cmd/go/internal/vcweb: simplify hgHandler cancellation

This uses the new Cancel and WaitDelay fields of os/exec.Cmd
(added in #50436) to interrupt or kill the 'hg serve' command
when its incoming http.Request is canceled.

This should keep the vcweb hg handler from getting stuck if 'hg serve'
hangs after the request either completes or is canceled.

Fixes #57597 (maybe).

Change-Id: I53cf58e8ab953fd48c0c37f596f99e885a036d9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/460997
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/vcweb/hg.go

@@ -6,6 +6,7 @@ package vcweb
 
 import (
 	"bufio"
+	"context"
 	"errors"
 	"io"
 	"log"
@@ -16,6 +17,7 @@ import (
 	"os/exec"
 	"strings"
 	"sync"
+	"time"
 )
 
 type hgHandler struct {
@@ -47,10 +49,25 @@ func (h *hgHandler) Handler(dir string, env []string, logger *log.Logger) (http.
 		// if "hg" works at all then "hg serve" works too, and we'll execute that as
 		// a subprocess, using a reverse proxy to forward the request and response.
 
-		cmd := exec.Command(h.hgPath, "serve", "--port", "0", "--address", "localhost", "--accesslog", os.DevNull, "--name", "vcweb", "--print-url")
+		ctx, cancel := context.WithCancel(req.Context())
+		defer cancel()
+
+		cmd := exec.CommandContext(ctx, h.hgPath, "serve", "--port", "0", "--address", "localhost", "--accesslog", os.DevNull, "--name", "vcweb", "--print-url")
 		cmd.Dir = dir
 		cmd.Env = append(env[:len(env):len(env)], "PWD="+dir)
 
+		cmd.Cancel = func() error {
+			err := cmd.Process.Signal(os.Interrupt)
+			if err != nil && !errors.Is(err, os.ErrProcessDone) {
+				err = cmd.Process.Kill()
+			}
+			return err
+		}
+		// This WaitDelay is arbitrary. After 'hg serve' prints its URL, any further
+		// I/O is only for debugging. (The actual output goes through the HTTP URL,
+		// not the standard I/O streams.)
+		cmd.WaitDelay = 10 * time.Second
+
 		stderr := new(strings.Builder)
 		cmd.Stderr = stderr
 
@@ -59,62 +76,46 @@ func (h *hgHandler) Handler(dir string, env []string, logger *log.Logger) (http.
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
-		readDone := make(chan struct{})
-		defer func() {
-			stdout.Close()
-			<-readDone
-		}()
-
-		hgURL := make(chan *url.URL, 1)
-		hgURLError := make(chan error, 1)
-		go func() {
-			defer close(readDone)
-			r := bufio.NewReader(stdout)
-			for {
-				line, err := r.ReadString('\n')
-				if err != nil {
-					return
-				}
-				u, err := url.Parse(strings.TrimSpace(line))
-				if err == nil {
-					hgURL <- u
-				} else {
-					hgURLError <- err
-				}
-				break
-			}
-			io.Copy(io.Discard, r)
-		}()
 
 		if err := cmd.Start(); err != nil {
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
+		var wg sync.WaitGroup
 		defer func() {
-			if err := cmd.Process.Signal(os.Interrupt); err != nil && !errors.Is(err, os.ErrProcessDone) {
-				cmd.Process.Kill()
-			}
+			cancel()
 			err := cmd.Wait()
 			if out := strings.TrimSuffix(stderr.String(), "interrupted!\n"); out != "" {
 				logger.Printf("%v: %v\n%s", cmd, err, out)
 			} else {
 				logger.Printf("%v", cmd)
 			}
+			wg.Wait()
 		}()
 
-		select {
-		case <-req.Context().Done():
-			logger.Printf("%v: %v", req.Context().Err(), cmd)
-			http.Error(w, req.Context().Err().Error(), http.StatusBadGateway)
+		r := bufio.NewReader(stdout)
+		line, err := r.ReadString('\n')
+		if err != nil {
 			return
-		case err := <-hgURLError:
+		}
+		// We have read what should be the server URL. 'hg serve' shouldn't need to
+		// write anything else to stdout, but it's not a big deal if it does anyway.
+		// Keep the stdout pipe open so that 'hg serve' won't get a SIGPIPE, but
+		// actively discard its output so that it won't hang on a blocking write.
+		wg.Add(1)
+		go func() {
+			io.Copy(io.Discard, r)
+			wg.Done()
+		}()
+
+		u, err := url.Parse(strings.TrimSpace(line))
+		if err != nil {
 			logger.Printf("%v: %v", cmd, err)
 			http.Error(w, err.Error(), http.StatusBadGateway)
 			return
-		case url := <-hgURL:
-			logger.Printf("proxying hg request to %s", url)
-			httputil.NewSingleHostReverseProxy(url).ServeHTTP(w, req)
 		}
+		logger.Printf("proxying hg request to %s", u)
+		httputil.NewSingleHostReverseProxy(u).ServeHTTP(w, req)
 	})
 
 	return handler, nil