Simplify shimProcess accept and wait

That we pull a value out of a channel and put it back is pretty weird.
Also we don't need a select in accept(). It's enough to just close the
listener when we learn the child is gone. (That will cancel the
Accept call.)

Change-Id: If520d9f405fa0b1ad6e3cd23e9ba8a35ff39ba75
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/60887
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

ssl/test/runner/runner.go

diff --git a/ssl/test/runner/runner.go b/ssl/test/runner/runner.go
index e7ee194..954f2a2 100644
--- a/ssl/test/runner/runner.go
+++ b/ssl/test/runner/runner.go
@@ -1248,8 +1248,11 @@
 )
 
 type shimProcess struct {
-	cmd            *exec.Cmd
-	waitChan       chan error
+	cmd *exec.Cmd
+	// done is closed when the process has exited. At that point, childErr may be
+	// read for the result.
+	done           chan struct{}
+	childErr       error
 	listener       *net.TCPListener
 	stdout, stderr bytes.Buffer
 }
@@ -1297,37 +1300,22 @@
 		return nil, err
 	}
 
-	shim.waitChan = make(chan error, 1)
-	go func() { shim.waitChan <- shim.cmd.Wait() }()
+	shim.done = make(chan struct{})
+	go func() {
+		shim.childErr = shim.cmd.Wait()
+		shim.listener.Close()
+		close(shim.done)
+	}()
 	return shim, nil
 }
 
 // accept returns a new TCP connection with the shim process, or returns an
 // error on timeout or shim exit.
 func (s *shimProcess) accept() (net.Conn, error) {
-	type connOrError struct {
-		conn net.Conn
-		err  error
+	if !useDebugger() {
+		s.listener.SetDeadline(time.Now().Add(*idleTimeout))
 	}
-	connChan := make(chan connOrError, 1)
-	go func() {
-		if !useDebugger() {
-			s.listener.SetDeadline(time.Now().Add(*idleTimeout))
-		}
-		conn, err := s.listener.Accept()
-		connChan <- connOrError{conn, err}
-		close(connChan)
-	}()
-	select {
-	case result := <-connChan:
-		return result.conn, result.err
-	case childErr := <-s.waitChan:
-		s.waitChan <- childErr
-		if childErr == nil {
-			return nil, fmt.Errorf("child exited early with no error")
-		}
-		return nil, fmt.Errorf("child exited early: %s", childErr)
-	}
+	return s.listener.Accept()
 }
 
 // wait finishes the test and waits for the shim process to exit.
@@ -1343,9 +1331,8 @@
 		defer waitTimeout.Stop()
 	}
 
-	err := <-s.waitChan
-	s.waitChan <- err
-	return err
+	<-s.done
+	return s.childErr
 }
 
 // close releases resources associated with the shimProcess. This is safe to