Ensure that wg.Add is called before wg.Wait

Now if Add is to be called it will be called before Wait, this is achieved
by doing following:
- if cancel() gets lock first and closes channelCh before spawnSync is called
we will exit right away
- if not than we will ensure that we hold a lock until syncers are spawned
so that cancel() will be blocked for this time and it will prevent whole Terminate() from
progressing

_assets/patches/geth/0016-fix-leveldb-issue.patch

@@ -1,8 +1,8 @@
-diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
-index cbdaa0ca2..60ff8dbc6 100644
---- a/eth/downloader/downloader.go
-+++ b/eth/downloader/downloader.go
-@@ -143,6 +143,8 @@
+diff --git i/eth/downloader/downloader.go w/eth/downloader/downloader.go
+index 43f0e3db9..b337f95c9 100644
+--- i/eth/downloader/downloader.go
++++ w/eth/downloader/downloader.go
+@@ -143,6 +143,8 @@ type Downloader struct {
  	quitCh   chan struct{} // Quit channel to signal termination
  	quitLock sync.RWMutex  // Lock to prevent double closes
  
@@ -11,7 +11,7 @@ index cbdaa0ca2..60ff8dbc6 100644
  	// Testing hooks
  	syncInitHook     func(uint64, uint64)  // Method to call upon initiating a new sync run
  	bodyFetchHook    func([]*types.Header) // Method to call upon starting a block body fetch
-@@ -403,7 +405,9 @@
+@@ -403,7 +405,9 @@ func (d *Downloader) synchronise(id string, hash common.Hash, td *big.Int, mode
  // specified peer and head hash.
  func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td *big.Int) (err error) {
  	d.mux.Post(StartEvent{})
@@ -21,7 +21,7 @@ index cbdaa0ca2..60ff8dbc6 100644
  		// reset on error
  		if err != nil {
  			d.mux.Post(FailedEvent{err})
-@@ -471,12 +475,17 @@
+@@ -471,14 +475,22 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td *big.I
  	} else if d.mode == FullSync {
  		fetchers = append(fetchers, d.processFullSyncContent)
  	}
@@ -33,15 +33,20 @@ index cbdaa0ca2..60ff8dbc6 100644
  // separate goroutines, returning the first error that appears.
 -func (d *Downloader) spawnSync(fetchers []func() error) error {
 +func (d *Downloader) spawnSync(errCancel error, fetchers []func() error) error {
++	d.cancelLock.Lock()
 +	select {
 +	case <-d.cancelCh:
++		d.cancelLock.Unlock()
 +		return errCancel
 +	default:
 +	}
  	errc := make(chan error, len(fetchers))
  	d.cancelWg.Add(len(fetchers))
++	d.cancelLock.Unlock()
  	for _, fn := range fetchers {
-@@ -539,6 +548,10 @@
+ 		fn := fn
+ 		go func() { defer d.cancelWg.Done(); errc <- fn() }()
+@@ -539,6 +551,10 @@ func (d *Downloader) Terminate() {
  
  	// Cancel any pending download requests
  	d.Cancel()
@@ -52,10 +57,10 @@ index cbdaa0ca2..60ff8dbc6 100644
  }
  
  // fetchHeight retrieves the head header of the remote peer to aid in estimating
-diff --git a/eth/handler.go b/eth/handler.go
-index c2426544f..08818a507 100644
---- a/eth/handler.go
-+++ b/eth/handler.go
+diff --git i/eth/handler.go w/eth/handler.go
+index 4069359c9..da9ebb243 100644
+--- i/eth/handler.go
++++ w/eth/handler.go
 @@ -230,6 +230,9 @@ func (pm *ProtocolManager) Stop() {
  	// Quit fetcher, txsyncLoop.
  	close(pm.quitSync)
@@ -66,10 +71,10 @@ index c2426544f..08818a507 100644
  	// Disconnect existing sessions.
  	// This also closes the gate for any new registrations on the peer set.
  	// sessions which are already established but not added to pm.peers yet
-diff --git a/eth/sync.go b/eth/sync.go
-index 2da1464bc..f14e84303 100644
---- a/eth/sync.go
-+++ b/eth/sync.go
+diff --git i/eth/sync.go w/eth/sync.go
+index e49e40087..4367434a6 100644
+--- i/eth/sync.go
++++ w/eth/sync.go
 @@ -135,7 +135,6 @@ func (pm *ProtocolManager) syncer() {
  	// Start and ensure cleanup of sync mechanisms
  	pm.fetcher.Start()
@@ -78,36 +83,10 @@ index 2da1464bc..f14e84303 100644
  
  	// Wait for different events to fire synchronisation operations
  	forceSync := time.NewTicker(forceSyncCycle)
-diff --git a/les/handler.go b/les/handler.go
-index 864abe605..67e459594 100644
---- a/les/handler.go
-+++ b/les/handler.go
-@@ -241,6 +241,9 @@ func (pm *ProtocolManager) Stop() {
-
- 	close(pm.quitSync) // quits syncer, fetcher
-
-+	// Stop downloader and make sure that all the running downloads are complete.
-+	pm.downloader.Terminate()
-+
- 	// Disconnect existing sessions.
- 	// This also closes the gate for any new registrations on the peer set.
- 	// sessions which are already established but not added to pm.peers yet
-diff --git a/les/sync.go b/les/sync.go
-index c0e17f97d..983dc7f36 100644
---- a/les/sync.go
-+++ b/les/sync.go
-@@ -36,7 +36,6 @@ func (pm *ProtocolManager) syncer() {
- 	// Start and ensure cleanup of sync mechanisms
- 	//pm.fetcher.Start()
- 	//defer pm.fetcher.Stop()
--	defer pm.downloader.Terminate()
-
- 	// Wait for different events to fire synchronisation operations
- 	//forceSync := time.Tick(forceSyncCycle)
-diff --git a/les/backend.go b/les/backend.go
-index 6a324cb0..e3844bf8 100644
---- a/les/backend.go
-+++ b/les/backend.go
+diff --git i/les/backend.go w/les/backend.go
+index 6a324cb04..e3844bf84 100644
+--- i/les/backend.go
++++ w/les/backend.go
 @@ -20,7 +20,6 @@ package les
  import (
  	"fmt"
@@ -124,3 +103,29 @@ index 6a324cb0..e3844bf8 100644
  	s.chainDb.Close()
  	close(s.shutdownChan)
  
+diff --git i/les/handler.go w/les/handler.go
+index 9627f392b..f2bbe899f 100644
+--- i/les/handler.go
++++ w/les/handler.go
+@@ -241,6 +241,9 @@ func (pm *ProtocolManager) Stop() {
+ 
+ 	close(pm.quitSync) // quits syncer, fetcher
+ 
++	// Stop downloader and make sure that all the running downloads are complete.
++	pm.downloader.Terminate()
++
+ 	// Disconnect existing sessions.
+ 	// This also closes the gate for any new registrations on the peer set.
+ 	// sessions which are already established but not added to pm.peers yet
+diff --git i/les/sync.go w/les/sync.go
+index c3d37e2f3..fc1f076c7 100644
+--- i/les/sync.go
++++ w/les/sync.go
+@@ -31,7 +31,6 @@ func (pm *ProtocolManager) syncer() {
+ 	// Start and ensure cleanup of sync mechanisms
+ 	//pm.fetcher.Start()
+ 	//defer pm.fetcher.Stop()
+-	defer pm.downloader.Terminate()
+ 
+ 	// Wait for different events to fire synchronisation operations
+ 	//forceSync := time.Tick(forceSyncCycle)

vendor/github.com/ethereum/go-ethereum/eth/downloader/downloader.go

@@ -481,13 +481,16 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td *big.I
 // spawnSync runs d.process and all given fetcher functions to completion in
 // separate goroutines, returning the first error that appears.
 func (d *Downloader) spawnSync(errCancel error, fetchers []func() error) error {
+	d.cancelLock.Lock()
 	select {
 	case <-d.cancelCh:
+		d.cancelLock.Unlock()
 		return errCancel
 	default:
 	}
 	errc := make(chan error, len(fetchers))
 	d.cancelWg.Add(len(fetchers))
+	d.cancelLock.Unlock()
 	for _, fn := range fetchers {
 		fn := fn
 		go func() { defer d.cancelWg.Done(); errc <- fn() }()