cmd/pprof/internal/commands: add command to open browser on windows

While we're at there, also add a message to prompt the user to install
Graphviz if "dot" command is not found.

Fixes #9178.

LGTM=adg, alex.brainman, cookieo9, rsc
R=rsc, adg, bradfitz, alex.brainman, cookieo9, smyrman
CC=golang-codereviews
https://golang.org/cl/180380043

src/cmd/pprof/internal/commands/commands.go

@@ -11,6 +11,7 @@ import (
 	"io"
 	"os"
 	"os/exec"
+	"runtime"
 	"strings"
 
 	"cmd/pprof/internal/plugin"
@@ -71,15 +72,27 @@ func PProf(c Completer, interactive **bool, svgpan **string) Commands {
 		"eog":    {c, report.Dot, invokeVisualizer(interactive, invokeDot("svg"), "svg", []string{"eog"}), false, "Visualize graph through eog"},
 		"evince": {c, report.Dot, invokeVisualizer(interactive, invokeDot("pdf"), "pdf", []string{"evince"}), false, "Visualize graph through evince"},
 		"gv":     {c, report.Dot, invokeVisualizer(interactive, invokeDot("ps"), "ps", []string{"gv --noantialias"}), false, "Visualize graph through gv"},
-		"web":    {c, report.Dot, invokeVisualizer(interactive, saveSVGToFile(svgpan), "svg", browsers), false, "Visualize graph through web browser"},
+		"web":    {c, report.Dot, invokeVisualizer(interactive, saveSVGToFile(svgpan), "svg", browsers()), false, "Visualize graph through web browser"},
 
 		// Visualize HTML directly generated by report.
-		"weblist": {c, report.WebList, invokeVisualizer(interactive, awayFromTTY("html"), "html", browsers), true, "Output annotated source in HTML for functions matching regexp or address"},
+		"weblist": {c, report.WebList, invokeVisualizer(interactive, awayFromTTY("html"), "html", browsers()), true, "Output annotated source in HTML for functions matching regexp or address"},
 	}
 }
 
-// List of web browsers to attempt for web visualization
-var browsers = []string{"chrome", "google-chrome", "firefox", "/usr/bin/open"}
+// browsers returns a list of commands to attempt for web visualization
+// on the current platform
+func browsers() []string {
+	cmds := []string{"chrome", "google-chrome", "firefox"}
+	switch runtime.GOOS {
+	case "darwin":
+		cmds = append(cmds, "/usr/bin/open")
+	case "windows":
+		cmds = append(cmds, "cmd /c start")
+	default:
+		cmds = append(cmds, "xdg-open")
+	}
+	return cmds
+}
 
 // NewCompleter creates an autocompletion function for a set of commands.
 func NewCompleter(cs Commands) Completer {
@@ -142,6 +155,10 @@ func awayFromTTY(format string) PostProcessor {
 func invokeDot(format string) PostProcessor {
 	divert := awayFromTTY(format)
 	return func(input *bytes.Buffer, output io.Writer, ui plugin.UI) error {
+		if _, err := exec.LookPath("dot"); err != nil {
+			ui.PrintErr("Cannot find dot, have you installed Graphviz?")
+			return err
+		}
 		cmd := exec.Command("dot", "-T"+format)
 		var buf bytes.Buffer
 		cmd.Stdin, cmd.Stdout, cmd.Stderr = input, &buf, os.Stderr
@@ -174,6 +191,7 @@ func invokeVisualizer(interactive **bool, format PostProcessor, suffix string, v
 		if err = format(input, tempFile, ui); err != nil {
 			return err
 		}
+		tempFile.Close() // on windows, if the file is Open, start cannot access it.
 		// Try visualizers until one is successful
 		for _, v := range visualizers {
 			// Separate command and arguments for exec.Command.