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/*
GoToSocial
Copyright (C) 2021 GoToSocial Authors admin@gotosocial.org
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Affero General Public License for more details.
You should have received a copy of the GNU Affero General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package federatingdb
import (
"context"
"errors"
"net/url"
"sync"
)
// Lock takes a lock for the object at the specified id. If an error
// is returned, the lock must not have been taken.
//
// The lock must be able to succeed for an id that does not exist in
// the database. This means acquiring the lock does not guarantee the
// entry exists in the database.
//
// Locks are encouraged to be lightweight and in the Go layer, as some
// processes require tight loops acquiring and releasing locks.
//
// Used to ensure race conditions in multiple requests do not occur.
func (f *federatingDB) Lock(c context.Context, id *url.URL) error {
// Before any other Database methods are called, the relevant `id`
// entries are locked to allow for fine-grained concurrency.
// Strategy: create a new lock, if stored, continue. Otherwise, lock the
// existing mutex.
if id == nil {
return errors.New("Lock: id was nil")
}
mu := &sync.Mutex{}
mu.Lock() // Optimistically lock if we do store it.
i, loaded := f.locks.LoadOrStore(id.String(), mu)
if loaded {
mu = i.(*sync.Mutex)
mu.Lock()
}
return nil
}
// Unlock makes the lock for the object at the specified id available.
// If an error is returned, the lock must have still been freed.
//
// Used to ensure race conditions in multiple requests do not occur.
func (f *federatingDB) Unlock(c context.Context, id *url.URL) error {
// Once Go-Fed is done calling Database methods, the relevant `id`
// entries are unlocked.
if id == nil {
return errors.New("Unlock: id was nil")
}
i, ok := f.locks.Load(id.String())
if !ok {
return errors.New("missing an id in unlock")
}
mu := i.(*sync.Mutex)
mu.Unlock()
return nil
}
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