Add configuration for async GitHub request (re)tries

Currently in some places we use synchronous retries and for checking
merge status on state updates we use a single asynchronous check. This
configuration can be used to combine the best of those 2 approaches -
multiple retries with increasing delays can be used to appropriately
handle both cases where GitHub achieves eventual consistency quickly and
the cases where it's achieved slowly (e.g. in low-traffic repositories).

sort.Slice is added in Go version 1.8. Travis defaults to Go version
1.7.4. Using 1.x in Travis conf forces Travis to use the latest minor
version, which is 1.8 right now.

Author: deiwin

.travis.yml

@@ -1,2 +1,4 @@
 language: go
+go:
+  - 1.x
 sudo: false

config.go

@@ -1,7 +1,9 @@
 package main
 
 import (
+	"sort"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/deiwin/gonfigure"
@@ -13,13 +15,17 @@ var (
 	secretProperty      = gonfigure.NewRequiredEnvProperty("GITHUB_SECRET")
 	// In the format defined in time.ParseDuration. E.g. "300ms", "-1.5h" or "2h45m".
 	githubAPIDelayProperty = gonfigure.NewEnvProperty("GITHUB_API_DELAY", "2s")
+	// A comma separated list of durations in the format defined in
+	// time.ParseDuration. E.g. "300ms,1.5h,2h45m".
+	githubAPITriesProperty = gonfigure.NewEnvProperty("GITHUB_API_TRIES", "0s,10s,30s,3m")
 )
 
 type Config struct {
-	Port           int
-	AccessToken    string
-	Secret         string
-	GithubAPIDelay time.Duration
+	Port               int
+	AccessToken        string
+	Secret             string
+	GithubAPIDelay     time.Duration
+	GithubAPITryDeltas []time.Duration
 }
 
 func NewConfig() Config {
@@ -34,9 +40,33 @@ func NewConfig() Config {
 	}
 
 	return Config{
-		Port:           port,
-		AccessToken:    accessTokenProperty.Value(),
-		Secret:         secretProperty.Value(),
-		GithubAPIDelay: githubAPIDelay,
+		Port:               port,
+		AccessToken:        accessTokenProperty.Value(),
+		Secret:             secretProperty.Value(),
+		GithubAPIDelay:     githubAPIDelay,
+		GithubAPITryDeltas: getDeltasFromDurationsString(githubAPITriesProperty.Value()),
 	}
 }
+
+func getDeltasFromDurationsString(durationsString string) []time.Duration {
+	durationStringList := strings.Split(durationsString, ",")
+	durationList := make([]time.Duration, len(durationStringList))
+	for i, durationString := range durationStringList {
+		var err error
+		durationList[i], err = time.ParseDuration(durationString)
+		if err != nil {
+			panic(err)
+		}
+	}
+	sort.Slice(durationList, func(i, j int) bool { return durationList[i] < durationList[j] })
+
+	deltas := make([]time.Duration, len(durationList))
+	for i, duration := range durationList {
+		if i == 0 {
+			deltas[i] = duration
+		} else {
+			deltas[i] = duration - durationList[i-1]
+		}
+	}
+	return deltas
+}

config_test.go

@@ -136,6 +136,87 @@ var _ = Describe("Config", func() {
 			})
 		})
 	})
+
+	Describe("GITHUB_API_TRIES", func() {
+		name := "GITHUB_API_TRIES"
+
+		Context("when set to sorted durations", func() {
+			durationsString := "0s,2m,2h,4h"
+			setEnvVars(requiredEnvVars)
+			setEnvVar(envVar{name: name, value: durationsString})
+
+			It("is passed as an array of duration deltas", func() {
+				conf := grh.NewConfig()
+				Expect(conf.GithubAPITryDeltas).To(Equal([]time.Duration{
+					0,
+					2 * time.Minute,
+					time.Hour + 58*time.Minute,
+					2 * time.Hour,
+				}))
+			})
+		})
+
+		Context("when set to unsorted durations", func() {
+			durationsString := "0s,2h,2m,4h"
+			setEnvVars(requiredEnvVars)
+			setEnvVar(envVar{name: name, value: durationsString})
+
+			It("is passed as a sorted array of duration deltas", func() {
+				conf := grh.NewConfig()
+				Expect(conf.GithubAPITryDeltas).To(Equal([]time.Duration{
+					0,
+					2 * time.Minute,
+					time.Hour + 58*time.Minute,
+					2 * time.Hour,
+				}))
+			})
+		})
+
+		Context("when contains an invalid duration", func() {
+			durationsString := "two minutes,2h"
+			setEnvVars(requiredEnvVars)
+			setEnvVar(envVar{name: name, value: durationsString})
+
+			It("panics", func() {
+				Expect(func() {
+					grh.NewConfig()
+				}).To(Panic())
+			})
+		})
+
+		Context("when contains an empty duration", func() {
+			durationsString := ",2h"
+			setEnvVars(requiredEnvVars)
+			setEnvVar(envVar{name: name, value: durationsString})
+
+			It("panics", func() {
+				Expect(func() {
+					grh.NewConfig()
+				}).To(Panic())
+			})
+		})
+
+		Context("when contains spaces", func() {
+			durationsString := "2m, 2h"
+			setEnvVars(requiredEnvVars)
+			setEnvVar(envVar{name: name, value: durationsString})
+
+			It("panics", func() {
+				Expect(func() {
+					grh.NewConfig()
+				}).To(Panic())
+			})
+		})
+
+		Context("when not set", func() {
+			setEnvVars(requiredEnvVars)
+
+			It("defaults to a value", func() {
+				conf := grh.NewConfig()
+				Expect(conf.GithubAPITryDeltas).NotTo(BeNil())
+			})
+		})
+	})
 })
 
 var setEnvVar = func(variable envVar) {