flush on WantWrite by youknowone · Pull Request #6717 · RustPython/RustPython

youknowone

📝 Walkthrough

Walkthrough

The SSL handshake logic in compat.rs is enhanced to more thoroughly drain the rustls write buffer when WantWrite occurs during socket mode, and to explicitly flush all pending TLS data after handshake completion in non-BIO mode.

Changes

Cohort / File(s)	Change Summary
SSL Handshake Buffer Flushing `crates/stdlib/src/ssl/compat.rs`	Modified `handle_handshake_complete` to loop and drain entire rustls write buffer on WantWrite instead of breaking early; added explicit flush of pending TLS data after handshake completion in non-BIO mode to ensure delivery to peer

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

Fix SSL handshake partial send #6635: Modifies SSL handshake write/flush logic in ssl/compat.rs to ensure rustls-generated TLS bytes are fully drained with WantWrite handling.
Share more ssl consts and fix openssl #6462: Changes TLS write-flushing and handshake read/write behavior in ssl/compat.rs through explicit flush/EOF handling and draining pending rustls writes.

Poem

🐰 The buffers flow like morning dew,
No data left behind in queue,
Loop and drain with gentle care,
Flush it all, both here and there! 💫

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'flush on WantWrite' is directly related to the main change, which involves flushing the rustls write buffer when a WantWrite error occurs during the SSL handshake.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

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📥 Commits

Reviewing files that changed from the base of the PR and between e91bc11 and 0d559d0.

📒 Files selected for processing (1)

crates/stdlib/src/ssl/compat.rs

🧰 Additional context used

📓 Path-based instructions (1)

**/*.rs

📄 CodeRabbit inference engine (.github/copilot-instructions.md)

**/*.rs: Follow the default rustfmt code style by running cargo fmt to format Rust code
Always run clippy to lint code (cargo clippy) before completing tasks and fix any warnings or lints introduced by changes
Follow Rust best practices for error handling and memory management
Use the macro system (pyclass, pymodule, pyfunction, etc.) when implementing Python functionality in Rust

Files:

crates/stdlib/src/ssl/compat.rs

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🔇 Additional comments (1)

crates/stdlib/src/ssl/compat.rs (1)

1240-1268: The WantWrite handling logic is correct and properly implements buffer draining with timeout protection.

The approach of catching WantWrite, performing a blocking flush, and continuing the loop ensures all TLS handshake data reaches the peer. The blocking_flush_all_pending implementation provides:

Timeout handling: Respects socket timeout via sock_select and returns timeout errors

Error resilience: Retries on transient blocking IO errors, returns on real errors

Bounded execution: Loops only while pending data exists, avoiding infinite loops

Edge case handling: Handles sent == 0 case where socket wasn't ready despite select

The data flow is correct:

send_all_bytes buffers unsent data in pending_tls_output on WantWrite

blocking_flush_all_pending blocks until that buffer is emptied (with timeout safeguard)

Loop continues to drain remaining rustls buffer

Final flush after loop handles any edge cases

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