Set the maximum number of ssh connection to a ssh remote target to 15

So that for most cases, when a folder has many child got expanded, we
won't get a lot of connection failures

core/os/device/remotessh/commands.go

@@ -32,21 +32,20 @@ import (
 	"github.com/google/gapid/core/os/device"
 	"github.com/google/gapid/core/os/shell"
 	"github.com/google/gapid/core/text"
-	"golang.org/x/crypto/ssh"
 )
 
 // remoteProcess is the interface to a running process, as started by a Target.
 type remoteProcess struct {
-	session *ssh.Session
 	wg      sync.WaitGroup
+	session *pooledSession
 }
 
 func (r *remoteProcess) Kill() error {
-	return r.session.Signal(ssh.SIGSEGV)
+	return r.session.kill()
 }
 
 func (r *remoteProcess) Wait(ctx context.Context) error {
-	ret := r.session.Wait()
+	ret := r.session.wait()
 	r.wg.Wait()
 	return ret
 }
@@ -57,17 +56,17 @@ type sshShellTarget struct{ b *binding }
 
 // Start starts the given command in the remote shell.
 func (t sshShellTarget) Start(cmd shell.Cmd) (shell.Process, error) {
-	session, err := t.b.connection.NewSession()
+	pooled, err := t.b.newPooledSession()
 	if err != nil {
 		return nil, err
 	}
 	p := &remoteProcess{
-		session: session,
+		session: pooled,
 		wg:      sync.WaitGroup{},
 	}
 
 	if cmd.Stdin != nil {
-		stdin, err := session.StdinPipe()
+		stdin, err := pooled.session.StdinPipe()
 		if err != nil {
 			return nil, err
 		}
@@ -78,7 +77,7 @@ func (t sshShellTarget) Start(cmd shell.Cmd) (shell.Process, error) {
 	}
 
 	if cmd.Stdout != nil {
-		stdout, err := session.StdoutPipe()
+		stdout, err := pooled.session.StdoutPipe()
 		if err != nil {
 			return nil, err
 		}
@@ -90,7 +89,7 @@ func (t sshShellTarget) Start(cmd shell.Cmd) (shell.Process, error) {
 	}
 
 	if cmd.Stderr != nil {
-		stderr, err := session.StderrPipe()
+		stderr, err := pooled.session.StderrPipe()
 		if err != nil {
 			return nil, err
 		}
@@ -121,7 +120,7 @@ func (t sshShellTarget) Start(cmd shell.Cmd) (shell.Process, error) {
 	}
 
 	val := prefix + cmd.Name + " " + strings.Join(cmd.Args, " ")
-	if err := session.Start(val); err != nil {
+	if err := pooled.session.Start(val); err != nil {
 		return nil, err
 	}
 

core/os/device/remotessh/device.go

@@ -49,6 +49,10 @@ type Device interface {
 	WriteFile(ctx context.Context, contents io.Reader, mode os.FileMode, destPath string) error
 }
 
+// MaxNumberOfSSHConnections defines the max number of ssh connections to each
+// ssh remote device that can be used to run commands concurrently.
+const MaxNumberOfSSHConnections = 15
+
 // binding represents an attached SSH client.
 type binding struct {
 	bind.Simple
@@ -59,6 +63,63 @@ type binding struct {
 	// We duplicate OS here because we need to use it
 	// before we get the rest of the information
 	os device.OSKind
+
+	// pool to limit the maximum number of connections
+	ch chan int
+}
+
+type pooledSession struct {
+	ch      chan int
+	session *ssh.Session
+}
+
+func (p *pooledSession) kill() error {
+	select {
+	case <-p.ch:
+	default:
+	}
+	<-p.ch
+	return p.session.Signal(ssh.SIGSEGV)
+}
+
+func (p *pooledSession) wait() error {
+	ret := p.session.Wait()
+	select {
+	case <-p.ch:
+	default:
+	}
+	return ret
+}
+
+func newBinding(conn *ssh.Client, conf *Configuration, env *shell.Env) *binding {
+	b := &binding{
+		connection:    conn,
+		configuration: conf,
+		env:           env,
+		ch:            make(chan int, MaxNumberOfSSHConnections),
+		Simple: bind.Simple{
+			To: &device.Instance{
+				Serial:        "",
+				Configuration: &device.Configuration{},
+			},
+			LastStatus: bind.Status_Online,
+		},
+	}
+	return b
+}
+
+func (b *binding) newPooledSession() (*pooledSession, error) {
+	b.ch <- int(0)
+	session, err := b.connection.NewSession()
+	if err != nil {
+		<-b.ch
+		err = fmt.Errorf("New SSH Session Error: %v, Current maximum number of ssh connections GAPID can issue to each remote device is: %v", err, MaxNumberOfSSHConnections)
+		return nil, err
+	}
+	return &pooledSession{
+		ch:      b.ch,
+		session: session,
+	}, nil
 }
 
 var _ Device = &binding{}
@@ -145,18 +206,7 @@ func GetConnectedDevice(ctx context.Context, c Configuration) (Device, error) {
 		env.Add(e)
 	}
 
-	b := &binding{
-		connection:    connection,
-		configuration: &c,
-		env:           env,
-		Simple: bind.Simple{
-			To: &device.Instance{
-				Serial:        "",
-				Configuration: &device.Configuration{},
-			},
-			LastStatus: bind.Status_Online,
-		},
-	}
+	b := newBinding(connection, &c, env)
 
 	kind := device.UnknownOS