Fix a Chainlink race using a cache with goroutines

common/ethereum/chainlink/chainlink.go

@@ -9,11 +9,16 @@ import (
 	"math/big"
 
 	ethAbi "github.com/ethereum/go-ethereum/accounts/abi"
+	"github.com/fluidity-money/fluidity-app/common/ethereum"
+	"github.com/fluidity-money/fluidity-app/lib/log"
+
 	ethCommon "github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/ethclient"
-	"github.com/fluidity-money/fluidity-app/common/ethereum"
 )
 
+// Context to use for logging
+const Context = "CHAINLINK"
+
 const chainlinkPriceFeedAbiString = `[
 	{
 		"inputs": [],
@@ -65,53 +70,28 @@ const chainlinkPriceFeedAbiString = `[
 
 var priceFeedAbi ethAbi.ABI
 
-var decimalsCache = make(map[ethCommon.Address]*big.Rat)
-
-func getFeedDecimals(client *ethclient.Client, priceFeedAddress ethCommon.Address) (*big.Rat, error) {
-	decimals, exists := decimalsCache[priceFeedAddress]
-
-	if exists {
-		return decimals, nil
-	}
-
-	decimalsRes, err := ethereum.StaticCall(
-		client,
-		priceFeedAddress,
-		priceFeedAbi,
-		"decimals",
-	)
-
-	if err != nil {
-		return nil, err
-	}
-
-	decimals_, err := ethereum.CoerceBoundContractResultsToUint8(decimalsRes)
+// GetPrice using a Chainlink feed, caching the results in an internal
+// server to make it thread safe
+func GetPrice(client *ethclient.Client, priceFeedAddress ethCommon.Address) (*big.Rat, error) {
+	log.Debug(func(k *log.Log) {
+		k.Context = Context
 
-	if err != nil {
-		return nil, fmt.Errorf(
-			"Failed to read decimals result! %w",
-			err,
+		k.Format(
+			"Using the Chainlink decimals cache to look up decimals for price feed %v",
+			priceFeedAddress,
 		)
-	}
-
-	ten := big.NewRat(10, 1)
+	})
 
-	decimals = ethereum.BigPow(ten, int(decimals_))
-	decimalsCache[priceFeedAddress] = decimals
-
-	return decimals, nil
-}
+	resp := make(chan *big.Rat)
 
-func GetPrice(client *ethclient.Client, priceFeedAddress ethCommon.Address) (*big.Rat, error) {
-	decimals, err := getFeedDecimals(client, priceFeedAddress)
-
-	if err != nil {
-		return nil, fmt.Errorf(
-			"Failed to get feed decimals! %w",
-			err,
-		)
+	decimalsServer <- request{
+		client:           client,
+		priceFeedAddress: priceFeedAddress,
+		resp:             resp,
 	}
 
+	decimals := <-resp
+
 	priceRes, err := ethereum.StaticCall(
 		client,
 		priceFeedAddress,

common/ethereum/chainlink/init.go

@@ -18,4 +18,6 @@ func init() {
 	if priceFeedAbi, err = ethAbi.JSON(priceFeedReader); err != nil {
 		panic(err)
 	}
+
+	go startDecimalsServer()
 }

common/ethereum/chainlink/server.go

@@ -0,0 +1,86 @@
+// Copyright 2022 Fluidity Money. All rights reserved. Use of this
+// source code is governed by a GPL-style license that can be found in the
+// LICENSE.md file.
+
+package chainlink
+
+import (
+	"math/big"
+
+	ethCommon "github.com/ethereum/go-ethereum/common"
+	"github.com/fluidity-money/fluidity-app/common/ethereum"
+	"github.com/fluidity-money/fluidity-app/lib/log"
+
+	"github.com/ethereum/go-ethereum/ethclient"
+)
+
+type request struct {
+	client           *ethclient.Client
+	priceFeedAddress ethCommon.Address
+	resp             chan *big.Rat
+}
+
+// decimalsServer to respond to requests with cached (or not, looked up)
+// responses
+var decimalsServer = make(chan request)
+
+func startDecimalsServer() {
+	decimalsCache := make(map[ethCommon.Address]*big.Rat)
+
+	for request := range decimalsServer {
+		var (
+			client           = request.client
+			priceFeedAddress = request.priceFeedAddress
+			respChan         = request.resp
+		)
+
+		decimals, exists := decimalsCache[priceFeedAddress]
+
+		if exists {
+			respChan <- decimals
+			continue
+		}
+
+		decimalsRes, err := ethereum.StaticCall(
+			client,
+			priceFeedAddress,
+			priceFeedAbi,
+			"decimals",
+		)
+
+		if err != nil {
+			log.Fatal(func(k *log.Log) {
+				k.Context = Context
+
+				k.Format(
+					"Failed to static call feed address %v",
+					priceFeedAddress,
+				)
+
+				k.Payload = err
+			})
+		}
+
+		decimals_, err := ethereum.CoerceBoundContractResultsToUint8(decimalsRes)
+
+		if err != nil {
+			log.Fatal(func(k *log.Log) {
+				k.Context = Context
+
+				k.Format(
+					"Failed to read decimals result for price feed address %v",
+					priceFeedAddress,
+				)
+
+				k.Payload = err
+			})
+		}
+
+		ten := big.NewRat(10, 1)
+
+		decimals = ethereum.BigPow(ten, int(decimals_))
+		decimalsCache[priceFeedAddress] = decimals
+
+		respChan <- decimals
+	}
+}