debugging-hanging-tests

Testing & Quality

Diagnosing and fixing hanging worker executor or integration tests. Use when a test hangs indefinitely, times out, or appears stuck during execution.

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Debugging Hanging Tests

Worker executor and integration tests can hang indefinitely due to unimplemented!() panics in async tasks, deadlocks, missing shard assignments, or other async runtime issues. This skill provides a systematic workflow for diagnosing and resolving these hangs.

Common Causes

Cause	Symptom
`unimplemented!()` panic in async task	Test hangs after a log line mentioning the unimplemented feature
Deadlock	Test hangs with no further log output
Missing shard assignment	Worker never starts executing
Channel sender dropped	Receiver awaits forever with no error
Infinite retry loop	Repeated log lines with the same error

Step 1: Add a Timeout

Add a #[timeout] attribute so the test fails with a clear error instead of hanging forever:

use test_r::test;
use test_r::timeout;

#[test]
#[timeout("30s")]
async fn my_hanging_test() {
    // ...
}

Choose a timeout generous enough for normal execution but short enough to fail quickly when hung (30s–60s for most tests, up to 120s for complex integration tests).

Step 2: Capture Full Output

Run the test with --nocapture and save all output to a file. The root cause often appears far before the point where the test hangs:

cargo test -p <crate> <test_name> -- --nocapture > tmp/test_output.txt 2>&1

Important: Always redirect to a file. The output can be thousands of lines, and the relevant error may be near the beginning while the hang occurs at the end.

Step 3: Search for Root Cause

Search the saved output file for these patterns, in order of likelihood:

grep -n "unimplemented" tmp/test_output.txt
grep -n "panic" tmp/test_output.txt
grep -n "ERROR" tmp/test_output.txt
grep -n "WARN" tmp/test_output.txt

What to look for

not yet implemented or unimplemented: An async task hit an unimplemented code path and panicked. The panic is silently swallowed by the async runtime, causing the caller to await forever.
panic: Similar to above — a panic in a spawned task won't propagate to the test.
ERROR with retry: A service call failing repeatedly, causing an infinite retry loop.
Repeated identical log lines: Indicates a retry loop or polling cycle that never succeeds.

Step 4: Fix the Root Cause

If caused by `unimplemented!()`

Implement the missing functionality, or if it's a test-only issue, provide a stub/mock.

If caused by a deadlock

Look for:

Multiple lock() calls on the same mutex in nested scopes
await while holding a lock guard
Circular lock dependencies between tasks

If caused by missing shard assignment

Check that the test setup properly initializes the shard manager and assigns shards before starting workers.

If caused by a dropped sender

Ensure all channel senders are kept alive for the duration the receiver needs them. Check for early returns or error paths that drop the sender.

Checklist

#[timeout("30s")] added to the hanging test
Test run with --nocapture, output saved to file
Output searched for unimplemented, panic, ERROR
Root cause identified and fixed
Test passes within the timeout
Remove the #[timeout] if it was only added for debugging (or keep it as a safety net)