280,6 +280,39 @@ func TestRunContext_GetBindsAndMounts(t *testing.T) {
 	})
 }
 
+func TestRunContext_GetGithubContextURL(t *testing.T) {
+	table := []struct {
+		instance  string
+		serverURL string
+		APIURL    string
+	}{
+		{instance: "", serverURL: "", APIURL: "/api/v1"},
+		{instance: "example.com", serverURL: "https://example.com", APIURL: "https://example.com/api/v1"},
+		{instance: "http://example.com", serverURL: "http://example.com", APIURL: "http://example.com/api/v1"},
+		{instance: "https://example.com", serverURL: "https://example.com", APIURL: "https://example.com/api/v1"},
+	}
+	for _, data := range table {
+		t.Run(data.instance, func(t *testing.T) {
+			rc := &RunContext{
+				EventJSON: "{}",
+				Config: &Config{
+					GitHubInstance: data.instance,
+				},
+				Run: &model.Run{
+					Workflow: &model.Workflow{
+						Name: "GitHubContextTest",
+					},
+				},
+			}
+
+			ghc := rc.getGithubContext(t.Context())
+
+			assert.Equal(t, data.serverURL, ghc.ServerURL)
+			assert.Equal(t, data.APIURL, ghc.APIURL)
+		})
+	}
+}
+
 func TestRunContext_GetGithubContextRef(t *testing.T) {
 	table := []struct {
 		event string