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
}
```
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