content: add cross-process ingest locking

Allow content stores to ingest content without coordination of a daemon
to manage locks. Supports coordinated ingest and cross-process ingest
status.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

content/locks.go

@@ -0,0 +1,50 @@
+package content
+
+import (
+	"errors"
+	"sync"
+
+	"github.com/nightlyone/lockfile"
+)
+
+// In addition to providing inter-process locks for content ingest, we also
+// define a global in process lock to prevent two goroutines writing to the
+// same file.
+//
+// This is prety unsophisticated for now. In the future, we'd probably like to
+// have more information about who is holding which locks, as well as better
+// error reporting.
+
+var (
+	// locks lets us lock in process, as well as output of process.
+	locks   = map[lockfile.Lockfile]struct{}{}
+	locksMu sync.Mutex
+)
+
+func tryLock(lock lockfile.Lockfile) error {
+	locksMu.Lock()
+	defer locksMu.Unlock()
+
+	if _, ok := locks[lock]; ok {
+		return errors.New("file in use")
+	}
+
+	if err := lock.TryLock(); err != nil {
+		return err
+	}
+
+	locks[lock] = struct{}{}
+	return nil
+}
+
+func unlock(lock lockfile.Lockfile) error {
+	locksMu.Lock()
+	defer locksMu.Unlock()
+
+	if _, ok := locks[lock]; !ok {
+		return nil
+	}
+
+	delete(locks, lock)
+	return lock.Unlock()
+}