conmon: implement logging to logPath

This adds a very simple implementation of logging within conmon, where
every buffer read from the masterfd of the container is also written to
the log file (with errors during writing to the log file ignored).

Signed-off-by: Aleksa Sarai <asarai@suse.de>

oci/oci.go

@@ -120,9 +120,13 @@ func (r *Runtime) CreateContainer(c *Container, cgroupParent string) error {
 	args = append(args, "-r", r.Path(c))
 	args = append(args, "-b", c.bundlePath)
 	args = append(args, "-p", filepath.Join(c.bundlePath, "pidfile"))
+	args = append(args, "-l", c.logPath)
 	if c.terminal {
 		args = append(args, "-t")
 	}
+	logrus.WithFields(logrus.Fields{
+		"args": args,
+	}).Debugf("running conmon: %s", r.conmonPath)
 
 	cmd := exec.Command(r.conmonPath, args...)
 	cmd.Dir = c.bundlePath
@@ -248,6 +252,19 @@ func (r *Runtime) ExecSync(c *Container, command []string, timeout int64) (resp
 		}
 	}()
 
+	logFile, err := ioutil.TempFile("", "ocid-log-"+c.name)
+	if err != nil {
+		return nil, ExecSyncError{
+			ExitCode: -1,
+			Err:      err,
+		}
+	}
+	logPath := logFile.Name()
+	defer func() {
+		logFile.Close()
+		os.RemoveAll(logPath)
+	}()
+
 	var args []string
 	args = append(args, "-c", c.name)
 	args = append(args, "-r", r.Path(c))
@@ -256,6 +273,7 @@ func (r *Runtime) ExecSync(c *Container, command []string, timeout int64) (resp
 	if c.terminal {
 		args = append(args, "-t")
 	}
+	args = append(args, "-l", logPath)
 
 	args = append(args, command...)
 
@@ -371,9 +389,25 @@ func (r *Runtime) ExecSync(c *Container, command []string, timeout int64) (resp
 		}
 	}
 
+	// The actual logged output is not the same as stdoutBuf and stderrBuf,
+	// which are used for getting error information. For the actual
+	// ExecSyncResponse we have to read the logfile.
+	// XXX: Currently runC dups the same console over both stdout and stderr,
+	//      so we can't differentiate between the two.
+
+	outputBytes, err := ioutil.ReadFile(logPath)
+	if err != nil {
+		return nil, ExecSyncError{
+			Stdout:   stdoutBuf,
+			Stderr:   stderrBuf,
+			ExitCode: -1,
+			Err:      err,
+		}
+	}
+
 	return &ExecSyncResponse{
-		Stdout:   stdoutBuf.Bytes(),
-		Stderr:   stderrBuf.Bytes(),
+		Stdout:   outputBytes,
+		Stderr:   nil,
 		ExitCode: 0,
 	}, nil
 }