markets: dcrdex candlesticks

[buck54321]

No description provided.

[chappjc]

Nice surprise. Works well and deployed with a couple tweaks to the rate server, explorer instances picking up the new feed: https://dcrdata.decred.org/market?chart=candlestick&xc=dcrdex&bin=1h

Mostly easy comments, but it does look like the dcr.reqs map is susceptible to a concurrent map read-write.

Also, when testing, I noticed a data race that was already on master:

WARNING: DATA RACE
Read at 0x00c000011750 by goroutine 104:
  github.com/decred/dcrdata/exchanges/v3.(*ExchangeBot).signalIndexUpdate()
      /home/jon/github/decred/dcrdata/exchanges/bot.go:575 +0x8a
  github.com/decred/dcrdata/exchanges/v3.(*ExchangeBot).Start()
      /home/jon/github/decred/dcrdata/exchanges/bot.go:483 +0xf50
  main._main·dwrap·3()
      /home/jon/github/decred/dcrdata/cmd/dcrdata/main.go:421 +0x64

Previous write at 0x00c000011750 by goroutine 136:
  github.com/decred/dcrdata/exchanges/v3.(*ExchangeBot).UpdateChannels()
      /home/jon/github/decred/dcrdata/exchanges/bot.go:554 +0x2a4
  github.com/decred/dcrdata/cmd/dcrdata/explorer.(*explorerUI).watchExchanges()
      /home/jon/github/decred/dcrdata/cmd/dcrdata/explorer/explorer.go:788 +0x8e
  github.com/decred/dcrdata/cmd/dcrdata/explorer.New·dwrap·2()
      /home/jon/github/decred/dcrdata/cmd/dcrdata/explorer/explorer.go:383 +0x39

Goroutine 104 (running) created at:
  main._main()
      /home/jon/github/decred/dcrdata/cmd/dcrdata/main.go:421 +0xa309
...

Goroutine 136 (running) created at:
  github.com/decred/dcrdata/cmd/dcrdata/explorer.New()
      /home/jon/github/decred/dcrdata/cmd/dcrdata/explorer/explorer.go:383 +0x212a
  main._main()
      /home/jon/github/decred/dcrdata/cmd/dcrdata/main.go:457 +0x47e8

I think it's straightforward to resolve:

diff --git a/exchanges/bot.go b/exchanges/bot.go
index 9f9c66ab..9b4621f7 100644
--- a/exchanges/bot.go
+++ b/exchanges/bot.go
@@ -216,15 +216,6 @@ type UpdateChannels struct {
        Quit     chan struct{}
 }
 
-// NewUpdateChannels creates a new initialized set of UpdateChannels.
-func NewUpdateChannels() *UpdateChannels {
-       return &UpdateChannels{
-               Exchange: make(chan *ExchangeUpdate, 16),
-               Index:    make(chan *IndexUpdate, 16),
-               Quit:     make(chan struct{}),
-       }
-}
-
 // The chart data structures that are encoded and cached are the
 // candlestickResponse and the depthResponse.
 type candlestickResponse struct {
@@ -562,6 +553,8 @@ func (bot *ExchangeBot) UpdateChannels() *UpdateChannels {
 
 // Send an update to any channels requested with bot.UpdateChannels().
 func (bot *ExchangeBot) signalExchangeUpdate(update *ExchangeUpdate) {
+       bot.mtx.RLock()
+       defer bot.mtx.RUnlock()
        for _, ch := range bot.updateChans {
                select {
                case ch <- update:
@@ -572,6 +565,8 @@ func (bot *ExchangeBot) signalExchangeUpdate(update *ExchangeUpdate) {
 }
 
 func (bot *ExchangeBot) signalIndexUpdate(update *IndexUpdate) {
+       bot.mtx.RLock()
+       defer bot.mtx.RUnlock()
        for _, ch := range bot.indexChans {
                select {
                case ch <- update:

NewUpdateChannels was unused, and despite being exported, I cannot imagine a use for it.