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/*
GoToSocial
Copyright (C) 2021-2022 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"
"sync/atomic"
"github.com/sirupsen/logrus"
)
// 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")
}
idStr := id.String()
// Acquire map lock
f.mutex.Lock()
// Get mutex, or create new
mu, ok := f.locks[idStr]
if !ok {
mu, ok = f.pool.Get().(*mutex)
if !ok {
logrus.Panic("Lock: pool entry was not a *mutex")
}
f.locks[idStr] = mu
}
// Unlock map, acquire mutex lock
f.mutex.Unlock()
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")
}
idStr := id.String()
// Check map for mutex
f.mutex.Lock()
mu, ok := f.locks[idStr]
f.mutex.Unlock()
if !ok {
return errors.New("missing an id in unlock")
}
// Unlock the mutex
mu.Unlock()
return nil
}
// mutex defines a mutex we can check the lock status of.
// this is not perfect, but it's good enough for a semi
// regular mutex cleanup routine
type mutex struct {
mu sync.Mutex
st uint32
}
// inUse returns if the mutex is in use
func (mu *mutex) inUse() bool {
return atomic.LoadUint32(&mu.st) == 1
}
// Lock acquire mutex lock
func (mu *mutex) Lock() {
mu.mu.Lock()
atomic.StoreUint32(&mu.st, 1)
}
// Unlock releases mutex lock
func (mu *mutex) Unlock() {
mu.mu.Unlock()
atomic.StoreUint32(&mu.st, 0)
}
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