Clarify SECURITY-3315 error code on client side

[jglick]

I wasted a fair amount of time trying to figure out why the CLI was not working on a test controller in WebSocket mode

$ java -jar jenkins-cli.jar -auth … -s … who-am-i
CLI handshake failed with status code 403
Content-Length: 0
Date: …
Ngrok-Agent-Ips: …
Vary: Accept-Encoding
X-Content-Type-Options: nosniff

before I realized that the problem was with the reverse proxy setup. There was an admin monitor showing that but it did not occur to me that this would have anything to do with WebSocket CLI. As of de45096, it does.

I had to inspect sources and turn on a fine logger on the controller to see this. As of #9591 we have the opportunity to make this clearer to people.

Testing done

None really. I did try

diff --git test/src/test/java/hudson/cli/CLIActionTest.java test/src/test/java/hudson/cli/CLIActionTest.java
index ebda2dec58..3a2edeeb04 100644
--- test/src/test/java/hudson/cli/CLIActionTest.java
+++ test/src/test/java/hudson/cli/CLIActionTest.java
@@ -43,7 +43,6 @@ import org.apache.commons.io.IOUtils;
 import org.apache.commons.io.input.NullInputStream;
 import org.apache.commons.io.output.CountingOutputStream;
 import org.apache.commons.io.output.TeeOutputStream;
-import org.junit.Ignore;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.TemporaryFolder;
@@ -136,9 +135,10 @@ public class CLIActionTest {
         assertEquals(code, proc.join());
     }
 
-    @Ignore("TODO flaky test") @Test public void authenticationFailed() throws Exception {
+    @Test public void authenticationFailed() throws Exception {
         j.jenkins.setSecurityRealm(j.createDummySecurityRealm());
         j.jenkins.setAuthorizationStrategy(new MockAuthorizationStrategy().grant(Jenkins.ADMINISTER).everywhere().toAuthenticated());
+        CLIAction.ALLOW_WEBSOCKET = false;
         var jar = tmp.newFile("jenkins-cli.jar");
         FileUtils.copyURLToFile(j.jenkins.getJnlpJars("jenkins-cli.jar").getURL(), jar);
         var baos = new ByteArrayOutputStream();

to see what the failed assertion said, but pending #9688 this test does not run at all.

Proposed changelog entries

• Clearer error message for the CLI in default -webSocket mode when a 403 error results from a reverse proxy misconfiguration.

Proposed upgrade guidelines

N/A

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6. New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
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Desired reviewers

@daniel-beck

Before the changes are marked as ready-for-merge:

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[jglick]

Note that if anyone had been using this as a @FunctionalInterface before, that would break as of jenkinsci/stapler#482. Not sure if this was only source-incompatible rather than binary-incompatible? https://docs.oracle.com/javase/specs/jls/se23/html/jls-13.html#jls-13.5.4 does not seem to mention SAMs.

[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.

/label ready-for-merge