2 Commits

Author	SHA1	Message	Date
Jason Song	bc0977f1c9	Refactor globallock (#31933) ... Follow #31908. The main refactor is that it has removed the returned context of `Lock`. The returned context of `Lock` in old code is to provide a way to let callers know that they have lost the lock. But in most cases, callers shouldn't cancel what they are doing even it has lost the lock. And the design would confuse developers and make them use it incorrectly. See the discussion history: https://github.com/go-gitea/gitea/pull/31813#discussion_r1732041513 and https://github.com/go-gitea/gitea/pull/31813#discussion_r1734078998 It's a breaking change, but since the new module hasn't been used yet, I think it's OK to not add the `pr/breaking` label. ## Design principles It's almost copied from #31908, but with some changes. ### Use spinlock even in memory implementation (unchanged) In actual use cases, users may cancel requests. `sync.Mutex` will block the goroutine until the lock is acquired even if the request is canceled. And the spinlock is more suitable for this scenario since it's possible to give up the lock acquisition. Although the spinlock consumes more CPU resources, I think it's acceptable in most cases. ### Do not expose the mutex to callers (unchanged) If we expose the mutex to callers, it's possible for callers to reuse the mutex, which causes more complexity. For example: ```go lock := GetLocker(key) lock.Lock() // ... // even if the lock is unlocked, we cannot GC the lock, // since the caller may still use it again. lock.Unlock() lock.Lock() // ... lock.Unlock() // callers have to GC the lock manually. RemoveLocker(key) ``` That's why https://github.com/go-gitea/gitea/pull/31813#discussion_r1721200549 In this PR, we only expose `ReleaseFunc` to callers. So callers just need to call `ReleaseFunc` to release the lock, and do not need to care about the lock's lifecycle. ```go release, err := locker.Lock(ctx, key) if err != nil { return err } // ... release() // if callers want to lock again, they have to re-acquire the lock. release, err := locker.Lock(ctx, key) // ... ``` In this way, it's also much easier for redis implementation to extend the mutex automatically, so that callers do not need to care about the lock's lifecycle. See also https://github.com/go-gitea/gitea/pull/31813#discussion_r1722659743 ### Use "release" instead of "unlock" (unchanged) For "unlock", it has the meaning of "unlock an acquired lock". So it's not acceptable to call "unlock" when failed to acquire the lock, or call "unlock" multiple times. It causes more complexity for callers to decide whether to call "unlock" or not. So we use "release" instead of "unlock" to make it clear. Whether the lock is acquired or not, callers can always call "release", and it's also safe to call "release" multiple times. But the code DO NOT expect callers to not call "release" after acquiring the lock. If callers forget to call "release", it will cause resource leak. That's why it's always safe to call "release" without extra checks: to avoid callers to forget to call it. ### Acquired locks could be lost, but the callers shouldn't stop Unlike `sync.Mutex` which will be locked forever once acquired until calling `Unlock`, for distributed lock, the acquired lock could be lost. For example, the caller has acquired the lock, and it holds the lock for a long time since auto-extending is working for redis. However, it lost the connection to the redis server, and it's impossible to extend the lock anymore. In #31908, it will cancel the context to make the operation stop, but it's not safe. Many operations are not revert-able. If they have been interrupted, then the instance goes corrupted. So `Lock` won't return `ctx` anymore in this PR. ### Multiple ways to use the lock 1. Regular way ```go release, err := Lock(ctx, key) if err != nil { return err } defer release() // ... ``` 2. Early release ```go release, err := Lock(ctx, key) if err != nil { return err } defer release() // ... // release the lock earlier release() // continue to do something else // ... ``` 3. Functional way ```go if err := LockAndDo(ctx, key, func(ctx context.Context) error { // ... return nil }); err != nil { return err } ```	2024-08-29 03:48:21 +00:00
Jason Song	1e4be0945b	Introduce globallock as distributed locks (#31908) ... To help #31813, but do not replace it, since this PR just introduces the new module but misses some work: - New option in settings. `#31813` has done it. - Use the locks in business logic. `#31813` has done it. So I think the most efficient way is to merge this PR first (if it's acceptable) and then finish #31813. ## Design principles ### Use spinlock even in memory implementation In actual use cases, users may cancel requests. `sync.Mutex` will block the goroutine until the lock is acquired even if the request is canceled. And the spinlock is more suitable for this scenario since it's possible to give up the lock acquisition. Although the spinlock consumes more CPU resources, I think it's acceptable in most cases. ### Do not expose the mutex to callers If we expose the mutex to callers, it's possible for callers to reuse the mutex, which causes more complexity. For example: ```go lock := GetLocker(key) lock.Lock() // ... // even if the lock is unlocked, we cannot GC the lock, // since the caller may still use it again. lock.Unlock() lock.Lock() // ... lock.Unlock() // callers have to GC the lock manually. RemoveLocker(key) ``` That's why https://github.com/go-gitea/gitea/pull/31813#discussion_r1721200549 In this PR, we only expose `ReleaseFunc` to callers. So callers just need to call `ReleaseFunc` to release the lock, and do not need to care about the lock's lifecycle. ```go _, release, err := locker.Lock(ctx, key) if err != nil { return err } // ... release() // if callers want to lock again, they have to re-acquire the lock. _, release, err := locker.Lock(ctx, key) // ... ``` In this way, it's also much easier for redis implementation to extend the mutex automatically, so that callers do not need to care about the lock's lifecycle. See also https://github.com/go-gitea/gitea/pull/31813#discussion_r1722659743 ### Use "release" instead of "unlock" For "unlock", it has the meaning of "unlock an acquired lock". So it's not acceptable to call "unlock" when failed to acquire the lock, or call "unlock" multiple times. It causes more complexity for callers to decide whether to call "unlock" or not. So we use "release" instead of "unlock" to make it clear. Whether the lock is acquired or not, callers can always call "release", and it's also safe to call "release" multiple times. But the code DO NOT expect callers to not call "release" after acquiring the lock. If callers forget to call "release", it will cause resource leak. That's why it's always safe to call "release" without extra checks: to avoid callers to forget to call it. ### Acquired locks could be lost Unlike `sync.Mutex` which will be locked forever once acquired until calling `Unlock`, in the new module, the acquired lock could be lost. For example, the caller has acquired the lock, and it holds the lock for a long time since auto-extending is working for redis. However, it lost the connection to the redis server, and it's impossible to extend the lock anymore. If the caller don't stop what it's doing, another instance which can connect to the redis server could acquire the lock, and do the same thing, which could cause data inconsistency. So the caller should know what happened, the solution is to return a new context which will be canceled if the lock is lost or released: ```go ctx, release, err := locker.Lock(ctx, key) if err != nil { return err } defer release() // ... DoSomething(ctx) // the lock is lost now, then ctx has been canceled. // Failed, since ctx has been canceled. DoSomethingElse(ctx) ``` ### Multiple ways to use the lock 1. Regular way ```go ctx, release, err := Lock(ctx, key) if err != nil { return err } defer release() // ... ``` 2. Early release ```go ctx, release, err := Lock(ctx, key) if err != nil { return err } defer release() // ... // release the lock earlier and reset the context back ctx = release() // continue to do something else // ... ``` 3. Functional way ```go if err := LockAndDo(ctx, key, func(ctx context.Context) error { // ... return nil }); err != nil { return err } ```	2024-08-26 22:27:57 +08:00