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internal/federation/federatingdb/lock.go

@@ -0,0 +1,70 @@
+/*
+   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.
+	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.
+
+	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
+}