posix: use real pipe(2) instead of socketpair(2)

The POSIX standard requires real UNIX pipes as in pipe(2). But on
systems supporting it (all modern ones), ksh uses socketpair(2)
instead to make it possible for certain commands to peek ahead
without consuming input from the pipe, which is not possible with
real pipes. See features/poll and sh/io.c.

But this can create undesired side effects: applications connected
to a pipe may test if they are connected to a pipe, which will fail
if they are connected to a socket. Also, on Linux:

  $ cat /etc/passwd | head -1 /dev/stdin
  head: cannot open '/dev/stdin' for reading: No such device or address

...which happens because, unlike most systems, Linux cannot open(2)
or openat(2) a socket (a limitation that is allowed by POSIX).

Unfortunately at least two things depend on the peekahead
capability of the _pipe_socketpair feature. One is the non-blocking
behaviour of the -n option of the 'read' built-in:

  -n      Causes at most n bytes to be  read  instead  of  a  full
          line, but will return when reading from a slow device as
          soon as any characters have been read.

The other thing that breaks is the <#pattern and <##pattern
redirection operators that basically grep standard input, which
inherently requires peekahead.

Standard UNIX pipes always block on read and it is not possible to
peek ahead, so these features inevitably break. Which means we
cannot simply use standard pipes without breaking compatibility.

But we can at least fix it in the POSIX mode so that cross-platform
scripts work more correctly.

src/cmd/ksh93/sh/io.c: sh_pipe():
- If _pipe_socketpair is detected at compile time, then use a real
  pipe via sh_rpipe() if the POSIX mode is active. (If
  _pipe_socketpair is not detected, a real pipe is always used.)

src/cmd/ksh93/data/builtins.c:
- sh.1 documents the slow-device behaviour of -n but 'read --man'
  did not. Add that, making it conditional on _pipe_socketpair.

Resolves: https://github.com/ksh93/ksh/issues/327

NEWS

@@ -3,6 +3,14 @@ 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.
 
+2022-06-09:
+
+- The POSIX mode has been amended to use a UNIX pipe(2) instead of a
+  socketpair(2) to connect commands in a pipeline, as the standard requires.
+  (When reading directly from a pipeline in posix mode, the <#pattern and
+  <##pattern redirection operators will not work and the -n option to the
+  read built-in will not return early when reading from a slow device.)
+
 2022-06-08:
 
 - If -B/--braceexpand is turned on in --posix mode, it now only allows brace

src/cmd/ksh93/Mamfile

@@ -1281,6 +1281,7 @@ make install
 			done aliases.o generated
 			make builtins.o
 				make data/builtins.c
+					prev FEATURE/poll implicit
 					prev FEATURE/cmds implicit
 					prev include/jobs.h implicit
 					prev include/builtins.h implicit

src/cmd/ksh93/data/builtins.c

@@ -29,6 +29,7 @@
 #include	"builtins.h"
 #include	"jobs.h"
 #include	"FEATURE/cmds"
+#include	"FEATURE/poll"
 #define	bltin(x)	(b_##x)
 /* The following is for builtins that do not accept -- options */
 #define	Bltin(x)	(B_##x)
@@ -1407,7 +1408,7 @@ const char sh_optpwd[] =
 ;
 
 const char sh_optread[] =
-"[-1c?\n@(#)$Id: read (ksh 93u+m) 2022-02-16 $\n]"
+"[-1c?\n@(#)$Id: read (ksh 93u+m) 2022-06-09 $\n]"
 "[--catalog?" SH_DICT "]"
 "[+NAME?read - read a line from standard input]"
 "[+DESCRIPTION?\bread\b reads a line from standard input and breaks it "
@@ -1433,7 +1434,13 @@ const char sh_optread[] =
 	"the line starting at index 0.]"
 "[C?Unset \avar\a and read  \avar\a as a compound variable.]"
 "[d]:[delim?Read until delimiter \adelim\a instead of to the end of line.]"
-"[n]#[count?Read at most \acount\a characters or (for binary fields) bytes.]"
+"[n]#[count?Read at most \acount\a characters or (for binary fields) bytes."
+#if _pipe_socketpair
+	" When reading from a slow device, "
+	"will return as soon as any characters have been read, "
+	"unless the \bposix\b shell option is on."
+#endif
+	"]"
 "[N]#[count?Read exactly \acount\a characters or (for binary fields) bytes.]"
 "[p?Read from the current co-process instead of standard input. "
 	"An end-of-file causes \bread\b to disconnect the co-process "

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	"2022-06-08"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2022-06-09"	/* 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.1

@@ -184,6 +184,12 @@ separated by
 .BR | .
 The standard output of each command but the last
 is connected by a
+.IR socketpair (2)
+or
+(if the
+.B posix
+shell option is on)
+by a
 .IR pipe (2)
 to the standard input of the next command.
 Each command,
@@ -7723,6 +7729,21 @@ disables fast filescan loops of type
 makes the \fB<>\fR redirection operator default to redirecting standard input
 if no file descriptor number precedes it;
 .IP \[bu]
+causes the shell to use a standard UNIX
+.IR pipe (2)
+instead of a
+.IR socketpair (2)
+to connect commands in a pipeline
+(when reading directly from a pipeline, the
+.BI <# pattern
+and
+.BI <## pattern
+redirection operators will not work and the
+.B -n
+option to the
+.B read
+built-in will not return early when reading from a slow device);
+.IP \[bu]
 disables the special floating point constants \fBInf\fR and \fBNaN\fR in
 arithmetic evaluation so that, e.g., \fB$((inf))\fR and \fB$((nan))\fR refer
 to the variables by those names;

src/cmd/ksh93/sh/io.c

@@ -913,6 +913,10 @@ int sh_iomovefd(register int fdold)
 int	sh_pipe(register int pv[])
 {
 	int fd[2];
+#ifdef pipe
+	if(sh_isoption(SH_POSIX))
+		return(sh_rpipe(pv));
+#endif
 	if(pipe(fd)<0 || (pv[0]=fd[0])<0 || (pv[1]=fd[1])<0)
 	{
 		errormsg(SH_DICT,ERROR_system(1),e_pipe);