Fix infinite loop in writing gzip header/footer

[Shnatsel]

Summary

Fix write::GzEncoder spinning forever when the wrapped writer returns Ok(0) while writing gzip header or footer bytes.

Problem

Most write paths in flate2 go through zio::Writer::dump, which treats Ok(0) for a non-empty buffer as io::ErrorKind::WriteZero.

write::GzEncoder has two gzip-specific direct write loops:

• writing the gzip header
• writing the final 8-byte gzip footer

Those loops previously retried after Ok(0) without making progress. A writer that repeatedly returns Ok(0) for a non-empty buffer could therefore cause GzEncoder::try_finish/finish/drop finalization to spin indefinitely.

Fix

Add a small helper for gzip’s direct writes that mirrors the shared writer behavior: Ok(0) on a non-empty buffer is converted to WriteZero.

Tests

Added regression coverage for both spin sites:

• zero-length progress while writing the gzip header
• zero-length progress while writing the gzip footer

Validation:

• cargo test --test zero-write
• cargo test gz::tests
• cargo test

Credits

The issue was discovered by GPT-5.5 xhigh

[Copilot]

Pull request overview

Fixes a potential infinite loop in flate2::write::GzEncoder when the wrapped Write impl returns Ok(0) while writing gzip header/footer bytes, aligning gzip-specific direct writes with zio::Writer::dump behavior (convert to WriteZero).

Changes:

• Add write_nonzero helper to convert Ok(0) on non-empty buffers into io::ErrorKind::WriteZero for gzip header/footer direct writes.
• Update gzip header write loop and final 8-byte footer write loop to use the new helper.
• Add regression tests intended to cover both “header zero-write” and “footer zero-write” cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/gz/write.rs	Prevents infinite retry loops by treating zero-progress writes during gzip header/footer emission as WriteZero.
tests/zero-write.rs	Adds integration tests for zero-progress writes during gzip header and footer finalization.

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[Byron]

Thanks a lot for the fix!

This looks good to me, just a few comments I'd like to add to make it clear where some magic numbers are coming from.

[Shnatsel]

Thanks for the quick review! Both your and copilot comments should now be addressed.

[Byron]

Great, let's merge this.