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Add new package cache lock modes The way locking worked before this PR is that only one cargo could write to the package cache at once (otherwise it could cause corruption). However, it allowed cargo's to read from the package cache while running a build under the assumption that writers are append-only and won't affect reading. This allows multiple builds to run concurrently, only blocking on the part where it is not possible to run concurrently (downloading to the cache). This introduces a new package cache locking strategy to support the ability to safely modify existing cache entries while other cargos are potentially reading from the cache. It has different locking modes: - `MutateExclusive` (new) — Held when cargo wants to modify existing cache entries (such as being introduced for garbage collection in #12634), and ensures only one cargo has access to the cache during that time. - `DownloadExclusive` (renamed) — This is a more specialized name for the lock that was before this PR. A caller should acquire this when downloading into the cache and doing resolution. It ensures that only one cargo can append to the cache, but allows other cargos to concurrently read from the cache. - `Shared` (new) — This is to preserve the old concurrent build behavior by allowing multiple concurrent cargos to hold this while a build is running when it is reading from the cache **Reviewing suggestions:** There are a few commits needed to help with testing which are first. The main commit has the following: - `src/cargo/util/cache_lock.rs` is an abstraction around package cache locks, and is the heart of the change. It should have comments and notes which should guide what it is doing. The `CacheLocker` is stored in `Config` along with all our other global stuff. - Every call to `config.acquire_package_cache_lock()` has been changed to explicitly state which lock mode it wants to lock the package cache in. - `Context::compile` is the key point where the `Shared` lock is acquired, ensuring that no mutation is done while the cache is being read. - `MutateExclusive` is not used in this PR, but is being added in preparation for #12634. - The non-blocking `try_acquire_package_cache_lock` API is not used in this PR, but is being added in preparation for #12634 to allow automatic gc to skip running if another cargo is already running (to avoid unnecessary blocking). - `src/cargo/util/flock.rs` has been updated with some code cleanup (removing unused stuff), adds support for non-blocking locks, and renames some functions to make their operation clearer. - `tests/testsuite/cache_lock.rs` contains tests for all the different permutations of ways of acquiring locks.