[JENKINS-33318] GHE server validation with private mode enabled

pom.xml

@@ -51,7 +51,8 @@
         <dependency>
             <groupId>org.jenkins-ci.plugins</groupId>
             <artifactId>github-api</artifactId>
-            <version>1.72.1</version>
+            <!-- Update when 1.75 is released -->
+            <version>1.75-SNAPSHOT</version>
         </dependency>
         <dependency>
             <groupId>org.jenkins-ci.plugins</groupId>

src/main/java/org/jenkinsci/plugins/github_branch_source/Endpoint.java

@@ -31,6 +31,7 @@
 import hudson.model.Descriptor;
 import hudson.util.FormValidation;
 
+import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.net.MalformedURLException;
 import java.net.URL;
@@ -115,14 +116,28 @@ public FormValidation doCheckApiUri(@QueryParameter String apiUri) {
                 URL api = new URL(apiUri);
                 GitHub github = GitHub.connectToEnterpriseAnonymously(api.toString());
                 github.checkApiUrlValidity();
-                return FormValidation.ok();
+                LOGGER.log(Level.FINE, "Trying to configure a GitHub Enterprise server");
+                // For example: https://api.github.com/ or https://github.mycompany.com/api/v3/ (with private mode disabled).
+                return FormValidation.ok("GitHub Enterprise server verified");
             } catch (MalformedURLException mue) {
-                return FormValidation.error("This does not look like a GitHub Enterprise API URL");
+                // For example: https:/api.github.com
+                LOGGER.log(Level.WARNING, "Trying to configure a GitHub Enterprise server: " + apiUri, mue.getCause());
+                return FormValidation.error("The endpoint does not look like a GitHub Enterprise (malformed URL)");
             } catch (JsonParseException jpe) {
-                return FormValidation.error("This does not look like a GitHub Enterprise API URL");
+                LOGGER.log(Level.WARNING, "Trying to configure a GitHub Enterprise server: " + apiUri, jpe.getCause());
+                return FormValidation.error("The endpoint does not look like a GitHub Enterprise (invalid JSON response)");
+            } catch (FileNotFoundException fnt) {
+                // For example: https://github.mycompany.com/server/api/v3/ gets a FileNotFoundException
+                LOGGER.log(Level.WARNING, "Getting HTTP Error 404 for " + apiUri);
+                return FormValidation.error("The endpoint does not look like a GitHub Enterprise (page not found");
             } catch (IOException e) {
+                // For example: https://github.mycompany.com/api/v3/ or https://github.mycompany.com/api/v3/mypath
+                if (e.getMessage().contains("private mode enabled")) {
+                    LOGGER.log(Level.FINE, e.getMessage());
+                    return FormValidation.warning("Private mode enabled, validation disabled");
+                }
                 LOGGER.log(Level.WARNING, e.getMessage());
-                return FormValidation.error("This does not look like a GitHub Enterprise API URL");
+                return FormValidation.error("The endpoint does not look like a GitHub Enterprise (verify network and/or try again later)");
             }
         }
 

src/main/resources/org/jenkinsci/plugins/github_branch_source/Endpoint/help-apiUri.html

@@ -1,3 +1,3 @@
 <p>
-    <a href="https://developer.github.com/v3/">GitHub API</a> endpoint such as <code>https://github.example.com/api/v3</code>.
+    <a href="https://developer.github.com/v3/">GitHub API</a> endpoint such as <code>https://github.example.com/api/v3/</code>.
 </p>