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SQLite: Only apply the ZFS -shm workaround on the first open of a database - #664
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Included review availability: This review used your included allowance. 2 included reviews remain after this review. Your included PR review attempts over the past 7 days set your current allowance at 5 reviews per hour. 📝 WalkthroughWalkthroughThe Linux SQLite constructor now tracks database paths in a process-wide synchronized set. It runs the existing shared-memory-file workaround only on the first open of each path. The catch-and-rethrow wrapper around the workaround was removed. ChangesSQLite shared-memory workaround
Priority: ➖ Normal Estimated code review effort: 2 (Simple) | ~10 minutes Change: Bug fix Suggested reviewers: Merge Risk: 🟡 Moderate · up to On ZFS, opening one database through two symlinked state-directory paths can still expose concurrent SQLite connections to a crash. Resolve or explicitly accept this risk before merging. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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Review comments at @src/libstore/sqlite.cc:
- Line 87: Keep the `shmFilesSynced` lock held throughout the `-shm`
synchronization workaround in `LocalStore::Config::openStore()`: store the lock
in a local variable before inserting `path`, and retain it until the
`AutoCloseFD` descriptor has closed and the workaround finishes. This makes
concurrent constructors wait before proceeding.
- Line 87: Update the shmFilesSynced gate to key entries by the symlink-resolved
database path, so aliases of the same database share the gate; keep the original
path for opening the -shm file.
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…abase Closing a file descriptor drops all POSIX locks that the process holds on that file. So opening and closing db.sqlite-shm while another connection to the same database is open in this process dropped SQLite's lock on that file. Another process could then truncate it, causing a SIGBUS in the first process. This showed up with the tarball cache, which opens multiple connections per process. We hold the lock on `shmFilesSynced` until the descriptor has been closed, so that a concurrent open of the same database in another thread cannot create a SQLite connection (and thus acquire POSIX locks on db.sqlite-shm) while the first thread still has the file open. Assisted-by: Claude Fable 5.1 <noreply@anthropic.com> (cherry picked from commit cc1cfbb)
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Motivation
Cherry-pick of cc1cfbb from #646 so the fix ships independently of the tarball cache work.
The ZFS
db.sqlite-shmworkaround inSQLite::SQLiteopens and closesdb.sqlite-shmon every database open. Closing a file descriptor drops all POSIX locks the process holds on that file, including the shared dead-man-switch lock SQLite holds on behalf of any other connection this process already has to the same database. Another process can then take the exclusive lock, conclude it is the only user, and truncate the-shmfile. The first process gets aSIGBUSthe next time it touches its WAL index mapping.Context
With this change the workaround runs only the first time a process opens a given database path, before any SQLite connection to it exists, so there is never a lock to drop.
This is a candidate explanation for Sentry issue 7626899056 (
SIGBUSinwalIndexAppendduring a WAL commit innix-daemon3.23.0). That crash requires the daemon child to have had two connections todb.sqliteopen, which happens when a local store is configured as a substituter or vialocal-overlay. Independently of that report, any process that opens a database twice is exposed, e.g. the fetcher cache in commands that fetch tarballs.🤖 Generated with Claude Code
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