Refactor collection_jitter and flush_jitter

use a common function between collection_jitter and flush_jitter. which
creates the same behavior between the two options.

going forward, both jitters will be random sleeps that get re-evaluated
at runtime for every interval (previously only collection_jitter did
this)

also fixes behavior so that both jitters will exit in the event of a
process exit.

closes #1296

internal/config/config.go

@@ -58,7 +58,6 @@ func NewConfig() *Config {
 			Interval:      internal.Duration{Duration: 10 * time.Second},
 			RoundInterval: true,
 			FlushInterval: internal.Duration{Duration: 10 * time.Second},
-			FlushJitter:   internal.Duration{Duration: 5 * time.Second},
 		},
 
 		Tags:          make(map[string]string),

internal/internal.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"log"
+	"math/big"
 	"os"
 	"os/exec"
 	"strconv"
@@ -228,3 +229,27 @@ func CompileFilter(filters []string) (glob.Glob, error) {
 	}
 	return out, err
 }
+
+// RandomSleep will sleep for a random amount of time up to max.
+// If the shutdown channel is closed, it will return before it has finished
+// sleeping.
+func RandomSleep(max time.Duration, shutdown chan struct{}) {
+	if max == 0 {
+		return
+	}
+	maxSleep := big.NewInt(max.Nanoseconds())
+
+	var sleepns int64
+	if j, err := rand.Int(rand.Reader, maxSleep); err == nil {
+		sleepns = j.Int64()
+	}
+
+	t := time.NewTimer(time.Nanosecond * time.Duration(sleepns))
+	select {
+	case <-t.C:
+		return
+	case <-shutdown:
+		t.Stop()
+		return
+	}
+}

internal/internal_test.go

@@ -137,3 +137,28 @@ func TestCompileFilter(t *testing.T) {
 	assert.True(t, f.Match("mem"))
 	assert.True(t, f.Match("network"))
 }
+
+func TestRandomSleep(t *testing.T) {
+	// test that zero max returns immediately
+	s := time.Now()
+	RandomSleep(time.Duration(0), make(chan struct{}))
+	elapsed := time.Since(s)
+	assert.True(t, elapsed < time.Millisecond)
+
+	// test that max sleep is respected
+	s = time.Now()
+	RandomSleep(time.Millisecond*50, make(chan struct{}))
+	elapsed = time.Since(s)
+	assert.True(t, elapsed < time.Millisecond*50)
+
+	// test that shutdown is respected
+	s = time.Now()
+	shutdown := make(chan struct{})
+	go func() {
+		time.Sleep(time.Millisecond * 100)
+		close(shutdown)
+	}()
+	RandomSleep(time.Second, shutdown)
+	elapsed = time.Since(s)
+	assert.True(t, elapsed < time.Millisecond*150)
+}