[gradle] Symlink user cache dir when RJE_UNSAFE_CACHE is set

private/rules/coursier.bzl

@@ -283,6 +283,7 @@ def _java_path(repository_ctx):
 def _generate_java_jar_command(repository_ctx, jar_path):
     coursier_opts = repository_ctx.os.environ.get("COURSIER_OPTS", "")
     coursier_opts = coursier_opts.split(" ") if len(coursier_opts) > 0 else []
+
     # if coursier OOMs from a large dependency tree, have it crash instead of hanging
     coursier_opts.append("-XX:+ExitOnOutOfMemoryError")
     java_path = _java_path(repository_ctx)

private/tools/java/com/github/bazelbuild/rules_jvm_external/resolver/gradle/GradleProject.java

@@ -65,12 +65,14 @@ public void setupProject() throws IOException {
   }
 
   public void connect(Path gradlePath) {
-    System.setProperty("gradle.user.home", gradleCacheDir.toAbsolutePath().toString());
+    // Use gradleCacheDir as gradle.user.home for complete isolation.
+    // When RJE_UNSAFE_CACHE is set, the user's caches are symlinked into this directory.
     System.setProperty("org.gradle.parallel", "true");
     connection =
         GradleConnector.newConnector()
             .forProjectDirectory(projectDir.toFile())
             .useInstallation(gradlePath.toFile())
+            .useGradleUserHomeDir(gradleCacheDir.toFile())
             .connect();
   }
 

private/tools/java/com/github/bazelbuild/rules_jvm_external/resolver/gradle/GradleResolver.java

@@ -450,7 +450,38 @@ private GradleProject setupFakeGradleProject(
       Path gradleCacheDir = fakeProjectDirectory.resolve(".gradle");
       Files.createDirectories(gradleCacheDir);
       if (useUnsafeCache) {
-        gradleCacheDir = Paths.get(System.getProperty("user.home"), ".gradle");
+        // Instead of changing gradleCacheDir, symlink the user's caches directory
+        // This avoids timing issues with gradle.user.home system property
+        Path userCaches = Paths.get(System.getProperty("user.home"), ".gradle", "caches");
+        if (Files.isDirectory(userCaches)) {
+          try {
+            Path cacheSymlink = gradleCacheDir.resolve("caches");
+            Files.createSymbolicLink(cacheSymlink, userCaches);
+            if (isVerbose()) {
+              eventListener.onEvent(
+                  new LogEvent(
+                      "gradle",
+                      "Using unsafe shared cache",
+                      "Symlinked " + userCaches + " -> " + cacheSymlink));
+            }
+          } catch (IOException e) {
+            // If symlinking fails, fall back to isolated cache
+            if (isVerbose()) {
+              eventListener.onEvent(
+                  new LogEvent(
+                      "gradle",
+                      "Failed to create cache symlink, using isolated cache",
+                      e.getMessage()));
+            }
+          }
+        } else if (isVerbose()) {
+          String reason = Files.exists(userCaches) ? "is not a directory" : "not found";
+          eventListener.onEvent(
+              new LogEvent(
+                  "gradle",
+                  "User gradle caches directory " + reason + ", using isolated cache",
+                  "Expected: " + userCaches));
+        }
       }
 
       return new GradleProject(

tests/com/github/bazelbuild/rules_jvm_external/resolver/gradle/BUILD.bazel

@@ -11,7 +11,7 @@ java_test(
         "//private/tools/java/com/github/bazelbuild/rules_jvm_external/resolver/gradle/data:gradle_build_templates",
     ],
     env = {
-        "RJE_VERBOSE": "true",
+        "RJE_VERBOSE": "1",
     },
     test_class = "com.github.bazelbuild.rules_jvm_external.resolver.gradle.GradleResolverTest",
     deps = [