49694 + doc: Allow using empty STTY= to freeze tty for a single command

Previously, doing this would just run stty with no arguments, which
normally causes it to print some terminal settings to stdout.

ChangeLog

@@ -1,3 +1,8 @@
+2022-01-30  Mikael Magnusson  <mikachu@gmail.com>
+
+	* 49694 + doc: Doc/Zsh/builtins.yo, Doc/Zsh/params.yo, Src/exec.c:
+	Allow using empty STTY= to freeze tty for a single command
+
 2022-01-29  Daniel Shahaf  <d.s@daniel.shahaf.name>
 
 	* unposted: Functions/VCS_Info/test-repo-git-rebase-apply,

Doc/Zsh/builtins.yo

@@ -1885,7 +1885,8 @@ unfreezing the tty does not guarantee settings made on the
 command line are preserved.  Strings of commands run between
 editing the command line will see a consistent tty state.
 See also the shell variable tt(STTY) for a means of initialising
-the tty before running external commands.
+the tty before running external commands and/or freezing the tty
+around a single command.
 )
 findex(type)
 item(tt(type) [ tt(-wfpamsS) ] var(name) ...)(

Doc/Zsh/params.yo

@@ -1570,6 +1570,11 @@ if it is in the environment of the shell but not explicitly assigned to in
 the input line. This avoids running stty at every external command by
 accidentally exporting it. Also note that tt(STTY) should not be used for
 window size specifications; these will not be local to the command.
+
+If the parameter is set and empty, all of the above applies except
+that tt(stty) is not run. This can be useful as a way to freeze the tty
+around a single command, blocking its changes to tty settings,
+similar to the tt(ttyctl) builtin.
 )
 vindex(TERM)
 item(tt(TERM) <S>)(

Src/exec.c

@@ -684,8 +684,10 @@ execute(LinkList args, int flags, int defpath)
 
     /* If the parameter STTY is set in the command's environment, *
      * we first run the stty command with the value of this       *
-     * parameter as it arguments.                                 */
-    if ((s = STTYval) && isatty(0) && (GETPGRP() == getpid())) {
+     * parameter as it arguments. If the parameter is empty, we   *
+     * do nothing, but this causes the terminal settings to be    *
+     * restored later which can be useful.                        */
+    if ((s = STTYval) && *s && isatty(0) && (GETPGRP() == getpid())) {
 	char *t = tricat("stty", " ", s);
 
 	STTYval = 0;	/* this prevents infinite recursion */