vppctl,cli: Improve non-interactive vppctl (VPP-944)

Short version: Make vppctl behave as expected when run
from scripts, or without a controlling terminal, and
especially when using it with VPP commands on its
command line ("non-interactively").

In particular, prevent the welcome banner and VPP CLI
prompt from being sent by VPP when being used in these
ways.

vppctl
------

- Improve vppctl's detection of non-interactive sessions.
- Pass non-interactiveness in the terminal type telnet option
  as a value distinct from "dumb" (which means non-ANSI capable.)
- Make tty setup handling more robust.
- Only send non-interactive command once we've sent the
  terminal type, to ensure correct event sequence; we need
  the VPP cli session to be in line-by-line mode.
- Ignore stdin when it looks something like /dev/null.
- Skip NUL bytes received from VPP.

VPP CLI
-------

- Detect "non-interactive" terminal types and set session
  parameters accordingly.
- Add an "interactive" flag that controls whether the welcome
  banner and CLI prompt are sent.
- Detect if telnet options processing switched us into line
  mode and act accordingly for the rest of the current input
  buffer. This was causing the command string to be echoed
  by the CLI editor code.
- For non-interactive sessions, send a NUL byte after the
  input buffer has been processed. This is because vppctl
  depends on seeing traffic before it will try to close the
  session; a command with no output would cause it to hang.
  NUL bytes are ignored by all decent terminals, but we have
  vppctl strip them out anyway.
- Prevent certain commands from running in non-interactive
  sessions since they manipulate interactive-related features.
- For interactive sessions, quench the prompt that prints on
  VPP shutdown.
- Detect and handle socket errors in the CLI; sessions were
  leaking.
- Pevent SIGPIPE from ever being raised; handle EPIPE instead.
  We don't need VPP to die just because a socket closed just
  before we try to write to it!
- Add a command to dump a list of current CLI sessions; mostly
  this was to detect session leakage, but it may have some
  general utility.

Change-Id: Ia147da013317180882c1d967b18eefb8519a55fb
Signed-off-by: Chris Luke <chrisy@flirble.org>

2 files changed, 404 insertions, 89 deletions

diff --git a/src/vlib/unix/cli.c b/src/vlib/unix/cli.c
index 0e035b1342b..ffe5de6d5b2 100644
--- a/src/vlib/unix/cli.c
+++ b/src/vlib/unix/cli.c
@@ -100,9 +100,6 @@
 /** Maximum terminal height we will accept */
 #define UNIX_CLI_MAX_TERMINAL_HEIGHT 512
 
-/** Unix standard in */
-#define UNIX_CLI_STDIN_FD 0
-
 
 /** A CLI banner line. */
 typedef struct
@@ -200,6 +197,18 @@ typedef struct
   /** Disable the pager? */
   u8 no_pager;
 
+  /** Whether the session is interactive or not.
+   * Controls things like initial banner, the CLI prompt etc.  */
+  u8 is_interactive;
+
+  /** Whether the session is attached to a socket. */
+  u8 is_socket;
+
+  /** If EPIPE has been detected, prevent further write-related
+   * activity on the descriptor.
+   */
+  u8 has_epipe;
+
   /** Pager buffer */
   u8 **pager_vector;
 
@@ -593,12 +602,33 @@ unix_vlib_cli_output_raw (unix_cli_file_t * cf,
 {
   int n = 0;
 
+  if (cf->has_epipe)		/* don't try writing anything */
+    return;
+
   if (vec_len (cf->output_vector) == 0)
-    n = write (uf->file_descriptor, buffer, buffer_bytes);
+    {
+      if (cf->is_socket)
+	/* If it's a socket we use MSG_NOSIGNAL to prevent SIGPIPE */
+	n = send (uf->file_descriptor, buffer, buffer_bytes, MSG_NOSIGNAL);
+      else
+	n = write (uf->file_descriptor, buffer, buffer_bytes);
+    }
 
   if (n < 0 && errno != EAGAIN)
     {
-      clib_unix_warning ("write");
+      if (errno == EPIPE)
+	{
+	  /* connection closed on us */
+	  unix_main_t *um = &unix_main;
+	  cf->has_epipe = 1;
+	  vlib_process_signal_event (um->vlib_main, cf->process_node_index,
+				     UNIX_CLI_PROCESS_EVENT_QUIT,
+				     uf->private_data);
+	}
+      else
+	{
+	  clib_unix_warning ("write");
+	}
     }
   else if ((word) n < (word) buffer_bytes)
     {
@@ -659,7 +689,9 @@ unix_cli_cli_prompt (unix_cli_file_t * cf, clib_file_t * uf)
 {
   unix_cli_main_t *cm = &unix_cli_main;
 
-  unix_vlib_cli_output_raw (cf, uf, cm->cli_prompt, vec_len (cm->cli_prompt));
+  if (cf->is_interactive)	/* Only interactive sessions get a prompt */
+    unix_vlib_cli_output_raw (cf, uf, cm->cli_prompt,
+			      vec_len (cm->cli_prompt));
 }
 
 /** @brief Output a pager prompt and show number of buffered lines */
@@ -1024,7 +1056,7 @@ unix_vlib_cli_output (uword cli_file_index, u8 * buffer, uword buffer_bytes)
  *         terminal sequences; @c 0 otherwise.
  */
 static u8
-unix_cli_terminal_type (u8 * term, uword len)
+unix_cli_terminal_type_ansi (u8 * term, uword len)
 {
   /* This may later be better done as a hash of some sort. */
 #define _(a) do { \
@@ -1042,6 +1074,45 @@ unix_cli_terminal_type (u8 * term, uword len)
   return 0;
 }
 
+/** Identify whether a terminal type is non-interactive.
+ *
+ * Compares the string given in @c term with a list of terminal types known
+ * to be non-interactive, as send by tools such as @c vppctl .
+ *
+ * This list contains, for example, @c vppctl.
+ *
+ * @param term A string with a terminal type in it.
+ * @param len The length of the string in @c term.
+ *
+ * @return @c 1 if the terminal type is recognized as being non-interactive;
+ *         @c 0 otherwise.
+ */
+static u8
+unix_cli_terminal_type_noninteractive (u8 * term, uword len)
+{
+  /* This may later be better done as a hash of some sort. */
+#define _(a) do { \
+    if (strncasecmp(a, (char *)term, (size_t)len) == 0) return 1; \
+  } while(0)
+
+  _("vppctl");
+#undef _
+
+  return 0;
+}
+
+/** Set a session to be non-interactive. */
+static void
+unix_cli_set_session_noninteractive (unix_cli_file_t * cf)
+{
+  /* Non-interactive sessions don't get these */
+  cf->is_interactive = 0;
+  cf->no_pager = 1;
+  cf->history_limit = 0;
+  cf->has_history = 0;
+  cf->line_mode = 1;
+}
+
 /** @brief Emit initial welcome banner and prompt on a connection. */
 static void
 unix_cli_file_welcome (unix_cli_main_t * cm, unix_cli_file_t * cf)
@@ -1052,6 +1123,12 @@ unix_cli_file_welcome (unix_cli_main_t * cm, unix_cli_file_t * cf)
   unix_cli_banner_t *banner;
   int i, len;
 
+  /* Mark the session as started if we get here */
+  cf->started = 1;
+
+  if (!(cf->is_interactive))	/* No banner for non-interactive sessions */
+    return;
+
   /*
    * Put the first bytes directly into the buffer so that further output is
    * queued until everything is ready. (oterwise initial prompt can appear
@@ -1082,7 +1159,6 @@ unix_cli_file_welcome (unix_cli_main_t * cm, unix_cli_file_t * cf)
   /* Prompt. */
   unix_cli_cli_prompt (cf, uf);
 
-  cf->started = 1;
 }
 
 /** @brief A failsafe triggered on a timer to ensure we send the prompt
@@ -1160,9 +1236,20 @@ unix_cli_process_telnet (unix_main_t * um,
 		  case TELOPT_TTYPE:
 		    if (input_vector[3] != 0)
 		      break;
-		    /* See if the terminal type is ANSI capable */
-		    cf->ansi_capable =
-		      unix_cli_terminal_type (input_vector + 4, i - 5);
+		    {
+		      /* See if the the terminal type is recognized */
+		      u8 *term = input_vector + 4;
+		      uword len = i - 5;
+
+		      /* See if the terminal type is ANSI capable */
+		      cf->ansi_capable =
+			unix_cli_terminal_type_ansi (term, len);
+
+		      /* See if the terminal type indicates non-interactive */
+		      if (unix_cli_terminal_type_noninteractive (term, len))
+			unix_cli_set_session_noninteractive (cf);
+		    }
+
 		    /* If session not started, we can release the pause */
 		    if (!cf->started)
 		      /* Send the welcome banner and initial prompt */
@@ -2122,10 +2209,12 @@ unix_cli_line_edit (unix_cli_main_t * cm, unix_main_t * um,
 					     vec_len (cf->input_vector) - i);
 	  if (matched < 0)
 	    {
+	      /* There was a partial match which means we need more bytes
+	       * than the input buffer currently has.
+	       */
 	      if (i)
 		{
-		  /* There was a partial match which means we need more bytes
-		   * than the input buffer currently has.
+		  /*
 		   * Since the bytes before here have been processed, shift
 		   * the remaining contents to the start of the input buffer.
 		   */
@@ -2136,6 +2225,16 @@ unix_cli_line_edit (unix_cli_main_t * cm, unix_main_t * um,
 	  break;
 
 	default:
+	  /* If telnet option processing switched us to line mode, get us
+	   * out of here!
+	   */
+	  if (cf->line_mode)
+	    {
+	      vec_delete (cf->input_vector, i, 0);
+	      cf->current_command = cf->input_vector;
+	      return 0;
+	    }
+
 	  /* process the action */
 	  if (!unix_cli_line_process_one (cm, um, cf, uf,
 					  cf->input_vector[i], action))
@@ -2165,11 +2264,14 @@ unix_cli_process_input (unix_cli_main_t * cm, uword cli_file_index)
   unformat_input_t input;
   int vlib_parse_eval (u8 *);
 
+  cm->current_input_file_index = cli_file_index;
+
 more:
   /* Try vlibplex first.  Someday... */
   if (0 && vlib_parse_eval (cf->input_vector) == 0)
     goto done;
 
+
   if (cf->line_mode)
     {
       /* just treat whatever we got as a complete line of input */
@@ -2190,20 +2292,16 @@ more:
       lv = format (lv, "%U[%d]: %v",
 		   format_timeval, 0 /* current bat-time */ ,
 		   0 /* current bat-format */ ,
-		   cli_file_index, cf->input_vector);
-      {
-	int rv __attribute__ ((unused)) =
-	  write (um->log_fd, lv, vec_len (lv));
-      }
+		   cli_file_index, cf->current_command);
+      int rv __attribute__ ((unused)) = write (um->log_fd, lv, vec_len (lv));
     }
 
-  /* Copy our input command to a new string */
+  /* Build an unformat structure around our command */
   unformat_init_vector (&input, cf->current_command);
 
   /* Remove leading white space from input. */
   (void) unformat (&input, "");
 
-  cm->current_input_file_index = cli_file_index;
   cf->pager_start = 0;		/* start a new pager session */
 
   if (unformat_check_input (&input) != UNFORMAT_END_OF_INPUT)
@@ -2222,9 +2320,14 @@ more:
 done:
   /* reset vector; we'll re-use it later  */
   if (cf->line_mode)
-    vec_reset_length (cf->input_vector);
+    {
+      vec_reset_length (cf->input_vector);
+      cf->current_command = 0;
+    }
   else
-    vec_reset_length (cf->current_command);
+    {
+      vec_reset_length (cf->current_command);
+    }
 
   if (cf->no_pager == 2)
     {
@@ -2251,6 +2354,15 @@ done:
   /* Any residual data in the input vector? */
   if (vec_len (cf->input_vector))
     goto more;
+
+  /* For non-interactive sessions send a NUL byte.
+   * Specifically this is because vppctl needs to see some traffic in
+   * order to move on to closing the session. Commands with no output
+   * would thus cause vppctl to hang indefinitely in non-interactive mode
+   * since there is also no prompt sent after the command completes.
+   */
+  if (!cf->is_interactive)
+    unix_vlib_cli_output_raw (cf, uf, (u8 *) "\0", 1);
 }
 
 /** Destroy a CLI session.
@@ -2270,7 +2382,7 @@ unix_cli_kill (unix_cli_main_t * cm, uword cli_file_index)
   uf = pool_elt_at_index (fm->file_pool, cf->clib_file_index);
 
   /* Quit/EOF on stdin means quit program. */
-  if (uf->file_descriptor == UNIX_CLI_STDIN_FD)
+  if (uf->file_descriptor == STDIN_FILENO)
     clib_longjmp (&um->vlib_main->main_loop_exit, VLIB_MAIN_LOOP_EXIT_CLI);
 
   vec_free (cf->current_command);
@@ -2342,11 +2454,30 @@ unix_cli_write_ready (clib_file_t * uf)
   cf = pool_elt_at_index (cm->cli_file_pool, uf->private_data);
 
   /* Flush output vector. */
-  n = write (uf->file_descriptor,
-	     cf->output_vector, vec_len (cf->output_vector));
+  if (cf->is_socket)
+    /* If it's a socket we use MSG_NOSIGNAL to prevent SIGPIPE */
+    n = send (uf->file_descriptor,
+	      cf->output_vector, vec_len (cf->output_vector), MSG_NOSIGNAL);
+  else
+    n = write (uf->file_descriptor,
+	       cf->output_vector, vec_len (cf->output_vector));
 
   if (n < 0 && errno != EAGAIN)
-    return clib_error_return_unix (0, "write");
+    {
+      if (errno == EPIPE)
+	{
+	  /* connection closed on us */
+	  unix_main_t *um = &unix_main;
+	  cf->has_epipe = 1;
+	  vlib_process_signal_event (um->vlib_main, cf->process_node_index,
+				     UNIX_CLI_PROCESS_EVENT_QUIT,
+				     uf->private_data);
+	}
+      else
+	{
+	  return clib_error_return_unix (0, "write");
+	}
+    }
 
   else if (n > 0)
     unix_cli_del_pending_output (uf, cf, n);
@@ -2393,6 +2524,24 @@ unix_cli_read_ready (clib_file_t * uf)
   return /* no error */ 0;
 }
 
+/** Called when a CLI session file descriptor has an error condition. */
+static clib_error_t *
+unix_cli_error_detected (clib_file_t * uf)
+{
+  unix_main_t *um = &unix_main;
+  unix_cli_main_t *cm = &unix_cli_main;
+  unix_cli_file_t *cf;
+
+  cf = pool_elt_at_index (cm->cli_file_pool, uf->private_data);
+  cf->has_epipe = 1;		/* prevent writes while the close is pending */
+  vlib_process_signal_event (um->vlib_main,
+			     cf->process_node_index,
+			     UNIX_CLI_PROCESS_EVENT_QUIT,
+			     /* event data */ uf->private_data);
+
+  return /* no error */ 0;
+}
+
 /** Store a new CLI session.
  * @param name The name of the session.
  * @param fd   The file descriptor for the session I/O.
@@ -2443,6 +2592,7 @@ unix_cli_file_add (unix_cli_main_t * cm, char *name, int fd)
 
   template.read_function = unix_cli_read_ready;
   template.write_function = unix_cli_write_ready;
+  template.error_function = unix_cli_error_detected;
   template.file_descriptor = fd;
   template.private_data = cf - cm->cli_file_pool;
 
@@ -2482,10 +2632,10 @@ unix_cli_listen_read_ready (clib_file_t * uf)
 
   cf_index = unix_cli_file_add (cm, client_name, client.fd);
   cf = pool_elt_at_index (cm->cli_file_pool, cf_index);
+  cf->is_socket = 1;
 
   /* No longer need CLIB version of socket. */
   clib_socket_free (&client);
-
   vec_free (client_name);
 
   /* if we're supposed to run telnet session in character mode (default) */
@@ -2512,6 +2662,9 @@ unix_cli_listen_read_ready (clib_file_t * uf)
       cf->history_limit = um->cli_history_limit;
       cf->has_history = cf->history_limit != 0;
 
+      /* This is an interactive session until we decide otherwise */
+      cf->is_interactive = 1;
+
       /* Make sure this session is in line mode */
       cf->line_mode = 0;
 
@@ -2521,9 +2674,8 @@ unix_cli_listen_read_ready (clib_file_t * uf)
       /* Setup the pager */
       cf->no_pager = um->cli_no_pager;
 
-      uf = pool_elt_at_index (fm->file_pool, cf->clib_file_index);
-
       /* Send the telnet options */
+      uf = pool_elt_at_index (fm->file_pool, cf->clib_file_index);
       unix_vlib_cli_output_raw (cf, uf, charmode_option,
 				ARRAY_LEN (charmode_option));
 
@@ -2550,7 +2702,7 @@ unix_cli_resize_interrupt (int signum)
   (void) signum;
 
   /* Terminal resized, fetch the new size */
-  if (ioctl (UNIX_CLI_STDIN_FD, TIOCGWINSZ, &ws) < 0)
+  if (ioctl (STDIN_FILENO, TIOCGWINSZ, &ws) < 0)
     {
       /* "Should never happen..." */
       clib_unix_warning ("TIOCGWINSZ");
@@ -2600,17 +2752,17 @@ unix_cli_config (vlib_main_t * vm, unformat_input_t * input)
   if (um->flags & UNIX_FLAG_INTERACTIVE)
     {
       /* Set stdin to be non-blocking. */
-      if ((flags = fcntl (UNIX_CLI_STDIN_FD, F_GETFL, 0)) < 0)
+      if ((flags = fcntl (STDIN_FILENO, F_GETFL, 0)) < 0)
 	flags = 0;
-      (void) fcntl (UNIX_CLI_STDIN_FD, F_SETFL, flags | O_NONBLOCK);
+      (void) fcntl (STDIN_FILENO, F_SETFL, flags | O_NONBLOCK);
 
-      cf_index = unix_cli_file_add (cm, "stdin", UNIX_CLI_STDIN_FD);
+      cf_index = unix_cli_file_add (cm, "stdin", STDIN_FILENO);
       cf = pool_elt_at_index (cm->cli_file_pool, cf_index);
       cm->stdin_cli_file_index = cf_index;
 
       /* If stdin is a tty and we are using chacracter mode, enable
        * history on the CLI and set the tty line discipline accordingly. */
-      if (isatty (UNIX_CLI_STDIN_FD) && um->cli_line_mode == 0)
+      if (isatty (STDIN_FILENO) && um->cli_line_mode == 0)
 	{
 	  /* Capture terminal resize events */
 	  memset (&sa, 0, sizeof (sa));
@@ -2619,7 +2771,7 @@ unix_cli_config (vlib_main_t * vm, unformat_input_t * input)
 	    clib_panic ("sigaction");
 
 	  /* Retrieve the current terminal size */
-	  ioctl (UNIX_CLI_STDIN_FD, TIOCGWINSZ, &ws);
+	  ioctl (STDIN_FILENO, TIOCGWINSZ, &ws);
 	  cf->width = ws.ws_col;
 	  cf->height = ws.ws_row;
 
@@ -2634,11 +2786,14 @@ unix_cli_config (vlib_main_t * vm, unformat_input_t * input)
 	  /* Setup the pager */
 	  cf->no_pager = um->cli_no_pager;
 
+	  /* This is an interactive session until we decide otherwise */
+	  cf->is_interactive = 1;
+
 	  /* We're going to be in char by char mode */
 	  cf->line_mode = 0;
 
 	  /* Save the original tty state so we can restore it later */
-	  tcgetattr (UNIX_CLI_STDIN_FD, &um->tio_stdin);
+	  tcgetattr (STDIN_FILENO, &um->tio_stdin);
 	  um->tio_isset = 1;
 
 	  /* Tweak the tty settings */
@@ -2647,23 +2802,23 @@ unix_cli_config (vlib_main_t * vm, unformat_input_t * input)
 	  tio.c_lflag &= ~(ECHO | ICANON | IEXTEN);
 	  tio.c_cc[VMIN] = 1;	/* 1 byte at a time */
 	  tio.c_cc[VTIME] = 0;	/* no timer */
-	  tcsetattr (UNIX_CLI_STDIN_FD, TCSAFLUSH, &tio);
+	  tcsetattr (STDIN_FILENO, TCSAFLUSH, &tio);
 
 	  /* See if we can do ANSI/VT100 output */
 	  term = (u8 *) getenv ("TERM");
 	  if (term != NULL)
-	    cf->ansi_capable = unix_cli_terminal_type (term,
-						       strlen ((char *)
-							       term));
+	    {
+	      int len = strlen ((char *) term);
+	      cf->ansi_capable = unix_cli_terminal_type_ansi (term, len);
+	      if (unix_cli_terminal_type_noninteractive (term, len))
+		unix_cli_set_session_noninteractive (cf);
+	    }
 	}
       else
 	{
 	notty:
-	  /* No tty, so make sure these things are off */
-	  cf->no_pager = 1;
-	  cf->history_limit = 0;
-	  cf->has_history = 0;
-	  cf->line_mode = 1;
+	  /* No tty, so make sure the session doesn't have tty-like features */
+	  unix_cli_set_session_noninteractive (cf);
 	}
 
       /* Send banner and initial prompt */
@@ -2726,8 +2881,8 @@ unix_cli_exit (vlib_main_t * vm)
   unix_main_t *um = &unix_main;
 
   /* If stdin is a tty and we saved the tty state, reset the tty state */
-  if (isatty (UNIX_CLI_STDIN_FD) && um->tio_isset)
-    tcsetattr (UNIX_CLI_STDIN_FD, TCSAFLUSH, &um->tio_stdin);
+  if (isatty (STDIN_FILENO) && um->tio_isset)
+    tcsetattr (STDIN_FILENO, TCSAFLUSH, &um->tio_stdin);
 
   return 0;
 }
@@ -2758,6 +2913,12 @@ unix_cli_quit (vlib_main_t * vm,
 	       unformat_input_t * input, vlib_cli_command_t * cmd)
 {
   unix_cli_main_t *cm = &unix_cli_main;
+  unix_cli_file_t *cf = pool_elt_at_index (cm->cli_file_pool,
+					   cm->current_input_file_index);
+
+  /* Cosmetic: suppress the final prompt from appearing before we die */
+  cf->is_interactive = 0;
+  cf->started = 1;
 
   vlib_process_signal_event (vm,
 			     vlib_current_process (vm),
@@ -2940,6 +3101,9 @@ unix_cli_show_history (vlib_main_t * vm,
 
   cf = pool_elt_at_index (cm->cli_file_pool, cm->current_input_file_index);
 
+  if (!cf->is_interactive)
+    return clib_error_return (0, "invalid for non-interactive sessions");
+
   if (cf->has_history && cf->history_limit)
     {
       i = 1 + cf->command_number - vec_len (cf->command_history);
@@ -2985,6 +3149,8 @@ unix_cli_show_terminal (vlib_main_t * vm,
   vlib_cli_output (vm, "Terminal height: %d\n", cf->height);
   vlib_cli_output (vm, "ANSI capable:    %s\n",
 		   cf->ansi_capable ? "yes" : "no");
+  vlib_cli_output (vm, "Interactive:     %s\n",
+		   cf->is_interactive ? "yes" : "no");
   vlib_cli_output (vm, "History enabled: %s%s\n",
 		   cf->has_history ? "yes" : "no", !cf->has_history
 		   || cf->history_limit ? "" :
@@ -3017,6 +3183,7 @@ unix_cli_show_terminal (vlib_main_t * vm,
  * Terminal width:  123
  * Terminal height: 48
  * ANSI capable:    yes
+ * Interactive:     yes
  * History enabled: yes
  * History limit:   50
  * Pager enabled:   yes
@@ -3032,6 +3199,87 @@ VLIB_CLI_COMMAND (cli_unix_cli_show_terminal, static) = {
 };
 /* *INDENT-ON* */
 
+/** CLI command to display a list of CLI sessions. */
+static clib_error_t *
+unix_cli_show_cli_sessions (vlib_main_t * vm,
+			    unformat_input_t * input,
+			    vlib_cli_command_t * cmd)
+{
+  //unix_main_t *um = &unix_main;
+  unix_cli_main_t *cm = &unix_cli_main;
+  clib_file_main_t *fm = &file_main;
+  unix_cli_file_t *cf;
+  clib_file_t *uf;
+  vlib_node_t *n;
+
+  vlib_cli_output (vm, "%-5s %-5s %-20s %s", "PNI", "FD", "Name", "Flags");
+
+#define fl(x, y) ( (x) ? toupper((y)) : tolower((y)) )
+  /* *INDENT-OFF* */
+  pool_foreach (cf, cm->cli_file_pool, ({
+    uf = pool_elt_at_index (fm->file_pool, cf->clib_file_index);
+    n = vlib_get_node (vm, cf->process_node_index);
+    vlib_cli_output (vm,
+		     "%-5d %-5d %-20v %c%c%c%c%c\n",
+		     cf->process_node_index,
+		     uf->file_descriptor,
+		     n->name,
+		     fl (cf->is_interactive, 'i'),
+		     fl (cf->is_socket, 's'),
+		     fl (cf->line_mode, 'l'),
+		     fl (cf->has_epipe, 'p'),
+		     fl (cf->ansi_capable, 'a'));
+  }));
+  /* *INDENT-ON* */
+#undef fl
+
+  return 0;
+}
+
+/*?
+ * Displays a summary of all the current CLI sessions.
+ *
+ * Typically used to diagnose connection issues with the CLI
+ * socket.
+ *
+ * @cliexpar
+ * @cliexstart{show cli-sessions}
+ * PNI   FD    Name                 Flags
+ * 343   0     unix-cli-stdin       IslpA
+ * 344   7     unix-cli-local:20    ISlpA
+ * 346   8     unix-cli-local:21    iSLpa
+ * @cliexend
+
+ * In this example we have the debug console of the running process
+ * on stdin/out, we have an interactive socket session and we also
+ * have a non-interactive socket session.
+ *
+ * Fields:
+ *
+ * - @em PNI: Process node index.
+ * - @em FD: Unix file descriptor.
+ * - @em Name: Name of the session.
+ * - @em Flags: Various flags that describe the state of the session.
+ *
+ * @em Flags have the following meanings; lower-case typically negates
+ * upper-case:
+ *
+ * - @em I Interactive session.
+ * - @em S Connected by socket.
+ * - @em s Not a socket, likely stdin.
+ * - @em L Line-by-line mode.
+ * - @em l Char-by-char mode.
+ * - @em P EPIPE detected on connection; it will close soon.
+ * - @em A ANSI-capable terminal.
+?*/
+/* *INDENT-OFF* */
+VLIB_CLI_COMMAND (cli_unix_cli_show_cli_sessions, static) = {
+  .path = "show cli-sessions",
+  .short_help = "Show current CLI sessions",
+  .function = unix_cli_show_cli_sessions,
+};
+/* *INDENT-ON* */
+
 /** CLI command to set terminal pager settings. */
 static clib_error_t *
 unix_cli_set_terminal_pager (vlib_main_t * vm,
@@ -3044,11 +3292,14 @@ unix_cli_set_terminal_pager (vlib_main_t * vm,
   unformat_input_t _line_input, *line_input = &_line_input;
   clib_error_t *error = 0;
 
+  cf = pool_elt_at_index (cm->cli_file_pool, cm->current_input_file_index);
+
+  if (!cf->is_interactive)
+    return clib_error_return (0, "invalid for non-interactive sessions");
+
   if (!unformat_user (input, unformat_line_input, line_input))
     return 0;
 
-  cf = pool_elt_at_index (cm->cli_file_pool, cm->current_input_file_index);
-
   while (unformat_check_input (line_input) != UNFORMAT_END_OF_INPUT)
     {
       if (unformat (line_input, "on"))
@@ -3100,11 +3351,14 @@ unix_cli_set_terminal_history (vlib_main_t * vm,
   u32 limit;
   clib_error_t *error = 0;
 
+  cf = pool_elt_at_index (cm->cli_file_pool, cm->current_input_file_index);
+
+  if (!cf->is_interactive)
+    return clib_error_return (0, "invalid for non-interactive sessions");
+
   if (!unformat_user (input, unformat_line_input, line_input))
     return 0;
 
-  cf = pool_elt_at_index (cm->cli_file_pool, cm->current_input_file_index);
-
   while (unformat_check_input (line_input) != UNFORMAT_END_OF_INPUT)
     {
       if (unformat (line_input, "on"))
@@ -3162,6 +3416,9 @@ unix_cli_set_terminal_ansi (vlib_main_t * vm,
 
   cf = pool_elt_at_index (cm->cli_file_pool, cm->current_input_file_index);
 
+  if (!cf->is_interactive)
+    return clib_error_return (0, "invalid for non-interactive sessions");
+
   if (unformat (input, "on"))
     cf->ansi_capable = 1;
   else if (unformat (input, "off"))
diff --git a/src/vpp/app/vppctl.c b/src/vpp/app/vppctl.c
index 980936f16ba..f81a0ce985a 100644
--- a/src/vpp/app/vppctl.c
+++ b/src/vpp/app/vppctl.c
@@ -41,12 +41,16 @@ volatile int window_resized = 0;
 struct termios orig_tio;
 
 static void
-send_ttype (clib_socket_t * s, int is_dumb)
+send_ttype (clib_socket_t * s, int is_interactive)
 {
+  char *term;
+
+  term = is_interactive ? getenv ("TERM") : "vppctl";
+  if (term == NULL)
+    term = "dumb";
+
   clib_socket_tx_add_formatted (s, "%c%c%c" "%c%s" "%c%c",
-				IAC, SB, TELOPT_TTYPE,
-				0, is_dumb ? "dumb" : getenv ("TERM"),
-				IAC, SE);
+				IAC, SB, TELOPT_TTYPE, 0, term, IAC, SE);
   clib_socket_tx (s);
 }
 
@@ -81,7 +85,8 @@ signal_handler_term (int signum)
 }
 
 static u8 *
-process_input (u8 * str, clib_socket_t * s, int is_interactive)
+process_input (u8 * str, clib_socket_t * s, int is_interactive,
+	       int *sent_ttype)
 {
   int i = 0;
 
@@ -104,7 +109,10 @@ process_input (u8 * str, clib_socket_t * s, int is_interactive)
 	      vec_free (sb);
 	      i += 2;
 	      if (opt == TELOPT_TTYPE)
-		send_ttype (s, !is_interactive);
+		{
+		  send_ttype (s, is_interactive);
+		  *sent_ttype = 1;
+		}
 	      else if (is_interactive && opt == TELOPT_NAWS)
 		send_naws (s);
 	    }
@@ -138,6 +146,8 @@ main (int argc, char *argv[])
   u8 *str = 0;
   u8 *cmd = 0;
   int do_quit = 0;
+  int is_interactive = 0;
+  int sent_ttype = 0;
 
 
   clib_mem_init (0, 64ULL << 10);
@@ -164,33 +174,47 @@ main (int argc, char *argv[])
   if (error)
     goto done;
 
-  /* Capture terminal resize events */
-  memset (&sa, 0, sizeof (struct sigaction));
-  sa.sa_handler = signal_handler_winch;
+  is_interactive = isatty (STDIN_FILENO) && cmd == 0;
 
-  if (sigaction (SIGWINCH, &sa, 0) < 0)
+  if (is_interactive)
     {
-      error = clib_error_return_unix (0, "sigaction");
-      goto done;
-    }
+      /* Capture terminal resize events */
+      memset (&sa, 0, sizeof (struct sigaction));
+      sa.sa_handler = signal_handler_winch;
+      if (sigaction (SIGWINCH, &sa, 0) < 0)
+	{
+	  error = clib_error_return_unix (0, "sigaction");
+	  goto done;
+	}
 
-  sa.sa_handler = signal_handler_term;
-  if (sigaction (SIGTERM, &sa, 0) < 0)
-    {
-      error = clib_error_return_unix (0, "sigaction");
-      goto done;
-    }
+      /* Capture SIGTERM to reset tty settings */
+      sa.sa_handler = signal_handler_term;
+      if (sigaction (SIGTERM, &sa, 0) < 0)
+	{
+	  error = clib_error_return_unix (0, "sigaction");
+	  goto done;
+	}
 
-  /* Save the original tty state so we can restore it later */
-  tcgetattr (STDIN_FILENO, &orig_tio);
+      /* Save the original tty state so we can restore it later */
+      if (tcgetattr (STDIN_FILENO, &orig_tio) < 0)
+	{
+	  error = clib_error_return_unix (0, "tcgetattr");
+	  goto done;
+	}
+
+      /* Tweak the tty settings */
+      tio = orig_tio;
+      /* echo off, canonical mode off, ext'd input processing off */
+      tio.c_lflag &= ~(ECHO | ICANON | IEXTEN);
+      tio.c_cc[VMIN] = 1;	/* 1 byte at a time */
+      tio.c_cc[VTIME] = 0;	/* no timer */
 
-  /* Tweak the tty settings */
-  tio = orig_tio;
-  /* echo off, canonical mode off, ext'd input processing off */
-  tio.c_lflag &= ~(ECHO | ICANON | IEXTEN);
-  tio.c_cc[VMIN] = 1;		/* 1 byte at a time */
-  tio.c_cc[VTIME] = 0;		/* no timer */
-  tcsetattr (STDIN_FILENO, TCSAFLUSH, &tio);
+      if (tcsetattr (STDIN_FILENO, TCSAFLUSH, &tio) < 0)
+	{
+	  error = clib_error_return_unix (0, "tcsetattr");
+	  goto done;
+	}
+    }
 
   efd = epoll_create1 (0);
 
@@ -199,8 +223,12 @@ main (int argc, char *argv[])
   event.data.fd = STDIN_FILENO;
   if (epoll_ctl (efd, EPOLL_CTL_ADD, STDIN_FILENO, &event) != 0)
     {
-      error = clib_error_return_unix (0, "epoll_ctl[%d]", STDIN_FILENO);
-      goto done;
+      /* ignore EPERM; it means stdin is something like /dev/null */
+      if (errno != EPERM)
+	{
+	  error = clib_error_return_unix (0, "epoll_ctl[%d]", STDIN_FILENO);
+	  goto done;
+	}
     }
 
   /* register socket */
@@ -240,6 +268,9 @@ main (int argc, char *argv[])
 	  int n;
 	  char c[100];
 
+	  if (!sent_ttype)
+	    continue;		/* not ready for this yet */
+
 	  n = read (STDIN_FILENO, c, sizeof (c));
 	  if (n > 0)
 	    {
@@ -250,6 +281,8 @@ main (int argc, char *argv[])
 	    }
 	  else if (n < 0)
 	    clib_warning ("read rv=%d", n);
+	  else			/* EOF */
+	    do_quit = 1;
 	}
       else if (event.data.fd == s->fd)
 	{
@@ -260,27 +293,49 @@ main (int argc, char *argv[])
 	  if (clib_socket_rx_end_of_file (s))
 	    break;
 
-	  str = process_input (str, s, cmd == 0);
+	  str = process_input (str, s, is_interactive, &sent_ttype);
 
 	  if (vec_len (str) > 0)
 	    {
-	      n = write (STDOUT_FILENO, str, vec_len (str));
-	      if (n < 0)
+	      int len = vec_len (str);
+	      u8 *p = str, *q = str;
+
+	      while (len)
 		{
-		  error = clib_error_return_unix (0, "write");
-		  goto done;
+		  /* Search for and skip NUL bytes */
+		  while (q < (p + len) && *q)
+		    q++;
+
+		  n = write (STDOUT_FILENO, p, q - p);
+		  if (n < 0)
+		    {
+		      error = clib_error_return_unix (0, "write");
+		      goto done;
+		    }
+
+		  while (q < (p + len) && !*q)
+		    q++;
+		  len -= q - p;
+		  p = q;
 		}
+
 	      vec_reset_length (str);
 	    }
 
 	  if (do_quit)
 	    {
-	      clib_socket_tx_add_formatted (s, "q\n");
+	      /* Ask the other end to close the connection */
+	      clib_socket_tx_add_formatted (s, "quit\n");
 	      clib_socket_tx (s);
 	      do_quit = 0;
 	    }
-	  if (cmd)
+	  if (cmd && sent_ttype)
 	    {
+	      /* We wait until after the TELNET TTYPE option has been sent.
+	       * That is to make sure the session at the VPP end has switched
+	       * to line-by-line mode, and thus avoid prompts and echoing.
+	       * Note that it does also disable further TELNET option processing.
+	       */
 	      clib_socket_tx_add_formatted (s, "%s\n", cmd);
 	      clib_socket_tx (s);
 	      vec_free (cmd);
@@ -302,12 +357,15 @@ done:
   if (efd > -1)
     close (efd);
 
+  if (is_interactive)
+    tcsetattr (STDIN_FILENO, TCSAFLUSH, &orig_tio);
+
   if (error)
     {
       clib_error_report (error);
       return 1;
     }
-  tcsetattr (STDIN_FILENO, TCSAFLUSH, &orig_tio);
+
   return 0;
 }