Windows: cache the stdio write mode for the duration of a lock session - #159562

Joel-Wwalker · 2026-07-19T12:21:26Z

On Windows, every write to stdout/stderr calls GetStdHandle and GetConsoleMode (and GetConsoleOutputCP for consoles) to decide how to write. For bulk writes through a locked handle that is most of the work; #154071 measured 14% of CPU time in is_console alone.

This caches the handle and the console verdict in the sys-level Stdout/Stderr and re-queries when a new lock session begins, as suggested in the issue. Holding a StdoutLock pins the stream; unlocked writes acquire the lock per call, so they re-query per write exactly as before, and SetStdHandle calls made between lock sessions keep working (the #40490 behavior is preserved).

Numbers from the issue's workload shape (16 KiB chunks to NUL, 4 GiB total, Windows 11):

mode	before	after
`stdout().lock()`	0.478 s (8,561 MiB/s)	0.120 s (34,350 MiB/s)
unlocked	0.467 s	0.293 s

The unlocked case also improves because one implicit lock session can contain several sys-level writes when LineWriter splits a chunk at newlines.

The console path cannot run in CI, so it was verified under a pseudoconsole: several writes through one lock session on a CP437 console came through WriteConsoleW intact (non-ASCII included), and after SetConsoleOutputCP plus a new lock session the refresh picked up the change.

The refresh hook is a small #[cfg(windows)] shim on StdoutRaw/StderrRaw; happy to change it to a method on every platform's sys type instead if that is preferred.

Fixes #154071

rustbot · 2026-07-19T12:21:31Z

r? @aapoalas

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rustbot · 2026-07-23T10:42:22Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

Joel-Wwalker · 2026-07-25T16:10:35Z

@rustbot ready

aapoalas

LGTM, thank you for the good work <3

@bors squash

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rust-bors · 2026-08-19T19:23:34Z

🔨 2 commits were squashed into f05562d.

rust-bors · 2026-08-19T19:23:58Z

📋 This PR cannot be approved because it has merge conflicts. Please resolve the merge conflicts by rebasing, and try again.

aapoalas · 2026-08-19T19:25:54Z

Oop, right: we have a merge conflict! Sorry @Joel-Wwalker could you resolve that? r=me once CI passes <3

@bors delegate+

rust-bors · 2026-08-19T19:25:56Z

✌️ @Joel-Wwalker, you can now approve this pull request!

If @aapoalas told you to "r=me" after making some further change, then please make that change and post @bors r=aapoalas.

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* Windows: cache the stdio write mode for the duration of a lock session Every write to stdout/stderr re-queried GetStdHandle and GetConsoleMode (and GetConsoleOutputCP for consoles) to decide how to write, which dominates bulk writes through a locked handle. Cache the handle and the console verdict in the sys-level Stdout/Stderr and re-query when a new lock session begins, so SetStdHandle calls made between lock sessions keep working. * Gate the stdio lock-session refresh to Windows On targets where refresh is a no-op, lock() now compiles to exactly the code it had before, instead of relying on the borrow-flag dance optimizing out.

rustbot · 2026-09-15T18:24:48Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

Joel-Wwalker · 2026-09-15T20:29:28Z

Rebased. The conflict was #160953 cleaning up return ret; in the same hunk; no other changes.

@bors r=aapoalas rollup=always

rust-bors · 2026-09-15T20:29:31Z

📌 Commit 0154b97 has been approved by aapoalas

It is now in the queue for this repository.

…uwer Rollup of 7 pull requests Successful merges: - #161873 (move bug and span_bug macros to rustc_span) - #162584 (Use File::*lock*() in rustc_data_structures::flock when possible) - #162704 (Parser: Uninterpolate when checking for const closures, try bikeshed blocks & in relevant diagnostic code) - #162787 (Fix suggestions for names captured by formatting macros) - #159562 (Windows: cache the stdio write mode for the duration of a lock session) - #162813 (Pre lint port cleanups) - #162822 (PassWrapper: adapt for removal of EABIVersion arg in LLVM 24)

Rollup merge of #159562 - Joel-Wwalker:154071-stdio-handle-cache, r=aapoalas Windows: cache the stdio write mode for the duration of a lock session On Windows, every write to stdout/stderr calls `GetStdHandle` and `GetConsoleMode` (and `GetConsoleOutputCP` for consoles) to decide how to write. For bulk writes through a locked handle that is most of the work; #154071 measured 14% of CPU time in `is_console` alone. This caches the handle and the console verdict in the sys-level `Stdout`/`Stderr` and re-queries when a new lock session begins, as [suggested in the issue](#154071 (comment)). Holding a `StdoutLock` pins the stream; unlocked writes acquire the lock per call, so they re-query per write exactly as before, and `SetStdHandle` calls made between lock sessions keep working (the #40490 behavior is preserved). Numbers from the issue's workload shape (16 KiB chunks to `NUL`, 4 GiB total, Windows 11): | mode | before | after | |---|---|---| | `stdout().lock()` | 0.478 s (8,561 MiB/s) | 0.120 s (34,350 MiB/s) | | unlocked | 0.467 s | 0.293 s | The unlocked case also improves because one implicit lock session can contain several sys-level writes when `LineWriter` splits a chunk at newlines. The console path cannot run in CI, so it was verified under a pseudoconsole: several writes through one lock session on a CP437 console came through `WriteConsoleW` intact (non-ASCII included), and after `SetConsoleOutputCP` plus a new lock session the refresh picked up the change. The `refresh` hook is a small `#[cfg(windows)]` shim on `StdoutRaw`/`StderrRaw`; happy to change it to a method on every platform's sys type instead if that is preferred. Fixes #154071

…uwer Rollup of 7 pull requests Successful merges: - rust-lang/rust#161873 (move bug and span_bug macros to rustc_span) - rust-lang/rust#162584 (Use File::*lock*() in rustc_data_structures::flock when possible) - rust-lang/rust#162704 (Parser: Uninterpolate when checking for const closures, try bikeshed blocks & in relevant diagnostic code) - rust-lang/rust#162787 (Fix suggestions for names captured by formatting macros) - rust-lang/rust#159562 (Windows: cache the stdio write mode for the duration of a lock session) - rust-lang/rust#162813 (Pre lint port cleanups) - rust-lang/rust#162822 (PassWrapper: adapt for removal of EABIVersion arg in LLVM 24)

rustbot assigned aapoalas Jul 19, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jul 19, 2026

aapoalas requested changes Jul 23, 2026

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rust-bors Bot force-pushed the 154071-stdio-handle-cache branch from 8fa6c14 to f05562d Compare August 19, 2026 19:23

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Joel-Wwalker force-pushed the 154071-stdio-handle-cache branch from f05562d to 0154b97 Compare September 15, 2026 18:24

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JonathanBrouwer mentioned this pull request Sep 15, 2026

Rollup of 7 pull requests #162827

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rust-bors Bot merged commit 556c0e6 into rust-lang:main Sep 16, 2026
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