When working with Google Cloud Storage (GCS) via gsutil, most file operations run smoothly. However, occasional cryptic error messages may appear that leave you scratching your head. One such error is:

Semaphore released too many times MiB]  99% Done
CommandException: 1 files/objects could not be copied/removed.
make: *** [Makefile:90: gs_upload] Error 1

At first glance, this error message can be confusing, especially when you see the mention of a semaphore. In computing, a semaphore is used to control access to a shared resource by multiple processes or threads. Seeing “released too many times” suggests some concurrency or synchronization hiccup.

In this article, we’ll explore possible causes behind this error, offer troubleshooting steps, and suggest best practices for managing concurrency when using gsutil.

The Error in Context

A typical scenario where this error might arise:

1. Running gsutil in a Makefile: You have a Make task like make gs_upload, which calls gsutil to upload files to a GCS bucket.
2. Multithreaded or Parallel Operations: Perhaps you used the -m (multithreaded) option with gsutil, or have environment variables configured to allow concurrency.

The error appears like:

Semaphore released too many times MiB]  99% Done
CommandException: 1 files/objects could not be copied/removed.
make: *** [Makefile:90: gs_upload] Error 1

Key takeaways:

• “Semaphore released too many times”: Typically signals a concurrency or resource management glitch.
• “1 files/objects could not be copied/removed.”: Indicates the actual failure impacted at least one file operation.
• Non-zero exit code: Terminates your Make task, causing the build or deployment to fail.

Possible Causes

1. Multithread / Multiprocess Bugs
   If gsutil or its underlying libraries encounter an unexpected state in concurrency, you could see a semaphore mismatch error.
2. Library or Environment Mismatch
   Sometimes Python environments and libraries that gsutil depends on (e.g., httplib2, oauth2client, six) may be out of sync, causing concurrency mismanagement.
3. Transient Network / Timeout Issues
   Intermittent network problems that force threads to fail in unexpected ways can lead to concurrency cleanups that don’t fully match the initial thread counts.
4. Running in a Constrained Environment
   If you’re running gsutil on a system (e.g., a CI container) with limited resources or unusual process constraints, concurrency synchronization may break under stress.

Troubleshooting Steps

1. Retry the Command without Multithreading

If you’re currently using -m (multithreaded) in your gsutil command, try removing it to see if the error goes away:

# Original (multithreaded)
gsutil -m cp -r ./local-folder gs://my-bucket/path/

# Try single-threaded
gsutil cp -r ./local-folder gs://my-bucket/path/

If single-thread mode succeeds, it suggests the concurrency logic was at least partly responsible.

2. Update gsutil and Dependencies

Sometimes concurrency bugs are resolved in newer releases:

gcloud components update

(or if installed via another package manager, use that method). This ensures you have the latest gsutil and libraries.

3. Inspect Your Environment

• Python Library Versions: If you’re using a standalone pip-installed version of gsutil, ensure that your environment doesn’t have mismatched library versions.
• System Limits: Check memory, CPU, and user process/thread limits (ulimit on Linux) to confirm your environment can handle the concurrency.

4. Check for Partial Uploads or Corrupted Files

In some cases, a file might be partially uploaded or removed, causing gsutil to throw an error. Inspect your GCS bucket to see if the file is partially present, then remove or rename it before retrying.

5. Reduce Parallelism

If you still need concurrency but want to reduce the chance of hitting concurrency bugs, tweak the parallel process and thread counts:

gsutil -o "GSUtil:parallel_process_count=1" -o "GSUtil:parallel_thread_count=5" \
    -m cp -r ./local-folder gs://my-bucket/path/

This cuts down concurrency while retaining some parallelism.

Makefile Considerations

When the error appears in a Makefile context:

gs_upload:
 gsutil -m cp -r ./local-folder gs://my-bucket/path/

Consider:

• Using .PHONY vs. real targets: If you rely on concurrency or multiple Make jobs, ensure you’re not inadvertently calling multiple gsutil commands in parallel.
• Serializing Steps: If your Make tasks are independent but run concurrently, add dependencies or force serialization with the -j1 flag in make to avoid concurrency overhead from both Make and gsutil.

Interpretation and Next Steps

The “Semaphore released too many times” error hints at a concurrency mismatch within gsutil or its underlying libraries. In many cases, simply retrying without -m or reducing concurrency resolves the issue. Keeping gsutil updated and verifying your environment’s library versions are also good practices.

If the error persists after these adjustments, you may want to:

• Check if your environment is particularly resource-constrained.
• File a bug report with detailed logs and environment information in the gsutil GitHub repository or via GCP support.

Conclusion

Encountering an error like “Semaphore released too many times” when using gsutil is typically a sign of concurrency or synchronization issues within the tool’s multithreading logic. By adjusting concurrency flags, updating software, and ensuring a stable environment, you can often mitigate or eliminate the error. While frustrating, this glitch reminds us that parallel operations can introduce race conditions or resource mismanagement in even the most robust cloud utilities.

With careful troubleshooting and environment management, you can continue to leverage gsutil for fast and reliable GCS operations—minus the semaphore headaches.