Add --cpuprofile option.

This commit provides a new --cpuprofile flag that can be used to specify a
file path to write CPU profile data into.  The resulting profile can then be
consumed by the 'go tool pprof' command.

btcd.go

@@ -10,6 +10,7 @@ import (
 	_ "net/http/pprof"
 	"os"
 	"runtime"
+	"runtime/pprof"
 )
 
 var (
@@ -44,7 +45,7 @@ func btcdMain() error {
 	// Show version at startup.
 	log.Infof("Version %s", version())
 
-	// See if we want to enable profiling.
+	// Enable http profiling server if requested.
 	if cfg.Profile != "" {
 		go func() {
 			listenAddr := net.JoinHostPort("", cfg.Profile)
@@ -56,6 +57,17 @@ func btcdMain() error {
 		}()
 	}
 
+	// Write cpu profile if requested.
+	if cfg.CpuProfile != "" {
+		f, err := os.Create(cfg.CpuProfile)
+		if err != nil {
+			log.Errorf("Unable to create cpu profile: %v", err)
+			return err
+		}
+		pprof.StartCPUProfile(f)
+		defer pprof.StopCPUProfile()
+	}
+
 	// Perform upgrades to btcd as new versions require it.
 	if err := doUpgrades(); err != nil {
 		log.Errorf("%v", err)

config.go

@@ -61,6 +61,7 @@ type config struct {
 	DisableCheckpoints bool          `long:"nocheckpoints" description:"Disable built-in checkpoints.  Don't do this unless you know what you're doing."`
 	DbType             string        `long:"dbtype" description:"Database backend to use for the Block Chain"`
 	Profile            string        `long:"profile" description:"Enable HTTP profiling on given port -- NOTE port must be between 1024 and 65536"`
+	CpuProfile         string        `long:"cpuprofile" description:"Write CPU profile to the specified file"`
 	DebugLevel         string        `short:"d" long:"debuglevel" description:"Logging level {trace, debug, info, warn, error, critical}"`
 }