Support for Custom URI Schemes in OAuth2 Redirect URIs (#37356)

Fix #34349

By the way, remove `(ctx *APIContext) HasAPIError() ` and `(ctx
*APIContext) GetErrMsg()` because they do nothing, the error handling
has been done in API's middeware

The existing OAuth2 tests were not quite right, refactored them together

services/forms/user_form_test.go

@@ -14,15 +14,9 @@ import (
 )
 
 func TestRegisterForm_IsDomainAllowed_Empty(t *testing.T) {
-	oldService := setting.Service
-	defer func() {
-		setting.Service = oldService
-	}()
-
+	defer test.MockVariableValue(&setting.Service)()
 	setting.Service.EmailDomainAllowList = nil
-
 	form := RegisterForm{}
-
 	assert.True(t, form.IsEmailDomainAllowed())
 }
 
@@ -87,3 +81,30 @@ func TestRegisterForm_IsDomainAllowed_BlockedEmail(t *testing.T) {
 		assert.Equal(t, v.valid, form.IsEmailDomainAllowed())
 	}
 }
+
+func TestDetectInvalidOAuth2ApplicationRedirectURI(t *testing.T) {
+	defer test.MockVariableValue(&setting.OAuth2.CustomSchemes)()
+	setting.OAuth2.CustomSchemes = []string{"my-app"}
+	assertValid := func(t *testing.T, s string, valid bool) {
+		ret := DetectInvalidOAuth2ApplicationRedirectURI([]string{s})
+		if valid {
+			assert.Empty(t, ret)
+		} else {
+			assert.Equal(t, s, ret)
+		}
+	}
+	assertValid(t, "my-app:", true)
+	assertValid(t, "my-app:/foo", true)
+	assertValid(t, "http://foo", true)
+	assertValid(t, "https://foo", true)
+
+	assertValid(t, "my-app", false)
+	assertValid(t, "ftp:", false)
+	assertValid(t, "ftp://foo", false)
+	assertValid(t, "https://[invalid", false)
+
+	ret := DetectInvalidOAuth2ApplicationRedirectURI([]string{"my-app:", "http://foo", "https://foo"})
+	assert.Empty(t, ret)
+	ret = DetectInvalidOAuth2ApplicationRedirectURI([]string{"my-app:", "http://foo", "invalid", "https://foo"})
+	assert.Equal(t, "invalid", ret)
+}