Race between GUI and hub in generateHashIDs.

[davecgh]

Running on top of PR #371 for the purposes of the matching up the line numbers, but it's the same on master.

WARNING: DATA RACE
Read at 0x00c0000e8e48 by goroutine 41:
  github.com/decred/dcrpool/pool.(*Endpoint).generateHashIDs()
      src/github.com/decred/dcrpool/pool/endpoint.go:225 +0x1d5
  github.com/decred/dcrpool/pool.(*Hub).FetchHashData()
      src/github.com/decred/dcrpool/pool/hub.go:655 +0x178
  github.com/decred/dcrpool/pool.(*Hub).FetchHashData-fm()
      <autogenerated>:1 +0x33
  github.com/decred/dcrpool/internal/gui.(*GUI).updateCacheAndNotifyWebsocketClients()
      src/github.com/decred/dcrpool/internal/gui/gui.go:316 +0xb38
  github.com/decred/dcrpool/internal/gui.(*GUI).Run.func2()
      src/github.com/decred/dcrpool/internal/gui/gui.go:395 +0x4f

Previous write at 0x00c0000e8e48 by goroutine 94:
  github.com/decred/dcrpool/pool.(*Client).handleAuthorizeRequest()
      src/github.com/decred/dcrpool/pool/client.go:271 +0xc64
  github.com/decred/dcrpool/pool.(*Client).process()
      src/github.com/decred/dcrpool/pool/client.go:842 +0x4fe
  github.com/decred/dcrpool/pool.(*Client).run.func2()
      src/github.com/decred/dcrpool/pool/client.go:1299 +0x33

Goroutine 41 (running) created at:
  github.com/decred/dcrpool/internal/gui.(*GUI).Run()
      src/github.com/decred/dcrpool/internal/gui/gui.go:394 +0x244
  main.realMain.func5()
      src/github.com/decred/dcrpool/dcrpool.go:223 +0x4f

Goroutine 94 (running) created at:
  github.com/decred/dcrpool/pool.(*Client).run()
      src/github.com/decred/dcrpool/pool/client.go:1298 +0x1bc
  github.com/decred/dcrpool/pool.(*Endpoint).connect.func3()
      src/github.com/decred/dcrpool/pool/endpoint.go:196 +0x33

[davecgh]

The culprit is that generateHashIDs accesses the account field of clients under the endpoint's client mutex:

	e.clientsMtx.Lock()
	defer e.clientsMtx.Unlock()
...
	for _, c := range e.clients {
		hashID := hashDataID(c.account, c.extraNonce1)
		ids[hashID] = struct{}{}
	}

But then handleAuthorizeRequest in client.go sets it without a mutex:

...
		c.account = account.UUID
		c.name = name

	case true:
		// Set a default account id.
		c.account = defaultAccountID