Add pane_pipe_pid with pipe fd and call setpgid to make it easier to kill.

cmd-pipe-pane.c

@@ -124,7 +124,7 @@ cmd_pipe_pane_exec(struct cmd *self, struct cmdq_item *item)
 	/* Fork the child. */
 	sigfillset(&set);
 	sigprocmask(SIG_BLOCK, &set, &oldset);
-	switch (fork()) {
+	switch ((wp->pipe_pid = fork())) {
 	case -1:
 		sigprocmask(SIG_SETMASK, &oldset, NULL);
 		cmdq_error(item, "fork error: %s", strerror(errno));
@@ -137,6 +137,9 @@ cmd_pipe_pane_exec(struct cmd *self, struct cmdq_item *item)
 		sigprocmask(SIG_SETMASK, &oldset, NULL);
 		close(pipe_fd[0]);
 
+		if (setpgid(0, 0) == -1)
+			_exit(1);
+
 		null_fd = open(_PATH_DEVNULL, O_WRONLY);
 		if (out) {
 			if (dup2(pipe_fd[1], STDIN_FILENO) == -1)

format.c

@@ -2225,6 +2225,17 @@ format_cb_pane_pipe(struct format_tree *ft)
 	return (NULL);
 }
 
+/* Callback for pane_pipe_pid. */
+static void *
+format_cb_pane_pipe_pid(struct format_tree *ft)
+{
+	char	*value = NULL;
+
+	if (ft->wp != NULL && ft->wp->pipe_fd != -1)
+		xasprintf(&value, "%ld", (long)ft->wp->pipe_pid);
+	return (value);
+}
+
 /* Callback for pane_right. */
 static void *
 format_cb_pane_right(struct format_tree *ft)
@@ -3311,6 +3322,9 @@ static const struct format_table_entry format_table[] = {
 	{ "pane_pipe", FORMAT_TABLE_STRING,
 	  format_cb_pane_pipe
 	},
+	{ "pane_pipe_pid", FORMAT_TABLE_STRING,
+	  format_cb_pane_pipe_pid
+	},
 	{ "pane_right", FORMAT_TABLE_STRING,
 	  format_cb_pane_right
 	},

tmux.1

@@ -6323,6 +6323,7 @@ The following variables are available, where appropriate:
 .It Li "pane_path" Ta "" Ta "Path of pane (can be set by application)"
 .It Li "pane_pid" Ta "" Ta "PID of first process in pane"
 .It Li "pane_pipe" Ta "" Ta "1 if pane is being piped"
+.It Li "pane_pipe_pid" Ta "" Ta "PID of pipe process, if any"
 .It Li "pane_right" Ta "" Ta "Right of pane"
 .It Li "pane_search_string" Ta "" Ta "Last search string in copy mode"
 .It Li "pane_start_command" Ta "" Ta "Command pane started with"

tmux.h

@@ -1213,6 +1213,7 @@ struct window_pane {
 	enum client_theme last_theme;
 
 	int		 pipe_fd;
+	pid_t		 pipe_pid;
 	struct bufferevent *pipe_event;
 	struct window_pane_offset pipe_offset;