some tweaks to prices.go (GetZECPrice, GetCurrentZECPrice)

Simplified locking; there is no need for a read-write mutex, this code
is not performance-critical. Removed some of the go routines because
they're not needed and the locking and concurrency are easier to reason
about without them.

NOTE: there is some test code left in here, search for "XXX testing" and
remove before committing! This test code makes things happen faster:
Instead of fetching prices every 15 minutes, do it every minute. Also,
write historical data every minute, instead of once per day.

Another change needed is to remove some of the logging. It's good for
now during testing, but it's too much for production.

common/prices.go

@@ -21,17 +21,17 @@ import (
 )
 
 var (
-	// Map of all historical prices. Date as "yyyy-mm-dd" to price in cents
+	// Map of all historical prices. Date as "yyyy-mm-dd" to price in USD
 	historicalPrices map[string]float64 = make(map[string]float64)
 
 	// The latest price
 	latestPrice float64 = -1
 
 	// Latest price was fetched at
-	latestPriceAt time.Time
+	latestPriceTime time.Time
 
-	// Mutex to control both historical and latest price
-	pricesRwMutex sync.RWMutex
+	// Mutex to guard both historical and latest price
+	mutex sync.Mutex
 
 	// Full path of the persistence file
 	pricesFileName string
@@ -78,7 +78,6 @@ func fetchAPIPrice(url string, resultPath []string) (float64, error) {
 
 func fetchCoinbasePrice() (float64, error) {
 	return fetchAPIPrice("https://api.coinbase.com/v2/exchange-rates?currency=ZEC", []string{"data", "rates", "USD"})
-
 }
 
 func fetchCoinCapPrice() (float64, error) {
@@ -89,98 +88,86 @@ func fetchBinancePrice() (float64, error) {
 	return fetchAPIPrice("https://api.binance.com/api/v3/avgPrice?symbol=ZECUSDC", []string{"price"})
 }
 
-func fetchHistoricalCoingeckoPrice(ts *time.Time) (float64, error) {
+func fetchHistoricalCoingeckoPrice(ts time.Time) (float64, error) {
 	dt := ts.Format("02-01-2006") // dd-mm-yyyy
-	url := fmt.Sprintf("https://api.coingecko.com/api/v3/coins/zcash/history?date=%s?id=zcash", dt)
+	url := fmt.Sprintf("https://api.coingecko.com/api/v3/coins/zcash/history?date=%s", dt)
 
 	return fetchAPIPrice(url, []string{"market_data", "current_price", "usd"})
 }
 
-// Median gets the median number in a slice of numbers
-func median(inp []float64) (median float64) {
-
-	// Start by sorting a copy of the slice
-	sort.Float64s(inp)
-
-	// No math is needed if there are no numbers
+// calcMedian calculates the median of a sorted slice of numbers
+func calcMedian(inp []float64) (median float64) {
 	// For even numbers we add the two middle numbers
 	// and divide by two using the mean function above
 	// For odd numbers we just use the middle number
-	l := len(inp)
-	if l == 0 {
-		return -1
-	} else if l%2 == 0 {
-		return (inp[l/2-1] + inp[l/2]) / 2
+	n := len(inp)
+	if n%2 == 0 {
+		return (inp[n/2-1] + inp[n/2]) / 2
 	} else {
-		return inp[l/2]
+		return inp[n/2]
 	}
 }
 
 // fetchPriceFromWebAPI will fetch prices from multiple places, discard outliers and return the
-// concensus price
+// concensus price. This function doesn't need the mutex.
 func fetchPriceFromWebAPI() (float64, error) {
 	// We'll fetch prices from all our endpoints, and use the median price from that
 	priceProviders := []func() (float64, error){fetchBinancePrice, fetchCoinCapPrice, fetchCoinbasePrice}
 
-	ch := make(chan float64)
-
 	// Get all prices
+	prices := make([]float64, 0)
 	for _, provider := range priceProviders {
-		go func(provider func() (float64, error)) {
 		price, err := provider()
-			if err != nil {
-				Log.WithFields(logrus.Fields{
-					"method":   "CurrentPrice",
-					"provider": runtime.FuncForPC(reflect.ValueOf(provider).Pointer()).Name(),
-					"error":    err,
-				}).Error("Service")
-
-				ch <- -1
-			} else {
+		if err == nil {
 			Log.WithFields(logrus.Fields{
 				"method":   "CurrentPrice",
 				"provider": runtime.FuncForPC(reflect.ValueOf(provider).Pointer()).Name(),
 				"price":    price,
 			}).Info("Service")
-
-				ch <- price
-			}
-		}(provider)
-	}
-
-	prices := make([]float64, 0)
-	for range priceProviders {
-		price := <-ch
-		if price > 0 {
 			prices = append(prices, price)
+		} else {
+			Log.WithFields(logrus.Fields{
+				"method":   "CurrentPrice",
+				"provider": runtime.FuncForPC(reflect.ValueOf(provider).Pointer()).Name(),
+				"error":    err,
+			}).Error("Service")
 		}
 	}
-
-	// sort
+	if len(prices) == 0 {
+		return -1, errors.New("no price providers are available")
+	}
 	sort.Float64s(prices)
 
 	// Get the median price
-	median1 := median(prices)
+	median := calcMedian(prices)
 
 	// Discard all values that are more than 20% outside the median
 	validPrices := make([]float64, 0)
 	for _, price := range prices {
-		if (math.Abs(price-median1) / median1) > 0.2 {
+		if (math.Abs(price-median) / median) > 0.2 {
 			Log.WithFields(logrus.Fields{
 				"method": "CurrentPrice",
-				"error":  fmt.Sprintf("Discarding price (%.2f) because too far away from median (%.2f", price, median1),
+				"error":  fmt.Sprintf("Discarding price (%.2f) because too far away from median (%.2f", price, median),
 			}).Error("Service")
 		} else {
 			validPrices = append(validPrices, price)
 		}
 	}
 
+	// At least 2 (valid) providers are required; we don't want to depend on just one
+	if len(validPrices) < 2 {
+		return -1, errors.New("insufficient price providers are available")
+	}
+
 	// If we discarded too many, return an error
 	if len(validPrices) < (len(prices)/2 + 1) {
 		return -1, errors.New("not enough valid prices")
-	} else {
-		return median(validPrices), nil
 	}
+	median = calcMedian(validPrices)
+	if median <= 0 {
+		return -1, errors.New("median price is <= 0")
+	}
+	return median, nil
 }
 
 func readHistoricalPricesFile() (map[string]float64, error) {
@@ -217,132 +204,98 @@ func writeHistoricalPricesMap() {
 	defer f.Close()
 
 	j := gob.NewEncoder(f)
-
-	{
-		// Read lock
-		pricesRwMutex.RLock()
-		err = j.Encode(historicalPrices)
-		pricesRwMutex.RUnlock()
-
-		if err != nil {
+	if err = j.Encode(historicalPrices); err != nil {
 		Log.Errorf("Couldn't encode historical prices: %v", err)
 		return
 	}
-	}
-
 	Log.WithFields(logrus.Fields{
 		"method": "HistoricalPrice",
 		"action": "Wrote historical prices file",
 	}).Info("Service")
 }
 
+// GetCurrentPrice is a top-level API, returns the latest price that we
+// have fetched if no more than 3 hours old, else an error. An error
+// should not occur unless we can't reach enough price oracles.
 func GetCurrentPrice() (float64, error) {
-	// Read lock
-	pricesRwMutex.RLock()
-	defer pricesRwMutex.RUnlock()
+	mutex.Lock()
+	defer mutex.Unlock()
+
+	if latestPriceTime.IsZero() {
+		return -1, errors.New("starting up, prices not available yet")
+	}
 
 	// If the current price is too old, don't return it.
-	if time.Since(latestPriceAt).Hours() > 3 {
+	if time.Since(latestPriceTime).Hours() > 3 {
 		return -1, errors.New("price too old")
 	}
 
 	return latestPrice, nil
 }
 
-func writeLatestPrice(price float64) {
-	{
-		// Read lock
-		pricesRwMutex.RLock()
-
-		// Check if the time has "rolled over", if yes then preserve the last price
-		// as the previous day's historical price
-		if latestPrice > 0 && latestPriceAt.Format("2006-01-02") != time.Now().Format("2006-01-02") {
-			// update the historical price.
-			// First, make a copy of the time
-			t := time.Unix(latestPriceAt.Unix(), 0)
-
-			go addHistoricalPrice(latestPrice, &t)
-		}
-		pricesRwMutex.RUnlock()
+// return the time in YYYY-MM-DD string format
+func day(t time.Time) string {
+	return t.Format("2006-01-02")
 }
 
-	// Write lock
-	pricesRwMutex.Lock()
-
-	latestPrice = price
-	latestPriceAt = time.Now()
-
-	pricesRwMutex.Unlock()
-}
-
-func GetHistoricalPrice(ts *time.Time) (float64, *time.Time, error) {
-	dt := ts.Format("2006-01-02")
+// GetHistoricalPrice returns the price for the given day, but only
+// accurate to day granularity.
+func GetHistoricalPrice(ts time.Time) (float64, *time.Time, error) {
+	dt := day(ts)
 	canonicalTime, err := time.Parse("2006-01-02", dt)
 	if err != nil {
 		return -1, nil, err
 	}
-
-	{
-		// Read lock
-		pricesRwMutex.RLock()
-		val, ok := historicalPrices[dt]
-		pricesRwMutex.RUnlock()
-
-		if ok {
+	mutex.Lock()
+	defer mutex.Unlock()
+	if val, ok := historicalPrices[dt]; ok {
 		return val, &canonicalTime, nil
 	}
-	}
-
-	{
 	// Check if this is the same as the current latest price
-
-		// Read lock
-		pricesRwMutex.RLock()
-		var price = latestPrice
-		var returnPrice = price > 0 && latestPriceAt.Format("2006-01-02") == dt
-		pricesRwMutex.RUnlock()
-
-		if returnPrice {
-			return price, &canonicalTime, nil
-		}
+	if latestPrice > 0 && day(latestPriceTime) == dt {
+		return latestPrice, &canonicalTime, nil
 	}
 
 	// Fetch price from web API
+	mutex.Unlock()
 	price, err := fetchHistoricalCoingeckoPrice(ts)
+	mutex.Lock()
 	if err != nil {
 		Log.Errorf("Couldn't read historical prices from Coingecko: %v", err)
 		return -1, nil, err
 	}
-
-	go addHistoricalPrice(price, ts)
-
-	return price, &canonicalTime, err
-}
-
-func addHistoricalPrice(price float64, ts *time.Time) {
 	if price <= 0 {
-		return
+		Log.Errorf("historical prices from Coingecko <= 0")
+		return -1, nil, errors.New("bad Coingecko result")
 	}
-	dt := ts.Format("2006-01-02")
+	// add to our cache so we don't have to hit Coingecko again
+	// for the same date
+	addHistoricalPrice(price, ts)
 
-	// Read Lock
-	pricesRwMutex.RLock()
-	_, ok := historicalPrices[dt]
-	pricesRwMutex.RUnlock()
+	return price, &canonicalTime, nil
+}
 
-	if !ok {
-		// Write lock
-		pricesRwMutex.Lock()
+// Add a price entry for the given day both to our map
+// and to the file (so we'll have it after a restart).
+// This caching allows us to hit coingecko less often,
+// and provides resilience when that site is down.
+//
+// There are two ways a historical price can be added:
+//   - When a client calls GetZECPrice to get a past price
+//   - When a new day begins, we'll save the previous day's price
+//
+func addHistoricalPrice(price float64, ts time.Time) {
+	dt := day(ts)
+	if _, ok := historicalPrices[dt]; !ok {
+		// an entry for this day doesn't exist, add it
 		historicalPrices[dt] = price
-		defer pricesRwMutex.Unlock()
-
-		go Log.WithFields(logrus.Fields{
+		Log.WithFields(logrus.Fields{
 			"method": "HistoricalPrice",
 			"action": "Add",
 			"date":   dt,
 			"price":  price,
 		}).Info("Service")
-		go writeHistoricalPricesMap()
+		writeHistoricalPricesMap()
 	}
 }
 
@@ -352,14 +305,14 @@ func StartPriceFetcher(dbPath string, chainName string) {
 	pricesFileName = filepath.Join(dbPath, chainName, "prices")
 
 	// Read the historical prices if available
+	mutex.Lock()
 	if prices, err := readHistoricalPricesFile(); err != nil {
 		Log.Errorf("Couldn't read historical prices, starting with empty map")
 	} else {
-		// Write lock
-		pricesRwMutex.Lock()
 		historicalPrices = prices
-		pricesRwMutex.Unlock()
+		Log.Infof("prices at start: %v", prices)
 	}
+	mutex.Unlock()
 
 	// Fetch the current price every 15 mins
 	go func() {
@@ -373,10 +326,20 @@ func StartPriceFetcher(dbPath string, chainName string) {
 					"price":  price,
 				}).Info("Service")
 
-				writeLatestPrice(price)
+				mutex.Lock()
+				// If the day has changed, save the previous day's
+				// historical price. Historical prices are per-day.
+				if day(latestPriceTime) != day(time.Now()) {
+					if latestPrice > 0 {
+						t := time.Unix(latestPriceTime.Unix(), 0)
+						addHistoricalPrice(latestPrice, t)
 					}
-
-			// Refresh every
+				}
+				latestPrice = price
+				latestPriceTime = time.Now()
+				mutex.Unlock()
+			}
+			// price data 15 minutes out of date is probably okay
 			time.Sleep(15 * time.Minute)
 		}
 	}()

docs/rtd/index.html

@@ -1404,7 +1404,7 @@ There is no staging or applying for these, very simple.</p></td>
                   <td>timestamp</td>
                   <td><a href="#uint64">uint64</a></td>
                   <td></td>
-                  <td><p>List of timestamps(in sec) at which the price is being requested </p></td>
+                  <td><p>timestamp (Unix time, seconds since 1970) at which the price is being requested </p></td>
                 </tr>
               
                 <tr>

frontend/service.go

@@ -183,7 +183,7 @@ func (s *lwdStreamer) GetZECPrice(ctx context.Context, in *walletrpc.PriceReques
 	}
 
 	ts := time.Unix(int64(in.Timestamp), 0)
-	price, timeFetched, err := common.GetHistoricalPrice(&ts)
+	price, timeFetched, err := common.GetHistoricalPrice(ts)
 
 	if err != nil {
 		return nil, err

walletrpc/service.pb.go

@@ -1196,7 +1196,7 @@ type PriceRequest struct {
 	sizeCache     protoimpl.SizeCache
 	unknownFields protoimpl.UnknownFields
 
-	// List of timestamps(in sec) at which the price is being requested
+	// timestamp (Unix time, seconds since 1970) at which the price is being requested
 	Timestamp uint64 `protobuf:"varint,1,opt,name=timestamp,proto3" json:"timestamp,omitempty"`
 	// 3 letter currency-code
 	Currency string `protobuf:"bytes,2,opt,name=currency,proto3" json:"currency,omitempty"`

walletrpc/service.proto

@@ -138,7 +138,7 @@ message GetAddressUtxosReplyList {
 }
 
 message PriceRequest {
-    // List of timestamps(in sec) at which the price is being requested
+    // timestamp (Unix time, seconds since 1970) at which the price is being requested
     uint64 timestamp = 1;
 
     // 3 letter currency-code