cache: fix backends shutting down when in use when used via the rc

Before this fix, if a long running task (eg a copy) was started by the
rc then the backend could expire before the copy had finished.

The typical symptom was with the dropbox backend giving "batcher is
shutting down" errors.

This patch fixes the problem by pinning the backend until the job has
finished.

See: https://forum.rclone.org/t/uploads-start-repeatedly-failing-after-a-while-using-rc-sync-copy-vs-rclone-copy-for-dropbox/38873/

fs/cache/cache.go

@@ -120,6 +120,14 @@ func Unpin(f fs.Fs) {
 	c.Unpin(fs.ConfigString(f))
 }
 
+// To avoid circular dependencies these are filled in by fs/rc/jobs/job.go
+var (
+	// JobGetJobID for internal use only
+	JobGetJobID func(context.Context) (int64, bool)
+	// JobOnFinish for internal use only
+	JobOnFinish func(int64, func()) (func(), error)
+)
+
 // Get gets an fs.Fs named fsString either from the cache or creates it afresh
 func Get(ctx context.Context, fsString string) (f fs.Fs, err error) {
 	// If we are making a long lived backend which lives longer
@@ -129,7 +137,22 @@ func Get(ctx context.Context, fsString string) (f fs.Fs, err error) {
 	newCtx := context.Background()
 	newCtx = fs.CopyConfig(newCtx, ctx)
 	newCtx = filter.CopyConfig(newCtx, ctx)
-	return GetFn(newCtx, fsString, fs.NewFs)
+	f, err = GetFn(newCtx, fsString, fs.NewFs)
+	if f == nil || (err != nil && err != fs.ErrorIsFile) {
+		return f, err
+	}
+	// If this is part of an rc job then pin the backend until it finishes
+	if JobOnFinish != nil && JobGetJobID != nil {
+		if jobID, ok := JobGetJobID(ctx); ok {
+			// fs.Debugf(f, "Pin for job %d", jobID)
+			Pin(f)
+			_, _ = JobOnFinish(jobID, func() {
+				// fs.Debugf(f, "Unpin for job %d", jobID)
+				Unpin(f)
+			})
+		}
+	}
+	return f, err
 }
 
 // GetArr gets []fs.Fs from []fsStrings either from the cache or creates it afresh

fs/rc/jobs/job.go

@@ -12,10 +12,17 @@ import (
 
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/accounting"
+	"github.com/rclone/rclone/fs/cache"
 	"github.com/rclone/rclone/fs/filter"
 	"github.com/rclone/rclone/fs/rc"
 )
 
+// Fill in these to avoid circular dependencies
+func init() {
+	cache.JobOnFinish = OnFinish
+	cache.JobGetJobID = GetJobID
+}
+
 // Job describes an asynchronous task started via the rc package
 type Job struct {
 	mu        sync.Mutex