cache: Implement --cache-db-wait-time flag

This can be used to make the cache wait for other running cache
backends to finish rather than erroring after 1 second.

backend/cache/cache.go

@@ -50,6 +50,8 @@ const (
 	DefCacheWrites = false
 	// DefCacheTmpWaitTime says how long should files be stored in local cache before being uploaded
 	DefCacheTmpWaitTime = "15m"
+	// DefCacheDbWaitTime defines how long the cache backend should wait for the DB to be available
+	DefCacheDbWaitTime = 1 * time.Second
 )
 
 // Globals
@@ -69,6 +71,7 @@ var (
 	cacheStoreWrites        = flags.BoolP("cache-writes", "", DefCacheWrites, "Will cache file data on writes through the FS")
 	cacheTempWritePath      = flags.StringP("cache-tmp-upload-path", "", "", "Directory to keep temporary files until they are uploaded to the cloud storage")
 	cacheTempWaitTime       = flags.StringP("cache-tmp-wait-time", "", DefCacheTmpWaitTime, "How long should files be stored in local cache before being uploaded")
+	cacheDbWaitTime         = flags.DurationP("cache-db-wait-time", "", DefCacheDbWaitTime, "How long to wait for the DB to be available - 0 is unlimited")
 )
 
 // Register with Fs

backend/cache/storage_persistent.go

@@ -122,7 +122,7 @@ func (b *Persistent) connect() error {
 	if err != nil {
 		return errors.Wrapf(err, "failed to create a data directory %q", b.dataPath)
 	}
-	b.db, err = bolt.Open(b.dbPath, 0644, &bolt.Options{Timeout: 1 * time.Second})
+	b.db, err = bolt.Open(b.dbPath, 0644, &bolt.Options{Timeout: *cacheDbWaitTime})
 	if err != nil {
 		return errors.Wrapf(err, "failed to open a cache connection to %q", b.dbPath)
 	}

docs/content/cache.md

@@ -397,3 +397,13 @@ Note that only one file is uploaded at a time and it can take longer to
 start the upload if a queue formed for this purpose.
 
 **Default**: 15m
+
+#### --cache-db-wait-time=DURATION ####
+
+Only one process can have the DB open at any one time, so rclone waits
+for this duration for the DB to become available before it gives an
+error.
+
+If you set it to 0 then it will wait forever.
+
+**Default**: 1s