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Posted to commits@asyncband.apache.org by "orthur2 (via GitHub)" <gi...@apache.org> on 2026/10/11 16:04:49 UTC
[PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
orthur2 opened a new pull request, #365:
URL: https://github.com/apache/asyncband/pull/365
## Summary
Closes #362.
Remove `MappedRwLockWriteGuard::downgrade` and `OwnedMappedRwLockWriteGuard::downgrade`, which are unsound. They were added in #65, first shipped in mea 0.4.2, and are still in 0.7.3. To keep shared access to a component, downgrade the original write guard first and then map it with `RwLockReadGuard::map` or `OwnedRwLockReadGuard::map`. The docs and the changelog describe this migration.
## Design Notes
The projection of a mapped write guard can go through interior mutability using exclusive access, as `Mutex::get_mut` does. After a downgrade, another reader can lock the same mutex and change that data while the guard still hands out `&U`. With a `RefCell<Box<_>>` it is even a use-after-free in single-threaded safe code. No bound on the types rules this out, and keeping the write permits would make the method pointless, so this follows tokio and lock_api in not offering it. A `downgrade_map` taking `FnOnce(&T) -> &U` would only shorten the migration above, so it can wait until someone needs it. The reproducers and discussion are in #358 and #362.
Two compile-fail doctests keep the removed methods from coming back. Two Miri tests in `unsafe_paths_test` take the migration path while a second reader locks a mutex inside the projection. No other Miri test keeps a projected read guard alive while another reader uses the lock, so these would catch a read projection derived through `&mut`.
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "jeffoodchain (via GitHub)" <gi...@apache.org>.
jeffoodchain commented on PR #365:
URL: https://github.com/apache/asyncband/pull/365#issuecomment-6111176191
yes ofc, would be happy to review. I will do this tomorrow. 🙏
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "orthur2 (via GitHub)" <gi...@apache.org>.
orthur2 commented on PR #365:
URL: https://github.com/apache/asyncband/pull/365#issuecomment-6111106328
@jeffoodchain @Soumya2004-java could you take a look when you have time?
GitHub only lets me request reviews from collaborators, so I couldn’t add you as reviewers, but you can still leave a review on the PR directly.
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "QwQBiG (via GitHub)" <gi...@apache.org>.
QwQBiG commented on code in PR #365:
URL: https://github.com/apache/asyncband/pull/365#discussion_r4255252964
##########
CHANGELOG.md:
##########
@@ -23,6 +23,10 @@ All notable changes to this project will be documented in this file.
## Unreleased
+### Breaking changes
Review Comment:
A separate PR for 0.7.4 sounds good to me, thanks! And I'll review it; we can keep #365 for after the release.
@tisonkun, I'd value your input.
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "jeffoodchain (via GitHub)" <gi...@apache.org>.
jeffoodchain commented on code in PR #365:
URL: https://github.com/apache/asyncband/pull/365#discussion_r4273913808
##########
CHANGELOG.md:
##########
@@ -23,6 +23,10 @@ All notable changes to this project will be documented in this file.
## Unreleased
+### Breaking changes
Review Comment:
Since the 0.7.4 release is already underway, I'll skip the deprecation PR. Merging #365 after 0.7.4 sounds good to me.
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "tisonkun (via GitHub)" <gi...@apache.org>.
tisonkun merged PR #365:
URL: https://github.com/apache/asyncband/pull/365
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "Soumya2004-java (via GitHub)" <gi...@apache.org>.
Soumya2004-java commented on PR #365:
URL: https://github.com/apache/asyncband/pull/365#issuecomment-6112002696
I reviewed the fix and it looks good to me. Removing both methods seems to address the issue, and the migration approach makes sense.
I saw the discussion about releasing v0.7.4 before merging this breaking change. That sounds reasonable.
I couldn't run the full test suite locally because Cargo couldn't resolve the crates.io download host, so I haven't independently verified the new tests under Miri.
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "orthur2 (via GitHub)" <gi...@apache.org>.
orthur2 commented on PR #365:
URL: https://github.com/apache/asyncband/pull/365#issuecomment-6123817270
@jeffoodchain Updated the new docs and both deref SAFETY comments as suggested.
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "tisonkun (via GitHub)" <gi...@apache.org>.
tisonkun commented on code in PR #365:
URL: https://github.com/apache/asyncband/pull/365#discussion_r4242260459
##########
CHANGELOG.md:
##########
@@ -23,6 +23,10 @@ All notable changes to this project will be documented in this file.
## Unreleased
+### Breaking changes
Review Comment:
I'm considering to release a 0.7.4 recently to include fixes like #361
Then we will merge this PR that includes breaking changes.
@QwQBiG do you happen to have some time the following week to handle this?
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "orthur2 (via GitHub)" <gi...@apache.org>.
orthur2 commented on code in PR #365:
URL: https://github.com/apache/asyncband/pull/365#discussion_r4242282501
##########
CHANGELOG.md:
##########
@@ -23,6 +23,10 @@ All notable changes to this project will be documented in this file.
## Unreleased
+### Breaking changes
Review Comment:
Sounds good. @QwQBiG, the release skill covers the whole process, from the tracking issue through the ATR vote to publication. Feel free to ask me if anything is unclear. I went through it for 0.7.3.
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "QwQBiG (via GitHub)" <gi...@apache.org>.
QwQBiG commented on code in PR #365:
URL: https://github.com/apache/asyncband/pull/365#discussion_r4245734330
##########
CHANGELOG.md:
##########
@@ -23,6 +23,10 @@ All notable changes to this project will be documented in this file.
## Unreleased
+### Breaking changes
Review Comment:
Sure, I can handle the 0.7.4 release. I'll confirm the release scope and follow the release skill.
Thanks @orthur2 for offering to help!
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "tisonkun (via GitHub)" <gi...@apache.org>.
tisonkun commented on code in PR #365:
URL: https://github.com/apache/asyncband/pull/365#discussion_r4257479886
##########
CHANGELOG.md:
##########
@@ -23,6 +23,10 @@ All notable changes to this project will be documented in this file.
## Unreleased
+### Breaking changes
Review Comment:
Either deprecating these methods or just merge #365 after 0.7.4 should be OK.
If anyone would like to send a deprecation PR, feel free to do :D
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "QwQBiG (via GitHub)" <gi...@apache.org>.
QwQBiG commented on PR #365:
URL: https://github.com/apache/asyncband/pull/365#issuecomment-6122295331
+1 LGTM
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "jeffoodchain (via GitHub)" <gi...@apache.org>.
jeffoodchain commented on code in PR #365:
URL: https://github.com/apache/asyncband/pull/365#discussion_r4245046296
##########
CHANGELOG.md:
##########
@@ -23,6 +23,10 @@ All notable changes to this project will be documented in this file.
## Unreleased
+### Breaking changes
Review Comment:
sorry I actually don't know much related to the releasing process. But I think it would be better to add a flag like `#[deprecated(note = "unsound, fixed in #365")]`? (just to avoid the bug from releasing)
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Re: [PR] fix(rwlock): remove downgrade from mapped write guards [asyncband]
Posted by "orthur2 (via GitHub)" <gi...@apache.org>.
orthur2 commented on code in PR #365:
URL: https://github.com/apache/asyncband/pull/365#discussion_r4247833487
##########
CHANGELOG.md:
##########
@@ -23,6 +23,10 @@ All notable changes to this project will be documented in this file.
## Unreleased
+### Breaking changes
Review Comment:
Good idea. A deprecation warning in 0.7.4 would point users at the migration before the methods go away. I’d put the migration in the note instead of a PR number, since that’s what users can act on, e.g. “downgrade the original write guard, then map the read guard”.
@QwQBiG @tisonkun should I send a small PR for 0.7.4 that deprecates both methods, or would you rather handle it as part of the release?
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