[gradle] Symlink user cache dir when RJE_UNSAFE_CACHE is set

[shs96c]

This can significantly improve the time it takes to perform resolutions.

[gemini-code-assist]

Summary of Changes

Hello @shs96c, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a key optimization for Gradle resolution performance within rules_jvm_external. It refines the mechanism for utilizing a shared Gradle user cache when RJE_UNSAFE_CACHE is active. The core change involves replacing the direct setting of the gradle.user.home system property with a more robust symbolic linking approach, connecting the temporary project's cache directory to the user's global Gradle cache. This aims to mitigate timing-related issues and ensures a more reliable shared cache experience, with built-in error handling and informative logging for fallback scenarios.

Highlights

• Performance Improvement: The primary goal of this change is to significantly improve Gradle resolution times by leveraging a shared user cache when RJE_UNSAFE_CACHE is enabled.
• Cache Handling Strategy: Instead of directly setting the gradle.user.home system property, the approach now involves creating a symbolic link from the temporary project's .gradle/caches directory to the user's global .gradle/caches directory. This avoids potential timing issues.
• Robust Symlinking with Fallback: The implementation includes checks for the existence of the user's cache directory and robust error handling for symbolic link creation. If symlinking fails or the user cache is not found, the system gracefully falls back to using an isolated cache.
• Verbose Logging: Added verbose logging to provide clear feedback on whether the unsafe shared cache is being utilized via symlinking or if the system has reverted to an isolated cache.

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[gemini-code-assist]

Code Review

This pull request changes the unsafe caching mechanism for Gradle resolutions to use symbolic links to the user's cache directory instead of setting gradle.user.home. This is a solid approach to improve performance while avoiding potential timing issues. The implementation correctly falls back to an isolated cache if creating the symlink fails. I have one suggestion to make the check for the user's cache directory more robust.

[gemini-code-assist]

Using Files.exists() here could lead to issues if userCaches points to a file instead of a directory. Files.exists() returns true for files, and createSymbolicLink would succeed, but Gradle would later fail when it expects a directory for its cache.

Using Files.isDirectory() is more robust as it ensures that we only attempt to symlink if the target is a directory. I've also updated the logging in the else branch to be more descriptive about why the symlink was not created.

        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));
        }