stty: support -I, --immediate to not wait for pending transmission

In some cases an initial drain may block indefinitely as discussed at:
http://lists.gnu.org/archive/html/coreutils/2016-01/msg00007.html

* src/stty.c (main): Use TCSANOW rather than TCSADRAIN if -I specified.
(usage): Document the new option.
* doc/coreutils.texi (stty invocation): Likewise.
* tests/misc/stty.sh: Ensure -I is supported.
* NEWS: Mention the new feature.

4 files changed, 30 insertions, 3 deletions

diff --git a/NEWS b/NEWS
index 72d69b8b5..756042b3e 100644
--- a/NEWS
+++ b/NEWS
@@ -47,6 +47,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   is reusable by most shells, with non-printable characters escaped
   with the POSIX proposed $'...' syntax.
 
+  stty now supports the -I, --immediate option, to apply settings
+  without waiting for transmission of any pending output.
+
 ** Changes in behavior
 
   base64 no longer supports hex or oct --wrap parameters,
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 62955a508..85477a05a 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -13808,6 +13808,18 @@ Print all current settings in a form that can be used as an argument to
 another @command{stty} command to restore the current settings.  This option
 may not be used in combination with any line settings.
 
+@item -I
+@itemx --immediate
+@opindex -I
+@opindex --immediate
+@cindex nonblocking @command{stty} setting
+Apply settings without first waiting for pending output to be transmitted.
+In some cases the system may be in a state where serial transmission
+is not possible.
+For example, if the system has received the @samp{DC3} character
+with @code{ixon} (software flow control) enabled, then @command{stty} would
+block without this option.
+
 @end table
 
 Many settings can be turned off by preceding them with a @samp{-}.
diff --git a/src/stty.c b/src/stty.c
index 37fbb25be..93f36de35 100644
--- a/src/stty.c
+++ b/src/stty.c
@@ -469,6 +469,7 @@ static struct option const longopts[] =
   {"all", no_argument, NULL, 'a'},
   {"save", no_argument, NULL, 'g'},
   {"file", required_argument, NULL, 'F'},
+  {"immediate", no_argument, NULL, 'I'},
   {GETOPT_HELP_OPTION_DECL},
   {GETOPT_VERSION_OPTION_DECL},
   {NULL, 0, NULL, 0}
@@ -522,7 +523,7 @@ usage (int status)
   else
     {
       printf (_("\
-Usage: %s [-F DEVICE | --file=DEVICE] [SETTING]...\n\
+Usage: %s [-F DEVICE | --file=DEVICE] [-I] [SETTING]...\n\
   or:  %s [-F DEVICE | --file=DEVICE] [-a|--all]\n\
   or:  %s [-F DEVICE | --file=DEVICE] [-g|--save]\n\
 "),
@@ -538,6 +539,9 @@ Print or change terminal characteristics.\n\
   -g, --save         print all current settings in a stty-readable form\n\
   -F, --file=DEVICE  open and use the specified DEVICE instead of stdin\n\
 "), stdout);
+      fputs (_("\
+  -I, --immediate    apply setting without waiting for pending transmission\n\
+"), stdout);
       fputs (HELP_OPTION_DESCRIPTION, stdout);
       fputs (VERSION_OPTION_DESCRIPTION, stdout);
       fputs (_("\
@@ -1080,6 +1084,7 @@ main (int argc, char **argv)
   bool noargs = true;
   char *file_name = NULL;
   const char *device_name;
+  int tcsetattr_options = TCSADRAIN;
 
   initialize_main (&argc, &argv);
   set_program_name (argv[0]);
@@ -1103,7 +1108,7 @@ main (int argc, char **argv)
      stty parses options, be sure it still works with combinations of
      short and long options, --, POSIXLY_CORRECT, etc.  */
 
-  while ((optc = getopt_long (argc - argi, argv + argi, "-agF:",
+  while ((optc = getopt_long (argc - argi, argv + argi, "-agF:I",
                               longopts, NULL))
          != -1)
     {
@@ -1125,6 +1130,10 @@ main (int argc, char **argv)
           file_name = optarg;
           break;
 
+        case 'I':
+          tcsetattr_options = TCSANOW;
+          break;
+
         case_GETOPT_HELP_CHAR;
 
         case_GETOPT_VERSION_CHAR (PROGRAM_NAME, AUTHORS);
@@ -1361,7 +1370,7 @@ main (int argc, char **argv)
          spurious difference in an uninitialized portion of the structure.  */
       static struct termios new_mode;
 
-      if (tcsetattr (STDIN_FILENO, TCSADRAIN, &mode))
+      if (tcsetattr (STDIN_FILENO, tcsetattr_options, &mode))
         error (EXIT_FAILURE, errno, "%s", quotef (device_name));
 
       /* POSIX (according to Zlotnick's book) tcsetattr returns zero if
diff --git a/tests/misc/stty.sh b/tests/misc/stty.sh
index 666ac0b26..88d5796cf 100755
--- a/tests/misc/stty.sh
+++ b/tests/misc/stty.sh
@@ -35,6 +35,9 @@ stty $(cat $saved_state) || fail=1
 # This would segfault prior to sh-utils-2.0j.
 stty erase - || fail=1
 
+# Ensure --immediate mode is supported
+stty -I erase - || fail=1
+
 # These would improperly ignore invalid options through coreutils 5.2.1.
 returns_ 1 stty -F 2>/dev/null || fail=1
 returns_ 1 stty -raw -F no/such/file 2>/dev/null || fail=1