cmd/getgo: prompt warning if an earlier installation exists

There is nothing else we can do other than just showing a
message that user need to remove it from their PATH not
to conflict with the current installation.

Fixes #21217.

Change-Id: Ie65385f4d536d5bb789387ba0229f54f2ee793f0
Reviewed-on: https://go-review.googlesource.com/51930
Run-TryBot: Jaana Burcu Dogan <jbd@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Kevin Burke <kev@inburke.com>

cmd/getgo/main.go

@@ -50,6 +50,7 @@ func main() {
 
 	if !*setupOnly {
 		runStep(welcome)
+		runStep(checkOthers)
 		runStep(chooseVersion)
 		runStep(downloadGo)
 	}

cmd/getgo/steps.go

@@ -29,10 +29,22 @@ func welcome(ctx context.Context) error {
 	return nil
 }
 
-func chooseVersion(ctx context.Context) error {
+func checkOthers(ctx context.Context) error {
-	// TODO: check if go is currently installed
 	// TODO: if go is currently installed install new version over that
+	path, err := whichGo(ctx)
+	if err != nil {
+		fmt.Printf("Cannot check if Go is already installed:\n%v\n", err)
+	}
+	if path == "" {
+		return nil
+	}
+	if path != installPath {
+		fmt.Printf("Go is already installed at %v; remove it from your PATH.\n", path)
+	}
+	return nil
+}
 
+func chooseVersion(ctx context.Context) error {
 	if *goVersion != "" {
 		return nil
 	}

cmd/getgo/system.go

@@ -6,8 +6,10 @@ package main
 
 import (
 	"bytes"
+	"context"
 	"os/exec"
 	"runtime"
+	"strings"
 )
 
 // arch contains either amd64 or 386.
@@ -27,3 +29,8 @@ var arch = func() string {
 	}
 	return "386"
 }()
+
+func findGo(ctx context.Context, cmd string) (string, error) {
+	out, err := exec.CommandContext(ctx, cmd, "go").CombinedOutput()
+	return strings.TrimSpace(string(out)), err
+}

cmd/getgo/system_unix.go

@@ -7,6 +7,7 @@
 package main
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -28,6 +29,10 @@ var installPath = func() string {
 	return filepath.Join(home, ".go")
 }()
 
+func whichGo(ctx context.Context) (string, error) {
+	return findGo(ctx, "which")
+}
+
 func isWindowsXP() bool {
 	return false
 }

cmd/getgo/system_windows.go

@@ -7,6 +7,7 @@
 package main
 
 import (
+	"context"
 	"log"
 	"os"
 	"syscall"
@@ -31,6 +32,10 @@ func isWindowsXP() bool {
 	return major < 6
 }
 
+func whichGo(ctx context.Context) (string, error) {
+	return findGo(ctx, "where")
+}
+
 // currentShell reports the current shell.
 // It might be "powershell.exe", "cmd.exe" or any of the *nix shells.
 //