Add tracking for exec session IDs

Exec sessions now have an ID generated and assigned to their PID
and stored in the database state. This allows us to track what
exec sessions are currently active.

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>

Closes: #412
Approved by: baude

libpod/container_api.go

@@ -5,8 +5,11 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strconv"
+	"time"
 
 	"github.com/docker/docker/daemon/caps"
+	"github.com/docker/docker/pkg/stringid"
 	"github.com/docker/docker/pkg/term"
 	"github.com/pkg/errors"
 	"github.com/projectatomic/libpod/libpod/driver"
@@ -235,7 +238,86 @@ func (c *Container) Exec(tty, privileged bool, env, cmd []string, user string) e
 		capList = caps.GetAllCapabilities()
 	}
 
-	return c.runtime.ociRuntime.execContainer(c, cmd, tty, user, capList, env)
+	// Generate exec session ID
+	// Ensure we don't conflict with an existing session ID
+	sessionID := stringid.GenerateNonCryptoID()
+	found := true
+	// This really ought to be a do-while, but Go doesn't have those...
+	for found {
+		found = false
+		for id, _ := range c.state.ExecSessions {
+			if id == sessionID {
+				found = true
+				break
+			}
+		}
+		if found == true {
+			sessionID = stringid.GenerateNonCryptoID()
+		}
+	}
+
+	execCmd, err := c.runtime.ociRuntime.execContainer(c, cmd, capList, env, tty, user, sessionID)
+	if err != nil {
+		return errors.Wrapf(err, "error creating exec command for container %s", c.ID())
+	}
+
+	if err := execCmd.Start(); err != nil {
+		return errors.Wrapf(err, "error starting exec command for container %s", c.ID())
+	}
+
+	pidFile := c.execPidPath(sessionID)
+	const pidWaitTimeout = 250
+
+	// Wait until runc makes the pidfile
+	// TODO: If runc errors before the PID file is created, we have to wait for timeout here
+	if err := WaitForFile(pidFile, pidWaitTimeout * time.Millisecond); err != nil {
+		logrus.Debugf("Timed out waiting for pidfile from runc for container %s exec", c.ID())
+
+		// Check if an error occurred in the process before we made a pidfile
+		// TODO: Wait() here is a poor choice - is there a way to see if
+		// a process has finished, instead of waiting for it to finish?
+		if err := execCmd.Wait(); err != nil {
+			return err
+		}
+
+		return errors.Wrapf(err, "timed out waiting for runc to create pidfile for exec session in container %s", c.ID())
+	}
+
+	// Pidfile exists, read it
+	contents, err := ioutil.ReadFile(pidFile)
+	if err != nil {
+		// We don't know the PID of the exec session
+		// However, it may still be alive
+		// TODO handle this better
+		return errors.Wrapf(err, "could not read pidfile for exec session %s in container %s", sessionID, c.ID())
+	}
+	pid, err := strconv.ParseInt(string(contents), 10, 32)
+	if err != nil {
+		// As above, we don't have a valid PID, but the exec session is likely still alive
+		// TODO handle this better
+		return errors.Wrapf(err, "error parsing PID of exec session %s in container %s", sessionID, c.ID())
+	}
+
+	// We have the PID, add it to state
+	if c.state.ExecSessions == nil {
+		c.state.ExecSessions = make(map[string]int)
+	}
+	c.state.ExecSessions[sessionID] = int(pid)
+	if err := c.save(); err != nil {
+		// Now we have a PID but we can't save it in the DB
+		// TODO handle this better
+		return errors.Wrapf(err, "error saving exec sessions %s for container %s", sessionID, c.ID())
+	}
+
+	waitErr := execCmd.Wait()
+
+	// Remove the exec session from state
+	delete(c.state.ExecSessions, sessionID)
+	if err := c.save(); err != nil {
+		logrus.Errorf("Error removing exec session %s from container %s state: %v", sessionID, c.ID(), err)
+	}
+
+	return waitErr
 }
 
 // Attach attaches to a container