os/exec: start checking for context cancelation in Start

Previously we started checking for context cancelation in Wait, but
that meant that when using StdoutPipe context cancelation never took
effect.

Fixes #16222.

Change-Id: I89cd26d3499a6080bf1a07718ce38d825561899e
Reviewed-on: https://go-review.googlesource.com/24650
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 70 insertions, 0 deletions

diff --git a/src/os/exec/exec_test.go b/src/os/exec/exec_test.go
index 41f9dfe1c6..4cc9847721 100644
--- a/src/os/exec/exec_test.go
+++ b/src/os/exec/exec_test.go
@@ -878,3 +878,73 @@ func TestContext(t *testing.T) {
 		t.Fatal("timeout waiting for child process death")
 	}
 }
+
+func TestContextCancel(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	c := helperCommandContext(t, ctx, "cat")
+
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	c.Stdin = r
+
+	stdout, err := c.StdoutPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	readDone := make(chan struct{})
+	go func() {
+		defer close(readDone)
+		var a [1024]byte
+		for {
+			n, err := stdout.Read(a[:])
+			if err != nil {
+				if err != io.EOF {
+					t.Errorf("unexpected read error: %v", err)
+				}
+				return
+			}
+			t.Logf("%s", a[:n])
+		}
+	}()
+
+	if err := c.Start(); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := r.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := io.WriteString(w, "echo"); err != nil {
+		t.Fatal(err)
+	}
+
+	cancel()
+
+	// Calling cancel should have killed the process, so writes
+	// should now fail.  Give the process a little while to die.
+	start := time.Now()
+	for {
+		if _, err := io.WriteString(w, "echo"); err != nil {
+			break
+		}
+		if time.Since(start) > time.Second {
+			t.Fatal("cancelling context did not stop program")
+		}
+		time.Sleep(time.Millisecond)
+	}
+
+	if err := w.Close(); err != nil {
+		t.Error("error closing write end of pipe: %v", err)
+	}
+	<-readDone
+
+	if err := c.Wait(); err == nil {
+		t.Error("program unexpectedly exited successfully")
+	} else {
+		t.Logf("exit status: %v", err)
+	}
+}