storage/filesystem: add R/W locking

This commit adds read/write locking for individual files, so that
concurrent requests (e.g. to read and write the same file) cannot
interfere with one another.

The locking is not very fine-grained at the moment, and can probably be
improved upon. But it does ensure consistency.

storage/filesystem.go

@@ -10,6 +10,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"strings"
+	"sync"
 
 	"github.com/rs/zerolog/log"
 
@@ -20,9 +21,13 @@ import (
 
 type filesystemBackend struct {
 	webdav.UserPrincipalBackend
+
 	path          string
 	caldavPrefix  string
 	carddavPrefix string
+
+	// maps file path to *sync.RWMutex
+	locks sync.Map
 }
 
 var (
@@ -47,6 +52,36 @@ func NewFilesystem(fsPath, caldavPrefix, carddavPrefix string, userPrincipalBack
 	return backend, backend, nil
 }
 
+func (b *filesystemBackend) RLock(filename string) {
+	lock_, _ := b.locks.LoadOrStore(filename, &sync.RWMutex{})
+	lock := lock_.(*sync.RWMutex)
+	lock.RLock()
+}
+
+func (b *filesystemBackend) RUnlock(filename string) {
+	lock_, ok := b.locks.Load(filename)
+	if !ok {
+		panic("attempt to unlock non-existing lock")
+	}
+	lock := lock_.(*sync.RWMutex)
+	lock.RUnlock()
+}
+
+func (b *filesystemBackend) Lock(filename string) {
+	lock_, _ := b.locks.LoadOrStore(filename, &sync.RWMutex{})
+	lock := lock_.(*sync.RWMutex)
+	lock.Lock()
+}
+
+func (b *filesystemBackend) Unlock(filename string) {
+	lock_, ok := b.locks.Load(filename)
+	if !ok {
+		panic("attempt to unlock non-existing lock")
+	}
+	lock := lock_.(*sync.RWMutex)
+	lock.Unlock()
+}
+
 func ensureLocalDir(path string) error {
 	if _, err := os.Stat(path); os.IsNotExist(err) {
 		err = os.MkdirAll(path, 0755)