Add backend code for detached exec

As part of the massive exec rework, I stubbed out a function for
non-detached exec, which is implemented here. It's largely
similar to the existing exec functions, but missing a few pieces.

This also involves implemented a new OCI runtime call for
detached exec. Again, very similar to the other functions, but
with a few missing pieces.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

4 files changed, 101 insertions, 8 deletions

diff --git a/libpod/container_exec.go b/libpod/container_exec.go
index 088bd0fd5..a38f22488 100644
--- a/libpod/container_exec.go
+++ b/libpod/container_exec.go
@@ -214,11 +214,52 @@ func (c *Container) ExecCreate(config *ExecConfig) (string, error) {
 }
 
 // ExecStart starts an exec session in the container, but does not attach to it.
-// Returns immediately upon starting the exec session.
+// Returns immediately upon starting the exec session, unlike other ExecStart
+// functions, which will only return when the exec session exits.
 func (c *Container) ExecStart(sessionID string) error {
-	// Will be implemented in part 2, migrating Start and implementing
-	// detached Start.
-	return define.ErrNotImplemented
+	if !c.batched {
+		c.lock.Lock()
+		defer c.lock.Unlock()
+
+		if err := c.syncContainer(); err != nil {
+			return err
+		}
+	}
+
+	// Verify that we are in a good state to continue
+	if !c.ensureState(define.ContainerStateRunning) {
+		return errors.Wrapf(define.ErrCtrStateInvalid, "can only start exec sessions when their container is running")
+	}
+
+	session, ok := c.state.ExecSessions[sessionID]
+	if !ok {
+		return errors.Wrapf(define.ErrNoSuchExecSession, "container %s has no exec session with ID %s", c.ID(), sessionID)
+	}
+
+	if session.State != define.ExecStateCreated {
+		return errors.Wrapf(define.ErrExecSessionStateInvalid, "can only start created exec sessions, while container %s session %s state is %q", c.ID(), session.ID(), session.State.String())
+	}
+
+	logrus.Infof("Going to start container %s exec session %s and attach to it", c.ID(), session.ID())
+
+	opts, err := prepareForExec(c, session)
+	if err != nil {
+		return err
+	}
+
+	pid, err := c.ociRuntime.ExecContainerDetached(c, session.ID(), opts, session.Config.AttachStdin)
+	if err != nil {
+		return err
+	}
+
+	c.newContainerEvent(events.Exec)
+	logrus.Debugf("Successfully started exec session %s in container %s", session.ID(), c.ID())
+
+	// Update and save session to reflect PID/running
+	session.PID = pid
+	session.State = define.ExecStateRunning
+
+	return c.save()
 }
 
 // ExecStartAndAttach starts and attaches to an exec session in a container.
diff --git a/libpod/oci.go b/libpod/oci.go
index da4030c14..7c5218319 100644
--- a/libpod/oci.go
+++ b/libpod/oci.go
@@ -68,10 +68,10 @@ type OCIRuntime interface {
 	AttachResize(ctr *Container, newSize remotecommand.TerminalSize) error
 
 	// ExecContainer executes a command in a running container.
-	// Returns an int (exit code), error channel (errors from attach), and
-	// error (errors that occurred attempting to start the exec session).
-	// This returns once the exec session is running - not once it has
-	// completed, as one might expect. The attach session will remain
+	// Returns an int (PID of exec session), error channel (errors from
+	// attach), and error (errors that occurred attempting to start the exec
+	// session). This returns once the exec session is running - not once it
+	// has completed, as one might expect. The attach session will remain
 	// running, in a goroutine that will return via the chan error in the
 	// return signature.
 	ExecContainer(ctr *Container, sessionID string, options *ExecOptions, streams *define.AttachStreams) (int, chan error, error)
@@ -81,6 +81,10 @@ type OCIRuntime interface {
 	// start, with a goroutine running in the background to handle attach).
 	// The HTTP attach itself maintains the same invariants as HTTPAttach.
 	ExecContainerHTTP(ctr *Container, sessionID string, options *ExecOptions, httpConn net.Conn, httpBuf *bufio.ReadWriter, streams *HTTPAttachStreams, cancel <-chan bool) (int, chan error, error)
+	// ExecContainerDetached executes a command in a running container, but
+	// does not attach to it. Returns the PID of the exec session and an
+	// error (if starting the exec session failed)
+	ExecContainerDetached(ctr *Container, sessionID string, options *ExecOptions, stdin bool) (int, error)
 	// ExecAttachResize resizes the terminal of a running exec session. Only
 	// allowed with sessions that were created with a TTY.
 	ExecAttachResize(ctr *Container, sessionID string, newSize remotecommand.TerminalSize) error
diff --git a/libpod/oci_conmon_exec_linux.go b/libpod/oci_conmon_exec_linux.go
index 3fc866796..51819f90a 100644
--- a/libpod/oci_conmon_exec_linux.go
+++ b/libpod/oci_conmon_exec_linux.go
@@ -143,6 +143,49 @@ func (r *ConmonOCIRuntime) ExecContainerHTTP(ctr *Container, sessionID string, o
 	return pid, attachChan, err
 }
 
+// ExecContainerDetached executes a command in a running container, but does
+// not attach to it.
+func (r *ConmonOCIRuntime) ExecContainerDetached(ctr *Container, sessionID string, options *ExecOptions, stdin bool) (int, error) {
+	if options == nil {
+		return -1, errors.Wrapf(define.ErrInvalidArg, "must provide exec options to ExecContainerHTTP")
+	}
+
+	var ociLog string
+	if logrus.GetLevel() != logrus.DebugLevel && r.supportsJSON {
+		ociLog = ctr.execOCILog(sessionID)
+	}
+
+	execCmd, pipes, err := r.startExec(ctr, sessionID, options, stdin, ociLog)
+	if err != nil {
+		return -1, err
+	}
+
+	defer func() {
+		pipes.cleanup()
+	}()
+
+	// Wait for Conmon to tell us we're ready to attach.
+	// We aren't actually *going* to attach, but this means that we're good
+	// to proceed.
+	if _, err := readConmonPipeData(pipes.attachPipe, ""); err != nil {
+		return -1, err
+	}
+
+	// Start the exec session
+	if err := writeConmonPipeData(pipes.startPipe); err != nil {
+		return -1, err
+	}
+
+	// Wait for conmon to succeed, when return.
+	if err := execCmd.Wait(); err != nil {
+		return -1, errors.Wrapf(err, "cannot run conmon")
+	}
+
+	pid, err := readConmonPipeData(pipes.syncPipe, ociLog)
+
+	return pid, err
+}
+
 // ExecAttachResize resizes the TTY of the given exec session.
 func (r *ConmonOCIRuntime) ExecAttachResize(ctr *Container, sessionID string, newSize remotecommand.TerminalSize) error {
 	controlFile, err := openControlFile(ctr, ctr.execBundlePath(sessionID))
diff --git a/libpod/oci_missing.go b/libpod/oci_missing.go
index 626740f72..4da16876c 100644
--- a/libpod/oci_missing.go
+++ b/libpod/oci_missing.go
@@ -130,6 +130,11 @@ func (r *MissingRuntime) ExecContainerHTTP(ctr *Container, sessionID string, opt
 	return -1, nil, r.printError()
 }
 
+// ExecContainerDetached is not available as the runtime is missing
+func (r *MissingRuntime) ExecContainerDetached(ctr *Container, sessionID string, options *ExecOptions, stdin bool) (int, error) {
+	return -1, r.printError()
+}
+
 // ExecAttachResize is not available as the runtime is missing.
 func (r *MissingRuntime) ExecAttachResize(ctr *Container, sessionID string, newSize remotecommand.TerminalSize) error {
 	return r.printError()