Solved : NPE if the server doesn't provide any HTTP content for an error

jib-core/src/main/java/com/google/cloud/tools/jib/registry/RegistryEndpointCaller.java

@@ -46,8 +46,7 @@
 class RegistryEndpointCaller<T> {
 
   /**
-   * <a
-   * href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/308">https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/308</a>.
+   * <a href = "https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/308">https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/308</a>.
    */
   @VisibleForTesting static final int STATUS_CODE_PERMANENT_REDIRECT = 308;
 
@@ -188,22 +187,28 @@ RegistryErrorException newRegistryErrorException(ResponseException responseExcep
     RegistryErrorExceptionBuilder registryErrorExceptionBuilder =
         new RegistryErrorExceptionBuilder(
             registryEndpointProvider.getActionDescription(), responseException);
-
-    try {
-      ErrorResponseTemplate errorResponse =
-          JsonTemplateMapper.readJson(responseException.getContent(), ErrorResponseTemplate.class);
-      for (ErrorEntryTemplate errorEntry : errorResponse.getErrors()) {
-        registryErrorExceptionBuilder.addReason(errorEntry);
+    if (responseException.getContent() != null) {
+      try {
+        ErrorResponseTemplate errorResponse =
+            JsonTemplateMapper.readJson(
+                responseException.getContent(), ErrorResponseTemplate.class);
+        for (ErrorEntryTemplate errorEntry : errorResponse.getErrors()) {
+          registryErrorExceptionBuilder.addReason(errorEntry);
+        }
+      } catch (IOException ex) {
+        registryErrorExceptionBuilder.addReason(
+            "registry returned error code "
+                + responseException.getStatusCode()
+                + "; possible causes include invalid or wrong reference. Actual error output follows:\n"
+                + responseException.getContent()
+                + "\n");
       }
-    } catch (IOException ex) {
+    } else {
       registryErrorExceptionBuilder.addReason(
           "registry returned error code "
               + responseException.getStatusCode()
-              + "; possible causes include invalid or wrong reference. Actual error output follows:\n"
-              + responseException.getContent()
-              + "\n");
+              + "; but did not return any details; possible causes include invalid or wrong reference, or proxy/firewall/VPN interfering \n");
     }
-
     return registryErrorExceptionBuilder.build();
   }
 

jib-core/src/test/java/com/google/cloud/tools/jib/registry/RegistryEndpointCallerTest.java

@@ -435,6 +435,24 @@ public void testNewRegistryErrorException_nonJsonErrorOutput() {
         registryException.getMessage());
   }
 
+  @Test
+  public void testNewRegistryErrorException_noOutputFromRegistry() {
+    ResponseException httpException = Mockito.mock(ResponseException.class);
+    // Registry returning null error output
+    Mockito.when(httpException.getContent()).thenReturn(null);
+    Mockito.when(httpException.getStatusCode()).thenReturn(404);
+
+    RegistryErrorException registryException =
+        endpointCaller.newRegistryErrorException(httpException);
+    Assert.assertSame(httpException, registryException.getCause());
+    Assert.assertEquals(
+        "Tried to actionDescription but failed because: registry returned error code 404; "
+            + "but did not return any details; possible causes include invalid or wrong reference, or proxy/firewall/VPN interfering \n"
+            + " | If this is a bug, please file an issue at "
+            + "https://github.com/GoogleContainerTools/jib/issues/new",
+        registryException.getMessage());
+  }
+
   /**
    * Verifies that a response with {@code httpStatusCode} throws {@link
    * RegistryUnauthorizedException}.