cli: spawn a pager when running on a terminal

This makes it a lot more convenient to deal with long outputs (such as
"nmcli c show id ...").

The implementation is essentially jacked from systemd. The bugs are
mine.

clients/cli/general.c

@@ -14,7 +14,7 @@
  * with this program; if not, write to the Free Software Foundation, Inc.,
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  *
- * Copyright 2010 - 2015 Red Hat, Inc.
+ * Copyright 2010 - 2017 Red Hat, Inc.
  */
 
 #include "nm-default.h"
@@ -1296,6 +1296,9 @@ do_overview (NmCli *nmc, int argc, char **argv)
 	/* Register polkit agent */
 	nmc_start_polkit_agent_start_try (nmc);
 
+	/* Optionally start paging the output. */
+	nmc_terminal_spawn_pager (&nmc->nmc_config);
+
 	/* The VPN connections don't have devices (yet?). */
 	p = nm_client_get_active_connections (nmc->client);
 	for (i = 0; i < p->len; i++) {

clients/cli/nmcli.c

@@ -16,7 +16,7 @@
  * with this program; if not, write to the Free Software Foundation, Inc.,
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  *
- * Copyright 2010 - 2015 Red Hat, Inc.
+ * Copyright 2010 - 2017 Red Hat, Inc.
  */
 
 #include "nm-default.h"
@@ -148,7 +148,6 @@ complete_fields (const char *prefix)
 	g_hash_table_destroy (h);
 }
 
-
 static void
 usage (void)
 {
@@ -547,7 +546,9 @@ nmc_init (NmCli *nmc)
 static void
 nmc_cleanup (NmCli *nmc)
 {
-	if (nmc->client) g_object_unref (nmc->client);
+	pid_t ret;
+
+	g_clear_object (&nmc->client);
 
 	g_string_free (nmc->return_text, TRUE);
 
@@ -561,6 +562,15 @@ nmc_cleanup (NmCli *nmc)
 
 	g_free (nmc->required_fields);
 
+	if (nmc->pager_pid > 0) {
+		fclose (stdout);
+		fclose (stderr);
+		do {
+			ret = waitpid (nmc->pager_pid, NULL, 0);
+		} while (ret == -1 && errno == EINTR);
+		nmc->pager_pid = 0;
+	}
+
 	nmc_polkit_agent_fini (nmc);
 }
 

clients/cli/nmcli.h

@@ -14,7 +14,7 @@
  * with this program; if not, write to the Free Software Foundation, Inc.,
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  *
- * Copyright 2010 - 2015 Red Hat, Inc.
+ * Copyright 2010 - 2017 Red Hat, Inc.
  */
 
 #ifndef NMC_NMCLI_H
@@ -129,6 +129,7 @@ typedef struct _NmCli {
 
 	NMCResultCode return_value;                       /* Return code of nmcli */
 	GString *return_text;                             /* Reason text */
+	pid_t pager_pid;                                  /* PID of a pager, if one was spawned */
 
 	int timeout;                                      /* Operation timeout */
 

clients/cli/utils.c

@@ -14,7 +14,8 @@
  * with this program; if not, write to the Free Software Foundation, Inc.,
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  *
- * Copyright 2010 - 2015 Red Hat, Inc.
+ * Copyright 2010 Lennart Poettering
+ * Copyright 2010 - 2017 Red Hat, Inc.
  */
 
 #include "nm-default.h"
@@ -28,6 +29,7 @@
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
+#include <sys/prctl.h>
 
 #include "nm-client-utils.h"
 #include "nm-meta-setting-access.h"
@@ -122,7 +124,7 @@ use_colors (NmcColorOption color_option)
 
 		if (G_UNLIKELY (cached == NMC_USE_COLOR_AUTO)) {
 			if (   g_strcmp0 (g_getenv ("TERM"), "dumb") == 0
-				|| !isatty (fileno (stdout)))
+				|| !isatty (STDOUT_FILENO))
 				cached = NMC_USE_COLOR_NO;
 			else
 				cached = NMC_USE_COLOR_YES;
@@ -1450,6 +1452,109 @@ nmc_print (const NmcConfig *nmc_config,
 
 /*****************************************************************************/
 
+static void
+pager_fallback (void)
+{
+	char buf[64];
+	int rb;
+
+	do {
+		rb = read (STDIN_FILENO, buf, sizeof (buf));
+		if (rb == -1) {
+			if (errno == EINTR) {
+				continue;
+			} else {
+				g_printerr (_("Error reading nmcli output: %s\n"), strerror (errno));
+				_exit(EXIT_FAILURE);
+			}
+		}
+		if (write (STDOUT_FILENO, buf, rb) == -1) {
+			g_printerr (_("Error writing nmcli output: %s\n"), strerror (errno));
+			_exit(EXIT_FAILURE);
+		}
+	} while (rb > 0);
+
+	_exit(EXIT_SUCCESS);
+}
+
+void
+nmc_terminal_spawn_pager (const NmcConfig *nmc_config)
+{
+	const char *pager = getenv ("PAGER");
+	pid_t parent_pid;
+	int fd[2];
+
+	if (   nm_cli.pager_pid > 0
+	    || nmc_config->print_output == NMC_PRINT_TERSE
+	    || !use_colors (nmc_config->use_colors)
+	    || g_strcmp0 (pager, "") == 0)
+		return;
+
+	if (pipe (fd) == -1) {
+		g_printerr (_("Failed to create pager pipe: %s\n"), strerror (errno));
+		return;
+	}
+
+	parent_pid = getpid ();
+
+	nm_cli.pager_pid = fork ();
+	if (nm_cli.pager_pid == -1) {
+		g_printerr (_("Failed to fork pager: %s\n"), strerror (errno));
+		close (fd[0]);
+		close (fd[1]);
+		return;
+	}
+
+	/* In the child start the pager */
+	if (nm_cli.pager_pid == 0) {
+		dup2 (fd[0], STDIN_FILENO);
+		close (fd[0]);
+		close (fd[1]);
+
+		setenv ("LESS", "FRSXMK", 1);
+		setenv ("LESSCHARSET", "utf-8", 1);
+
+		/* Make sure the pager goes away when the parent dies */
+		if (prctl (PR_SET_PDEATHSIG, SIGTERM) < 0)
+			_exit (EXIT_FAILURE);
+
+		/* Check whether our parent died before we were able
+		 * to set the death signal */
+		if (getppid () != parent_pid)
+			_exit (EXIT_SUCCESS);
+
+		if (pager) {
+			execlp (pager, pager, NULL);
+			execl ("/bin/sh", "sh", "-c", pager, NULL);
+		}
+
+		/* Debian's alternatives command for pagers is
+		 * called 'pager'. Note that we do not call
+		 * sensible-pagers here, since that is just a
+		 * shell script that implements a logic that
+		 * is similar to this one anyway, but is
+		 * Debian-specific. */
+		execlp ("pager", "pager", NULL);
+
+		execlp ("less", "less", NULL);
+		execlp ("more", "more", NULL);
+
+		pager_fallback ();
+		/* not reached */
+	}
+
+	/* Return in the parent */
+	if (dup2 (fd[1], STDOUT_FILENO) < 0)
+		g_printerr (_("Failed to duplicate pager pipe: %s\n"), strerror (errno));
+	if (dup2 (fd[1], STDERR_FILENO) < 0)
+		g_printerr (_("Failed to duplicate pager pipe: %s\n"), strerror (errno));
+
+	close (fd[0]);
+	close (fd[1]);
+}
+
+/*****************************************************************************/
+
 static const char *
 get_value_to_print (NmcColorOption color_option,
                     const NmcOutputField *field,
@@ -1514,6 +1619,9 @@ print_required_fields (const NmcConfig *nmc_config,
 	gboolean field_names = of_flags & NMC_OF_FLAG_FIELD_NAMES;
 	gboolean section_prefix = of_flags & NMC_OF_FLAG_SECTION_PREFIX;
 
+	/* Optionally start paging the output. */
+	nmc_terminal_spawn_pager (nmc_config);
+
 	/* --- Main header --- */
 	if ((main_header_add || main_header_only) && pretty) {
 		gs_free char *line = NULL;

clients/cli/utils.h

@@ -14,7 +14,7 @@
  * with this program; if not, write to the Free Software Foundation, Inc.,
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  *
- * Copyright 2010 - 2015 Red Hat, Inc.
+ * Copyright 2010 - 2017 Red Hat, Inc.
  */
 
 #ifndef NMC_UTILS_H
@@ -40,6 +40,8 @@ gboolean nmc_parse_args (nmc_arg_t *arg_arr, gboolean last, int *argc, char ***a
 char *ssid_to_hex (const char *str, gsize len);
 void nmc_terminal_erase_line (void);
 void nmc_terminal_show_progress (const char *str);
+void nmc_terminal_spawn_pager (const NmcConfig *nmc_config);
+gboolean nmc_term_use_colors (NmcColorOption color_option);
 const char *nmc_term_color_sequence (NMMetaTermColor color);
 const char *nmc_term_format_sequence (NMMetaTermFormat format);
 NMMetaTermColor nmc_term_color_parse_string (const char *str, GError **error);