Add flag for block profiling

Fixes #1672

Test by adding `--app.block.profile.rate=1`, running scope in
multi-tenant mode and browsing to http://<scope-url>/debug/pprof/

prog/app.go

@@ -7,6 +7,7 @@ import (
 	_ "net/http/pprof"
 	"net/url"
 	"os"
+	"runtime"
 	"strconv"
 	"strings"
 	"time"
@@ -199,6 +200,7 @@ func pipeRouterFactory(userIDer multitenant.UserIDer, pipeRouterURL, consulInf s
 func appMain(flags appFlags) {
 	setLogLevel(flags.logLevel)
 	setLogFormatter(flags.logPrefix)
+	runtime.SetBlockProfileRate(flags.blockProfileRate)
 
 	defer log.Info("app exiting")
 	rand.Seed(time.Now().UnixNano())

prog/main.go

@@ -111,6 +111,8 @@ type appFlags struct {
 	memcachedExpiration time.Duration
 	userIDHeader        string
 
+	blockProfileRate int
+
 	awsCreateTables bool
 	consulInf       string
 }
@@ -197,6 +199,8 @@ func main() {
 	flag.StringVar(&flags.app.memcachedService, "app.memcached.service", "memcached", "SRV service used to discover memcache servers.")
 	flag.StringVar(&flags.app.userIDHeader, "app.userid.header", "", "HTTP header to use as userid")
 
+	flag.IntVar(&flags.app.blockProfileRate, "app.block.profile.rate", 0, "If more than 0, enable block profiling. The profiler aims to sample an average of one blocking event per rate nanoseconds spent blocked.")
+
 	flag.BoolVar(&flags.app.awsCreateTables, "app.aws.create.tables", false, "Create the tables in DynamoDB")
 	flag.StringVar(&flags.app.consulInf, "app.consul.inf", "", "The interface who's address I should advertise myself under in consul")