Cygwin: workaround for ksh to execute #!-less scripts with itself

On Cygwin, ksh does not execute scripts without a #! path in a fork
of the ksh process as it does on other systems. Reproducer (run
from ksh):

  $ cat test.sh
  echo "${BASH_VERSION:-not bash}"
  echo "${.sh.version}"
  $ chmod +x test.sh
  $ ./test.sh
  4.4.12(3)-release
  ./test.sh: line 2: ${.sh.version}: bad substitution

The script was executed in bash instead of ksh.

After this fix, the output on Cygwin is like ksh on other systems:

  not bash
  Version AJM 93u+m/1.1.0-alpha+dev 2022-01-26

This also fixes a number of regression test failures, as quite a
few tests create and execute temp scripts without a hashbang path.

Analysis: On Cygwin, execve(2) happily executes shell scripts
without a #! path with /bin/sh (which is bash --posix). However,
ksh relies on execve(2) executing binaries or #! only, as it uses
an ENOEXEC failure to decide whether to fork and execute a #!-less
shell script with a reinitialized copy of itself via exscript().

src/cmd/ksh93/sh/path.c: path_spawn():
- Look at the magic first two bytes of the file; if it is "MZ"
  (Mark Zbikowski, originator of the .exe format) or "#!", continue
  as normal, otherwise simulate an ENOEXEC failure from execve(2)
  which will cause ksh to fall back on #!-less script execution.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-01-26:
+
+- On Cygwin, ksh now executes scripts that do not have a #! path itself,
+  like it does on other systems, instead of with /bin/sh.
+
 2022-01-24:
 
 - Fixed a crashing bug in history expansion that could occur when using the "&"

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-beta.2"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2022-01-24"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2022-01-26"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/path.c

@@ -2,7 +2,7 @@
 *                                                                      *
 *               This software is part of the ast package               *
 *          Copyright (c) 1982-2012 AT&T Intellectual Property          *
-*          Copyright (c) 2020-2021 Contributors to ksh 93u+m           *
+*          Copyright (c) 2020-2022 Contributors to ksh 93u+m           *
 *                      and is licensed under the                       *
 *                 Eclipse Public License, Version 1.0                  *
 *                    by AT&T Intellectual Property                     *
@@ -1214,6 +1214,28 @@ pid_t path_spawn(const char *opath,register char **argv, char **envp, Pathcomp_t
 		path = sp;
 	}
 #endif /* SHELLMAGIC */
+#if __CYGWIN__
+	/*
+	 * On Cygwin, execve(2) happily executes shell scripts without a #! path with /bin/sh (which is bash --posix).
+	 * However, ksh relies on execve(2) executing binaries or #! only, as it uses an ENOEXEC failure to decide
+	 * whether to fork and execute a #!-less shell script with a reinitialized copy of itself via exscript() below.
+	 * So, simulate that failure if the file is not a Windows executable or a script with a #! path.
+	 */
+	if((n = sh_open(opath,O_RDONLY,0)) >= 0)
+	{
+		uint16_t mz;
+		r = !(read(n,&mz,2)==2 && (mz==0x5A4D || mz==0x2123));  /* "MZ" or "#!" */
+		sh_close(n);
+	}
+	else
+		r = 0;
+	if(r)
+	{
+		pid = -1;
+		errno = ENOEXEC;
+	}
+	else
+#endif
 #if SHOPT_PFSH
 	if(spawn && !sh_isoption(SH_PFSH))
 		pid = _spawnveg(opath, &argv[0], envp, spawn>>1);