Fix a blocking-pool deadlock in nativelink_util::fs::read_dir [1.6-patch-5]

[cormacrelf]

What and why

The previous implementation acquired a blocking-pool permit and then, from inside that blocking-pool thread, called Handle::current().block_on(tokio::fs::read_dir(path)). But tokio::fs::read_dir is itself implemented as a spawn_blocking, so this nested block_on required a second blocking-pool thread to be available to make progress — every call to read_dir needed two pool threads at once instead of one.

Under enough concurrent callers (e.g. upload_directory's unbounded directory-tree fan-out on an action with many inputs), all blocking-pool threads could end up parked waiting on inner tasks that could never get a thread to run on, freezing every fs:: operation in the process. I believe this may have caused deadlocks on actions with a lot of inputs (700+).

Fix: acquire the permit, then simply .await tokio::fs::read_dir directly instead of block_on-ing it — no nested blocking-pool thread is required.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How was this verified?

New regression test read_dir_needs_only_one_blocking_thread in nativelink-util/tests/fs_test.rs: builds a runtime with max_blocking_threads(1) and asserts fs::read_dir completes within a 5s timeout. The pre-fix implementation deadlocks in this configuration since it requires two blocking-pool threads at once.

Risk

Low. Only changes how read_dir waits on tokio::fs::read_dir. Removes a layer of blocking in async, so the final code is simpler.

AI assistance

Opus did most of the work here.

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

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Project	Deployment	Actions	Updated
nativelink	Ready	Preview	Oct 6, 2026 5:02am UTC
nativelink-aidm	Ready	Preview	Oct 6, 2026 5:02am UTC

[amankrx]

Hi @cormacrelf, Can you please fix the pre-commit hook test that's failing right now?