Add patch to fix race condition in `Downloader.Cancel`

- Submitted patch upstream in https://github.com/ethereum/go-ethereum/pull/16585
- Fixes Jenkins build in https://jenkins.status.im/job/status-go/job/race-check/48/

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

@@ -1,5 +1,5 @@
 diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
-index 7ede530a9..60ff8dbc6 100644
+index cbdaa0ca2..60ff8dbc6 100644
 --- a/eth/downloader/downloader.go
 +++ b/eth/downloader/downloader.go
 @@ -143,6 +143,8 @@
@@ -11,15 +11,7 @@ index 7ede530a9..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
-@@ -228,6 +230,7 @@
- 		},
- 		trackStateReq: make(chan *stateReq),
- 	}
-+	dl.downloads.Add(1) // Add a sentinel value to avoid waiting with zero count and getting a race condition signaled when terminating Downloader before starting sync
- 	go dl.qosTuner()
- 	go dl.stateFetcher()
- 	return dl
-@@ -403,7 +406,9 @@
+@@ -403,7 +405,9 @@
  // specified peer and head hash.
  func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td *big.Int) (err error) {
  	d.mux.Post(StartEvent{})
@@ -29,14 +21,31 @@ index 7ede530a9..60ff8dbc6 100644
  		// reset on error
  		if err != nil {
  			d.mux.Post(FailedEvent{err})
-@@ -539,6 +544,13 @@
+@@ -471,12 +475,17 @@
+ 	} else if d.mode == FullSync {
+ 		fetchers = append(fetchers, d.processFullSyncContent)
+ 	}
+-	return d.spawnSync(fetchers)
++	return d.spawnSync(errCancelHeaderFetch, fetchers)
+ }
+ 
+ // spawnSync runs d.process and all given fetcher functions to completion in
+ // 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 {
++	select {
++	case <-d.cancelCh:
++		return errCancel
++	default:
++	}
+ 	errc := make(chan error, len(fetchers))
+ 	d.cancelWg.Add(len(fetchers))
+ 	for _, fn := range fetchers {
+@@ -539,6 +548,10 @@
  
  	// Cancel any pending download requests
  	d.Cancel()
 +
-+	// Remove sentinel value from WaitGroup before calling Wait()
-+	go func() { d.downloads.Done() }()
-+
 +	// Wait, so external dependencies aren't destroyed
 +	// until the download processing is done.
 +	d.downloads.Wait()

_assets/patches/geth/0027-geth-185-fix-race-in-downloader-cancel.patch

@@ -0,0 +1,13 @@
+diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
+index eb64f05a..3e8c9cb1 100644
+--- a/eth/downloader/downloader.go
++++ b/eth/downloader/downloader.go
+@@ -230,6 +230,7 @@ func New(mode SyncMode, stateDb ethdb.Database, mux *event.TypeMux, chain BlockC
+ 		},
+ 		trackStateReq: make(chan *stateReq),
+ 	}
++	dl.cancelWg.Add(1)  // Add a sentinel to avoid getting flagged as a race condition in case Cancel is called before spawnSync
+ 	dl.downloads.Add(1) // Add a sentinel value to avoid waiting with zero count and getting a race condition signaled when terminating Downloader before starting sync
+ 	go dl.qosTuner()
+ 	go dl.stateFetcher()
+ 

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

@@ -230,7 +230,6 @@ func New(mode SyncMode, stateDb ethdb.Database, mux *event.TypeMux, chain BlockC
 		},
 		trackStateReq: make(chan *stateReq),
 	}
-	dl.downloads.Add(1) // Add a sentinel value to avoid waiting with zero count and getting a race condition signaled when terminating Downloader before starting sync
 	go dl.qosTuner()
 	go dl.stateFetcher()
 	return dl
@@ -476,12 +475,17 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td *big.I
 	} else if d.mode == FullSync {
 		fetchers = append(fetchers, d.processFullSyncContent)
 	}
-	return d.spawnSync(fetchers)
+	return d.spawnSync(errCancelHeaderFetch, fetchers)
 }
 
 // spawnSync runs d.process and all given fetcher functions to completion in
 // 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 {
+	select {
+	case <-d.cancelCh:
+		return errCancel
+	default:
+	}
 	errc := make(chan error, len(fetchers))
 	d.cancelWg.Add(len(fetchers))
 	for _, fn := range fetchers {
@@ -545,9 +549,6 @@ func (d *Downloader) Terminate() {
 	// Cancel any pending download requests
 	d.Cancel()
 
-	// Remove sentinel value from WaitGroup before calling Wait()
-	go func() { d.downloads.Done() }()
-
 	// Wait, so external dependencies aren't destroyed
 	// until the download processing is done.
 	d.downloads.Wait()