Jakarta REST client: do not produce '//' when base URI ends in '/'

httpclient5-jakarta-rest-client/src/main/java/org/apache/hc/client5/http/rest/RestInvocationHandler.java

@@ -157,7 +157,18 @@ private Object executeRequest(final ResourceMethod rm,
                     pathSegments.add(pathTemplate.getSegment());
                 }
             }
-            uriBuilder.appendPathSegments(pathSegments);
+            if (!pathSegments.isEmpty()) {
+                // A base URI ending in '/' (as many are written) has an empty last path
+                // segment. Appending after it would produce '//'; drop it first, exactly
+                // as ResourceIfaceScanner#join already does when joining a resource class's
+                // path with its method's path.
+                final List<String> baseSegments = uriBuilder.getPathSegments();
+                if (!baseSegments.isEmpty() && baseSegments.get(baseSegments.size() - 1).isEmpty()) {
+                    baseSegments.remove(baseSegments.size() - 1);
+                }
+                baseSegments.addAll(pathSegments);
+                uriBuilder.setPathSegments(baseSegments);
+            }
             uriBuilder.addParameters(queryParams);
             requestUri = uriBuilder.build();
         } catch (final URISyntaxException ex) {

httpclient5-jakarta-rest-client/src/test/java/org/apache/hc/client5/http/rest/RestClientIntegrationTest.java

@@ -160,6 +160,24 @@ void readsFormUrlEncodedResponseAsNameValuePairs() throws Exception {
         assertThat(pairs.get(1).getValue()).isEqualTo("en");
     }
 
+    @Test
+    void getBook_withTrailingSlashBaseUri_doesNotProduceDoubleSlash() throws Exception {
+        // A base URI ending in '/', as many are written, must not combine with the resource
+        // path to produce '//books/...': the server only has a context registered at
+        // "/books", so a leading double slash would not match it and the call would fail.
+        final URI trailingSlashBaseUri = new URI(baseUri.toString() + "/");
+        final BookResource client = RestClientBuilder.newBuilder()
+                .baseUri(trailingSlashBaseUri)
+                .httpClient(httpClient)
+                .build(BookResource.class);
+
+        final Book fetched = client.getBook("abc-123", "en", "token-1");
+
+        assertThat(fetched).isNotNull();
+        assertThat(fetched.id).isEqualTo("abc-123");
+        assertThat(fetched.lang).isEqualTo("en");
+    }
+
     @Test
     void rejectsNonFormResponseForNameValuePairReturnType() {
         final BookResource client = RestClientBuilder.newBuilder()