mirror of
https://github.com/superseriousbusiness/gotosocial.git
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117 lines
3 KiB
Go
117 lines
3 KiB
Go
/*
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GoToSocial
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Copyright (C) 2021 GoToSocial Authors admin@gotosocial.org
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU Affero General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Affero General Public License for more details.
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You should have received a copy of the GNU Affero General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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package federatingdb
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import (
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"context"
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"errors"
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"net/url"
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"sync"
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"sync/atomic"
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)
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// Lock takes a lock for the object at the specified id. If an error
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// is returned, the lock must not have been taken.
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//
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// The lock must be able to succeed for an id that does not exist in
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// the database. This means acquiring the lock does not guarantee the
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// entry exists in the database.
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//
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// Locks are encouraged to be lightweight and in the Go layer, as some
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// processes require tight loops acquiring and releasing locks.
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//
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// Used to ensure race conditions in multiple requests do not occur.
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func (f *federatingDB) Lock(c context.Context, id *url.URL) error {
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// Before any other Database methods are called, the relevant `id`
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// entries are locked to allow for fine-grained concurrency.
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// Strategy: create a new lock, if stored, continue. Otherwise, lock the
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// existing mutex.
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if id == nil {
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return errors.New("Lock: id was nil")
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}
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idStr := id.String()
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// Acquire map lock
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f.mutex.Lock()
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// Get mutex, or create new
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mu, ok := f.locks[idStr]
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if !ok {
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mu = f.pool.Get().(*mutex)
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f.locks[idStr] = mu
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}
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// Unlock map, acquire mutex lock
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f.mutex.Unlock()
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mu.Lock()
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return nil
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}
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// Unlock makes the lock for the object at the specified id available.
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// If an error is returned, the lock must have still been freed.
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//
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// Used to ensure race conditions in multiple requests do not occur.
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func (f *federatingDB) Unlock(c context.Context, id *url.URL) error {
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// Once Go-Fed is done calling Database methods, the relevant `id`
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// entries are unlocked.
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if id == nil {
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return errors.New("Unlock: id was nil")
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}
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idStr := id.String()
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// Check map for mutex
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f.mutex.Lock()
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mu, ok := f.locks[idStr]
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f.mutex.Unlock()
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if !ok {
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return errors.New("missing an id in unlock")
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}
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// Unlock the mutex
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mu.Unlock()
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return nil
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}
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// mutex defines a mutex we can check the lock status of.
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// this is not perfect, but it's good enough for a semi
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// regular mutex cleanup routine
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type mutex struct {
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mu sync.Mutex
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st uint32
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}
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// inUse returns if the mutex is in use
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func (mu *mutex) inUse() bool {
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return atomic.LoadUint32(&mu.st) == 1
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}
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// Lock acquire mutex lock
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func (mu *mutex) Lock() {
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mu.mu.Lock()
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atomic.StoreUint32(&mu.st, 1)
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}
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// Unlock releases mutex lock
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func (mu *mutex) Unlock() {
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mu.mu.Unlock()
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atomic.StoreUint32(&mu.st, 0)
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}
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