Invocation: fix infinite loop on 'ksh +s'

When starting ksh +s, it gets stuck in an infinite loop continually
trying to parse its own binary as a shell script and rejecting it:

$ arch/linux.i386-64/bin/ksh +s
arch/linux.i386-64/bin/ksh: arch/linux.i386-64/bin/ksh: cannot execute [Exec format error]
arch/linux.i386-64/bin/ksh: arch/linux.i386-64/bin/ksh: cannot execute [Exec format error]
arch/linux.i386-64/bin/ksh: arch/linux.i386-64/bin/ksh: cannot execute [Exec format error]
arch/linux.i386-64/bin/ksh: arch/linux.i386-64/bin/ksh: cannot execute [Exec format error]
arch/linux.i386-64/bin/ksh: arch/linux.i386-64/bin/ksh: cannot execute [Exec format error]
[...]
$ echo 'echo "this is stdin"' | arch/linux.i386-64/bin/ksh +s
arch/linux.i386-64/bin/ksh: arch/linux.i386-64/bin/ksh: cannot execute [Exec format error]
(no loop, but still ksh trying to parse itself)

src/cmd/ksh93/sh/init.c: sh_init():
- When forcing on the '-s' option upon finding no command
  arguments, also update sh.offoptions, a.k.a. shp->offoptions.
  This avoids the inconsistent state causing this problem.

  In main.c, there is:

  if(sh_isoption(SH_SFLAG))
      fdin = 0;
  else
      (code to open $0 as a file)

  This was entering the else block because sh_isoption(SH_SFLAG)
  was returning 0, and $0 is set to the ksh binary as it is
  supposed to when no other script is provided. When I looked for
  why sh_isoption was returning 0, I found main.c's

  for(i=0; i<elementsof(shp->offoptions.v); i++)
      shp->options.v[i] &= ~shp->offoptions.v[i];

  Before this loop, shp->offoptions tracks which options were
  explicitly disabled by the user on the command line. The effect
  of this loop is to make "explicitly disabled" take precedence
  over "implicitly enabled". My patch removes the registration of
  the +s option.

Fixes: https://github.com/ksh93/ksh/issues/150
Co-authored-by: Martijn Dekker <martijn@inlv.org>

NEWS

@@ -3,6 +3,14 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-01-03:
+
+- The invocation
+	$ ksh +s
+  caused an infinite loop and corrupted ~/.sh_history. This is now fixed so
+  that the '-s' option is automatically turned on if there are no non-option
+  command arguments, as documented in Bolsky & Korn (1995), p. 261.
+
 2020-10-22:
 
 - Fixed: 'typeset -F0', 'typeset -E0', and 'typeset -X0' floating point

src/cmd/ksh93/data/builtins.c

@@ -1522,7 +1522,7 @@ USAGE_LICENSE
 	"[+-?Executing \bcommand -p\b \a...\a .]"
 	"}"
 "[s?Read the commands from standard input.  The positional parameters will be "
-	"initialized from \aarg\a.]"
+	"initialized from \aarg\a. This option is forced on if no \aarg\a is given.]"
 "[D\f:dump-strings\f?Do not execute the script, but output the set of double "
 	"quoted strings preceded by a \b$\b.  These strings are needed for "
 	"localization of the script to different locales.]"

src/cmd/ksh93/include/defs.h

@@ -264,7 +264,7 @@ struct shared
 	Dt_t		*transdict; \
 	char		ifstable[256]; \
 	unsigned long	test; \
-	Shopt_t		offoptions; \
+	Shopt_t		offoptions;	/* options that were explicitly disabled by the user on the command line */ \
 	Shopt_t		glob_options; \
 	Namval_t	*typeinit; \
 	Namfun_t	nvfun; \

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-10-22"
+#define SH_RELEASE	"93u+m 2021-01-03"

src/cmd/ksh93/sh/init.c

@@ -1355,7 +1355,10 @@ Shell_t *sh_init(register int argc,register char *argv[], Shinit_f userinit)
 		shp->st.dolv=argv+(argc-1)-shp->st.dolc;
 		shp->st.dolv[0] = argv[0];
 		if(shp->st.dolc < 1)
+		{
 			sh_onoption(SH_SFLAG);
+			off_option(&shp->offoptions,SH_SFLAG);
+		}
 		if(!sh_isoption(SH_SFLAG))
 		{
 			shp->st.dolc--;