ipc: read out executor output

If executor constantly prints something, it will eventually hang
since the pipe will overflow. Read out executor output to prevent the hang.

ipc/ipc.go

@@ -296,13 +296,13 @@ func closeMapping(f *os.File, mem []byte) error {
 }
 
 type command struct {
-	timeout time.Duration
-	cmd     *exec.Cmd
-	flags   uint64
-	dir     string
-	rp      *os.File
-	inrp    *os.File
-	outwp   *os.File
+	timeout  time.Duration
+	cmd      *exec.Cmd
+	flags    uint64
+	dir      string
+	readDone chan []byte
+	inrp     *os.File
+	outwp    *os.File
 }
 
 func makeCommand(bin []string, timeout time.Duration, flags uint64, inFile *os.File, outFile *os.File) (*command, error) {
@@ -330,7 +330,6 @@ func makeCommand(bin []string, timeout time.Duration, flags uint64, inFile *os.F
 		return nil, fmt.Errorf("failed to create pipe: %v", err)
 	}
 	defer wp.Close()
-	c.rp = rp
 
 	// Input command pipe.
 	inrp, inwp, err := os.Pipe()
@@ -348,6 +347,8 @@ func makeCommand(bin []string, timeout time.Duration, flags uint64, inFile *os.F
 	defer outrp.Close()
 	c.outwp = outwp
 
+	c.readDone = make(chan []byte, 1)
+
 	cmd := exec.Command(bin[0], bin[1:]...)
 	cmd.ExtraFiles = []*os.File{inFile, outFile, outrp, inwp}
 	cmd.Env = []string{}
@@ -355,6 +356,28 @@ func makeCommand(bin []string, timeout time.Duration, flags uint64, inFile *os.F
 	if flags&FlagDebug == 0 {
 		cmd.Stdout = wp
 		cmd.Stderr = wp
+		go func(c *command) {
+			// Read out output in case executor constantly prints something.
+			const BufSize = 128 << 10
+			output := make([]byte, BufSize)
+			size := 0
+			for {
+				n, err := rp.Read(output[size:])
+				if n > 0 {
+					size += n
+					if size >= BufSize*3/4 {
+						copy(output, output[size-BufSize/2:size])
+						size = BufSize / 2
+					}
+				}
+				if err != nil {
+					rp.Close()
+					c.readDone <- output[:size]
+					close(c.readDone)
+					return
+				}
+			}
+		}(c)
 	} else {
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stdout
@@ -379,9 +402,6 @@ func (c *command) close() {
 	}
 	fileutil.UmountAll(c.dir)
 	os.RemoveAll(c.dir)
-	if c.rp != nil {
-		c.rp.Close()
-	}
 	if c.inrp != nil {
 		c.inrp.Close()
 	}
@@ -415,7 +435,7 @@ func (c *command) kill() {
 func (c *command) exec() (output []byte, failed, hanged, restart bool, err0 error) {
 	var tmp [1]byte
 	if _, err := c.outwp.Write(tmp[:]); err != nil {
-		output, _ = ioutil.ReadAll(c.rp)
+		output = <-c.readDone
 		err0 = fmt.Errorf("failed to write control pipe: %v", err)
 		return
 	}
@@ -432,7 +452,6 @@ func (c *command) exec() (output []byte, failed, hanged, restart bool, err0 erro
 			hang <- false
 		}
 	}()
-	//!!! handle c.rp overflow
 	_, readErr := c.inrp.Read(tmp[:])
 	close(done)
 	if readErr == nil {
@@ -441,12 +460,9 @@ func (c *command) exec() (output []byte, failed, hanged, restart bool, err0 erro
 	}
 	err0 = fmt.Errorf("executor did not answer")
 	c.kill()
-	var err error
-	output, err = ioutil.ReadAll(c.rp)
-	if err = c.cmd.Wait(); <-hang && err != nil {
+	output = <-c.readDone
+	if err := c.cmd.Wait(); <-hang && err != nil {
 		hanged = true
-	}
-	if err != nil {
 		output = append(output, []byte(err.Error())...)
 		output = append(output, '\n')
 	}