cli-plugins: terminate plugin when CLI exits

Previously, long lived CLI plugin processes weren't
properly handled
(see: #4402)
resulting in plugin processes being left behind
running, after the CLI process exits.

This commit changes the plugin handling code to open
an abstract unix socket before running the plugin and
passing it to the plugin process, and changes the
signal handling on the CLI side to close this socket
which tells the plugin that it should exit.

This implementation makes use of sockets instead of
simply setting PDEATHSIG on the plugin process
so that it will work on both BSDs, assorted UNIXes
and Windows.

Signed-off-by: Laura Brehm <laurabrehm@hey.com>

Author: laurazard

cli-plugins/plugin/plugin.go

@@ -2,7 +2,10 @@ package plugin
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
+	"io"
+	"net"
 	"os"
 	"sync"
 
@@ -14,6 +17,11 @@ import (
 	"github.com/spf13/cobra"
 )
 
+// CliPluginCloseSocketEnvKey is used to pass the plugin being
+// executed the abstract socket name it should listen on to know
+// when the CLI has exited.
+const CliPluginCloseSocketEnvKey = "CLI_PLUGIN_EXIT_SOCKET"
+
 // PersistentPreRunE must be called by any plugin command (or
 // subcommand) which uses the cobra `PersistentPreRun*` hook. Plugins
 // which do not make use of `PersistentPreRun*` do not need to call
@@ -24,12 +32,45 @@ import (
 // called.
 var PersistentPreRunE func(*cobra.Command, []string) error
 
+// closeOnParentSocketClose connects to the socket specified
+// by the CLI_PLUGIN_EXIT_SOCKET env var, if present, and attempts
+// to read from it until it receives an EOF, which signals that
+// the CLI is going to exit and the plugin should also exit.
+func closeOnParentSocketClose() {
+	socketAddr, ok := os.LookupEnv(CliPluginCloseSocketEnvKey)
+	if !ok {
+		// if a plugin compiled against a more recent version of docker/cli
+		// is executed by an older CLI binary, ignore missing environment
+		// variable and behave as usual
+		return
+	}
+	addr, err := net.ResolveUnixAddr("unix", socketAddr)
+	if err != nil {
+		return
+	}
+	cliCloseConn, err := net.DialUnix("unix", nil, addr)
+	if err != nil {
+		return
+	}
+
+	go func() {
+		for {
+			b := make([]byte, 1)
+			_, err := cliCloseConn.Read(b)
+			if errors.Is(err, io.EOF) {
+				os.Exit(0)
+			}
+		}
+	}()
+}
+
 // RunPlugin executes the specified plugin command
 func RunPlugin(dockerCli *command.DockerCli, plugin *cobra.Command, meta manager.Metadata) error {
 	tcmd := newPluginCommand(dockerCli, plugin, meta)
 
 	var persistentPreRunOnce sync.Once
 	PersistentPreRunE = func(_ *cobra.Command, _ []string) error {
+		closeOnParentSocketClose()
 		var err error
 		persistentPreRunOnce.Do(func() {
 			var opts []command.InitializeOpt

cmd/docker/docker.go

@@ -2,18 +2,22 @@ package main
 
 import (
 	"fmt"
+	"net"
 	"os"
 	"os/exec"
+	"os/signal"
 	"strings"
 	"syscall"
 
 	"github.com/docker/cli/cli"
 	pluginmanager "github.com/docker/cli/cli-plugins/manager"
+	"github.com/docker/cli/cli-plugins/plugin"
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/cli/cli/command/commands"
 	cliflags "github.com/docker/cli/cli/flags"
 	"github.com/docker/cli/cli/version"
-	"github.com/docker/cli/cmd/docker/internal/appcontext"
+	platformsignals "github.com/docker/cli/cmd/docker/internal/signals"
+	"github.com/docker/distribution/uuid"
 	"github.com/docker/docker/api/types/versions"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -187,17 +191,58 @@ func setValidateArgs(dockerCli command.Cli, cmd *cobra.Command) {
 	})
 }
 
+func setupPluginSocket() (*net.UnixListener, error) {
+	uuid := uuid.Generate().String()
+	pluginCloseSocketAddr := net.UnixAddr{Name: "@docker_cli_" + uuid, Net: "unix"}
+	return net.ListenUnix("unix", &pluginCloseSocketAddr)
+}
+
 func tryPluginRun(dockerCli command.Cli, cmd *cobra.Command, subcommand string, envs []string) error {
 	plugincmd, err := pluginmanager.PluginRunCommand(dockerCli, subcommand, cmd)
 	if err != nil {
 		return err
 	}
 	plugincmd.Env = append(envs, plugincmd.Env...)
 
-	go func() {
-		// override SIGTERM handler so we let the plugin shut down first
-		<-appcontext.Context().Done()
-	}()
+	listener, err := setupPluginSocket()
+	// if we somehow failed to set up the socket to alert the plugin
+	// when the CLI process wants to exit, fallback to old behaviour
+	if err == nil {
+		defer listener.Close()
+		plugincmd.Env = append(plugincmd.Env, plugin.CliPluginCloseSocketEnvKey+"="+listener.Addr().String())
+
+		go func() {
+			signals := make(chan os.Signal, 2048)
+			signal.Notify(signals, platformsignals.TerminationSignals...)
+
+			const exitLimit = 3
+			retries := 0
+
+			var conn *net.UnixConn
+			go func() {
+				conn, err = listener.AcceptUnix()
+				if err != nil {
+					// TODO(laurazard): change me, halt execution or fallback to
+					// previous behaviour if we couldn't set up socket connection
+					println("got a weird error!", err.Error())
+				}
+			}()
+
+			go func() {
+				for {
+					<-signals
+					if err := conn.Close(); err != nil {
+						logrus.Errorf("failed to signal plugin to close: %v", err)
+					}
+					retries++
+					if retries >= exitLimit {
+						logrus.Errorf("got %d SIGTERM/SIGINTs, exiting without cleanup", retries)
+						os.Exit(1)
+					}
+				}
+			}()
+		}()
+	}
 
 	if err := plugincmd.Run(); err != nil {
 		statusCode := 1

cmd/docker/internal/appcontext/appcontext.go

@@ -0,0 +1,14 @@
+//go:build !windows
+// +build !windows
+
+package signals
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// TerminationSignals represents the list of signals we
+// want to special-case handle, on this platform.
+var TerminationSignals = []os.Signal{unix.SIGTERM, unix.SIGINT}

cmd/docker/internal/appcontext/appcontext_unix.go

@@ -0,0 +1,7 @@
+package signals
+
+import "os"
+
+// TerminationSignals represents the list of signals we
+// want to special-case handle, on this platform.
+var TerminationSignals = []os.Signal{os.Interrupt}