Add job queue (fixes #629)

Request to terminate currently ongoing downloads and jump to the bumped file
incoming in 3, 2, 1.

Also, has a slightly strange effect where we pop a job off the queue, but
the copyChannel is still busy and blocks, though it gets moved to the
progress slice in the jobqueue, and looks like it's in progress which it isn't
as it's waiting to be picked up from the copyChan.

As a result, the progress emitter doesn't register on the task, and hence the file
doesn't have a progress bar, but cannot be replaced by a bump.

I guess I can fix progress bar issue by moving the progressEmiter.Register just
before passing the file to the copyChan, but then we are back to the initial
problem of a file with a progress bar, but no progress happening as it's stuck
 on write to copyChan

I checked if there is a way to check for channel writeability (before popping)
but got struck by lightning just for bringing the idea up in #go-nuts.

My ideal scenario would be to check if copyChan is writeable, pop job from the
queue and shove it down handleFile. This way jobs would stay in the queue while
they cannot be handled, meaning that the `Bump` could bring your file up higher.

internal/model/model.go

@@ -79,6 +79,8 @@ const (
 type service interface {
 	Serve()
 	Stop()
+	Jobs() ([]protocol.FileInfoTruncated, []protocol.FileInfoTruncated) // In progress, Queued
+	Bump(string)
 }
 
 type Model struct {
@@ -416,22 +418,46 @@ func (m *Model) NeedSize(folder string) (files int, bytes int64) {
 	return
 }
 
-// NeedFiles returns the list of currently needed files, stopping at maxFiles
-// files. Limit <= 0 is ignored.
-func (m *Model) NeedFolderFilesLimited(folder string, maxFiles int) []protocol.FileInfoTruncated {
+// NeedFiles returns the list of currently needed files in progress, queued,
+// and to be queued on next puller iteration. Also takes a soft cap which is
+// only respected when adding files from the model rather than the runner queue.
+func (m *Model) NeedFolderFiles(folder string, max int) ([]protocol.FileInfoTruncated, []protocol.FileInfoTruncated, []protocol.FileInfoTruncated) {
 	defer m.leveldbPanicWorkaround()
 
 	m.fmut.RLock()
 	defer m.fmut.RUnlock()
 	if rf, ok := m.folderFiles[folder]; ok {
-		fs := make([]protocol.FileInfoTruncated, 0, maxFiles)
-		rf.WithNeedTruncated(protocol.LocalDeviceID, func(f protocol.FileIntf) bool {
-			fs = append(fs, f.(protocol.FileInfoTruncated))
-			return maxFiles <= 0 || len(fs) < maxFiles
-		})
-		return fs
+		progress := []protocol.FileInfoTruncated{}
+		queued := []protocol.FileInfoTruncated{}
+		rest := []protocol.FileInfoTruncated{}
+		seen := map[string]struct{}{}
+
+		runner, ok := m.folderRunners[folder]
+		if ok {
+			progress, queued = runner.Jobs()
+			seen = make(map[string]struct{}, len(progress)+len(queued))
+			for _, file := range progress {
+				seen[file.Name] = struct{}{}
+			}
+			for _, file := range queued {
+				seen[file.Name] = struct{}{}
+			}
+		}
+		left := max - len(progress) - len(queued)
+		if max < 1 || left > 0 {
+			rf.WithNeedTruncated(protocol.LocalDeviceID, func(f protocol.FileIntf) bool {
+				left--
+				ft := f.(protocol.FileInfoTruncated)
+				_, ok := seen[ft.Name]
+				if !ok {
+					rest = append(rest, ft)
+				}
+				return max < 1 || left > 0
+			})
+		}
+		return progress, queued, rest
 	}
-	return nil
+	return nil, nil, nil
 }
 
 // Index is called when a new device is connected and we receive their full index.
@@ -1336,7 +1362,7 @@ func (m *Model) RemoteLocalVersion(folder string) uint64 {
 	return ver
 }
 
-func (m *Model) availability(folder string, file string) []protocol.DeviceID {
+func (m *Model) availability(folder, file string) []protocol.DeviceID {
 	// Acquire this lock first, as the value returned from foldersFiles can
 	// gen heavily modified on Close()
 	m.pmut.RLock()
@@ -1359,6 +1385,17 @@ func (m *Model) availability(folder string, file string) []protocol.DeviceID {
 	return availableDevices
 }
 
+// Bump the given files priority in the job queue
+func (m *Model) Bump(folder, file string) {
+	m.pmut.RLock()
+	defer m.pmut.RUnlock()
+
+	runner, ok := m.folderRunners[folder]
+	if ok {
+		runner.Bump(file)
+	}
+}
+
 func (m *Model) String() string {
 	return fmt.Sprintf("model@%p", m)
 }