command -x: fix efficiency; always run external cmd (re: acf84e96)

This commit fixes 'command -x' to adapt to OS limitations with
regards to data alignment in the arguments list. A feature test is
added that detects if the OS aligns the argument on 32-bit or
64-bit boundaries or not at all, allowing 'command -x' to avoid
E2BIG errors while maximising efficiency.

Also, as of now, 'command -x' is a way to bypass built-ins and
run/query an external command. Built-ins do not limit the length of
their argument list, so '-x' never made sense to use for them. And
because '-x' hangs on Linux and macOS on every ksh93 release
version to date (see acf84e96), few use it, so there is little
reason not to make this change.

Finally, this fixes a longstanding bug that caused the minimum exit
status of 'command -x' to be 1 if a command with many arguments was
divided into several command invocations. This is done by replacing
broken flaggery with a new SH_XARG state flag bit.

src/cmd/ksh93/features/externs:
- Add new C feature test detecting byte alignment in args list.
  The test writes a #define ARG_ALIGN_BYTES with the amount of
  bytes the OS aligns arguments to, or zero for no alignment.

src/cmd/ksh93/include/defs.h:
- Add new SH_XARG state bit indicating 'command -x' is active.

src/cmd/ksh93/sh/path.c: path_xargs():
- Leave extra 2k in the args buffer instead of 1k, just to be sure;
  some commands add large environment variables these days.
- Fix a bug in subtracting the length of existing arguments and
  environment variables. 'size -= strlen(cp)-1;' subtracts one less
  than the size of cp, which makes no sense; what is necessary is
  to substract the length plus one to account for the terminating
  zero byte, i.e.: 'size -= strlen(cp)+1'.
- Use the ARG_ALIGN_BYTES feature test result to match the OS's
  data alignment requirements.
- path_spawn(): E2BIG: Change to checking SH_XARG state bit.

src/cmd/ksh93/bltins/whence.c: b_command():
- Allow combining -x with -p, -v and -V with the expected results
  by setting P_FLAG to act like 'whence -p'. E.g., as of now,
	command -xv printf
  is equivalent to
	whence -p printf
  but note that 'whence' has no equivalent of 'command -pvx printf'
  which searches $(getconf PATH) for a command.
- When -x will run a command, now set the new SH_XARG state flag.

src/cmd/ksh93/sh/xec.c: sh_exec():
- Change to using the new SH_XARG state bit.
- Skip the check for built-ins if SH_XARG is active, so that
  'command -x' now always runs an external command.

src/lib/libcmd/date.c, src/lib/libcmd/uname.c:
- These path-bound builtins sometimes need to run the external
  system command by the same name, but they did that by hardcoding
  an unportable direct path. Now that 'command -x' runs an external
  command, change this to using 'command -px' to guarantee using
  the known-good external system utility in the default PATH.
- In date.c, fix the format string passed to 'command -px date'
  when setting the date; it was only compatible with BSD systems.
  Use the POSIX variant on non-BSD systems.

src/lib/libcmd/date.c

@@ -218,37 +218,38 @@ typedef struct Fmt
 static int
 settime(Shbltin_t* context, const char* cmd, Time_t now, int adjust, int network)
 {
-	char*		s;
 	char**		argv;
-	char*		args[5];
+	char*		args[7];
 	char		buf[1024];
 
 	if (!adjust && !network)
 		return tmxsettime(now);
 	argv = args;
-	s = "/usr/bin/date";
-	if (!streq(cmd, s) && (!eaccess(s, X_OK) || !eaccess(s+=4, X_OK)))
+	*argv++ = "command";
+	*argv++ = "-px";
+	*argv++ = "date";
+	if (streq(astconf("UNIVERSE", NiL, NiL), "att"))
 	{
-		*argv++ = s;
-		if (streq(astconf("UNIVERSE", NiL, NiL), "att"))
-		{
-			tmxfmt(buf, sizeof(buf), "%m%d%H" "%M%Y.%S", now);
-			if (adjust)
-				*argv++ = "-a";
-		}
-		else
-		{
-			tmxfmt(buf, sizeof(buf), "%Y%m%d%H" "%M.%S", now);
-			if (network)
-				*argv++ = "-n";
-			if (tm_info.flags & TM_UTC)
-				*argv++ = "-u";
-		}
-		*argv++ = buf;
-		*argv = 0;
-		if (!sh_run(context, argv - args, args))
-			return 0;
+		tmxfmt(buf, sizeof(buf), "%m%d%H" "%M%Y.%S", now);
+		if (adjust)
+			*argv++ = "-a";
 	}
+	else
+	{
+#if __FreeBSD__ || __NetBSD__ || __OpenBSD__ || __bsdi__ || __DragonFly__
+		tmxfmt(buf, sizeof(buf), "%Y%m%d%H" "%M.%S", now);
+#else
+		tmxfmt(buf, sizeof(buf), "%m%d%H" "%M%Y.%S", now);
+#endif
+		if (network)
+			*argv++ = "-n";
+		if (tm_info.flags & TM_UTC)
+			*argv++ = "-u";
+	}
+	*argv++ = buf;
+	*argv = 0;
+	if (!sh_run(context, argv - args, args))
+		return 0;
 	return -1;
 }
 

src/lib/libcmd/uname.c

@@ -37,7 +37,7 @@ USAGE_LICENSE
 "	separated, on a single line. When more than one option is specified"
 "	the output is in the order specified by the \b-A\b option below."
 "	Unsupported option values are listed as \a[option]]\a. If any unknown"
-"	options are specified then the local \b/usr/bin/uname\b is called.]"
+"	options are specified, the OS default \buname\b(1) is called.]"
 "[+?If any \aname\a operands are specified then the \bsysinfo\b(2) values"
 "	for each \aname\a are listed, separated by space, on one line."
 "	\bgetconf\b(1), a pre-existing \astandard\a interface, provides"
@@ -325,14 +325,14 @@ b_uname(int argc, char** argv, Shbltin_t* context)
 			sethost = opt_info.arg;
 			continue;
 		case ':':
-			s = "/usr/bin/uname";
-			if (!streq(argv[0], s) && (!eaccess(s, X_OK) || !eaccess(s+=4, X_OK)))
 			{
-				argv[0] = s;
-				return sh_run(context, argc, argv);
+				char **new_argv = (char **)stakalloc((argc + 3) * sizeof(char*));
+				new_argv[0] = "command";
+				new_argv[1] = "-px";
+				for (n = 0; n <= argc; n++)
+					new_argv[n + 2] = argv[n];
+				return sh_run(context, argc + 2, new_argv);
 			}
-			error(2, "%s", opt_info.arg);
-			break;
 		case '?':
 			error(ERROR_usage(2), "%s", opt_info.arg);
 			break;