cli-plugins: terminate plugin when CLI exits

Previously, long lived CLI plugin processes weren't
properly handled
(see: https://github.com/docker/cli/issues/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>

cli-plugins/plugin/plugin.go

@@ -1,8 +1,12 @@
 package plugin
 
 import (
+	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"io"
+	"net"
 	"os"
 	"sync"
 
@@ -14,6 +18,11 @@ import (
 	"github.com/spf13/cobra"
 )
 
+// CLIPluginSocketEnvKey is used to pass the plugin being
+// executed the abstract socket name it should listen on to know
+// when the CLI has exited.
+const CLIPluginSocketEnvKey = "DOCKER_CLI_PLUGIN_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
@@ -24,14 +33,56 @@ import (
 // called.
 var PersistentPreRunE func(*cobra.Command, []string) error
 
+// closeOnCLISocketClose connects to the socket specified
+// by the DOCKER_CLI_PLUGIN_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 closeOnCLISocketClose(cancel func()) {
+	socketAddr, ok := os.LookupEnv(CLIPluginSocketEnvKey)
+	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() {
+		b := make([]byte, 1)
+		for {
+			_, err := cliCloseConn.Read(b)
+			if errors.Is(err, io.EOF) {
+				cancel()
+			}
+		}
+	}()
+}
+
 // 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 {
+	PersistentPreRunE = func(cmd *cobra.Command, _ []string) error {
 		var err error
 		persistentPreRunOnce.Do(func() {
+			cmdContext := cmd.Context()
+			// TODO: revisit and make sure this check makes sense
+			// see: https://github.com/docker/cli/pull/4599#discussion_r1422487271
+			if cmdContext == nil {
+				cmdContext = context.TODO()
+			}
+			ctx, cancel := context.WithCancel(cmdContext)
+			cmd.SetContext(ctx)
+			closeOnCLISocketClose(cancel)
+
 			var opts []command.InitializeOpt
 			if os.Getenv("DOCKER_CLI_PLUGIN_USE_DIAL_STDIO") != "" {
 				opts = append(opts, withPluginClientConn(plugin.Name()))