Simplify replacement code for preadv and pwritev.

preadv() and pwritev() are not standardized by POSIX, but appeared in
NetBSD in 1999 and were adopted by at least OpenBSD, FreeBSD,
DragonFlyBSD, Linux, AIX, illumos and macOS.  We don't use them much
yet, but an active proposal uses them heavily.

In 15, we had two replacement implementations for other OSes: one based
on lseek() + -v function if available for true vector I/O, and the other
based on a loop over p- function.

The former would be an obstacle to hypothetical future multi-threaded
code sharing file descriptors, while the latter would not, since commit
cf112c12.  Furthermore, the number of targeted systems that could
benefit from the former's potential upside has dwindled to just one
niche OS, since macOS added the functions and we de-supported HP-UX.
That doesn't seem like a good trade-off.

Therefore, drop the lseek()-based variant, and also the pg_ prefix now
that the file position portability hazard is gone.

At the time of writing, the only systems in our build farm that lack
native preadv/pwritev and thus use fallback code are:

 * Solaris (but not illumos)
 * macOS before release 11.0
 * Windows

With this commit, the above systems will now use the *same* fallback
code, the version that loops over pread()/pwrite().  Windows already
used that (though a later proposal may include true vector I/O for
Windows), so this decision really only affects Solaris, until it gets
around to adding these system calls.

Also remove some useless includes while here.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

configure

@@ -16039,7 +16039,7 @@ fi
 LIBS_including_readline="$LIBS"
 LIBS=`echo "$LIBS" | sed -e 's/-ledit//g' -e 's/-lreadline//g'`
 
-for ac_func in backtrace_symbols clock_gettime copyfile fdatasync getifaddrs getpeerucred inet_pton kqueue mbstowcs_l memset_s posix_fallocate ppoll pthread_is_threaded_np readv setproctitle setproctitle_fast strchrnul strsignal syncfs sync_file_range uselocale wcstombs_l writev
+for ac_func in backtrace_symbols clock_gettime copyfile fdatasync getifaddrs getpeerucred inet_pton kqueue mbstowcs_l memset_s posix_fallocate ppoll pthread_is_threaded_np setproctitle setproctitle_fast strchrnul strsignal syncfs sync_file_range uselocale wcstombs_l
 do :
   as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
 ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"

configure.ac

@@ -1803,7 +1803,6 @@ AC_CHECK_FUNCS(m4_normalize([
 	posix_fallocate
 	ppoll
 	pthread_is_threaded_np
-	readv
 	setproctitle
 	setproctitle_fast
 	strchrnul
@@ -1812,7 +1811,6 @@ AC_CHECK_FUNCS(m4_normalize([
 	sync_file_range
 	uselocale
 	wcstombs_l
-	writev
 ]))
 
 # These typically are compiler builtins, for which AC_CHECK_FUNCS fails.

src/backend/storage/file/fd.c

@@ -3762,7 +3762,7 @@ data_sync_elevel(int elevel)
 }
 
 /*
- * A convenience wrapper for pg_pwritev() that retries on partial write.  If an
+ * A convenience wrapper for pwritev() that retries on partial write.  If an
  * error is returned, it is unspecified how much has been written.
  */
 ssize_t
@@ -3782,7 +3782,7 @@ pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 	for (;;)
 	{
 		/* Write as much as we can. */
-		part = pg_pwritev(fd, iov, iovcnt, offset);
+		part = pwritev(fd, iov, iovcnt, offset);
 		if (part < 0)
 			return -1;
 

src/include/pg_config.h.in

@@ -412,9 +412,6 @@
 /* Define to 1 if you have the <readline/readline.h> header file. */
 #undef HAVE_READLINE_READLINE_H
 
-/* Define to 1 if you have the `readv' function. */
-#undef HAVE_READV
-
 /* Define to 1 if you have the `rl_completion_matches' function. */
 #undef HAVE_RL_COMPLETION_MATCHES
 
@@ -643,9 +640,6 @@
 /* Define to 1 if you have the <winldap.h> header file. */
 #undef HAVE_WINLDAP_H
 
-/* Define to 1 if you have the `writev' function. */
-#undef HAVE_WRITEV
-
 /* Define to 1 if you have the `X509_get_signature_nid' function. */
 #undef HAVE_X509_GET_SIGNATURE_NID
 

src/include/port/pg_iovec.h

@@ -39,16 +39,12 @@ struct iovec
 /* Define a reasonable maximum that is safe to use on the stack. */
 #define PG_IOV_MAX Min(IOV_MAX, 32)
 
-#if HAVE_DECL_PREADV
-#define pg_preadv preadv
-#else
-extern ssize_t pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset);
+#if !HAVE_DECL_PREADV
+extern ssize_t preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset);
 #endif
 
-#if HAVE_DECL_PWRITEV
-#define pg_pwritev pwritev
-#else
-extern ssize_t pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset);
+#if !HAVE_DECL_PWRITEV
+extern ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset);
 #endif
 
 #endif							/* PG_IOVEC_H */

src/port/preadv.c

@@ -8,33 +8,19 @@
  * IDENTIFICATION
  *	  src/port/preadv.c
  *
- * Note that this implementation changes the current file position, unlike
- * the POSIX-like function, so we use the name pg_preadv().
- *
  *-------------------------------------------------------------------------
  */
 
 
 #include "postgres.h"
 
-#ifdef WIN32
-#include <windows.h>
-#else
 #include <unistd.h>
-#endif
 
 #include "port/pg_iovec.h"
 
 ssize_t
-pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 {
-#ifdef HAVE_READV
-	if (iovcnt == 1)
-		return pread(fd, iov[0].iov_base, iov[0].iov_len, offset);
-	if (lseek(fd, offset, SEEK_SET) < 0)
-		return -1;
-	return readv(fd, iov, iovcnt);
-#else
 	ssize_t		sum = 0;
 	ssize_t		part;
 
@@ -54,5 +40,4 @@ pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 			return sum;
 	}
 	return sum;
-#endif
 }

src/port/pwritev.c

@@ -8,33 +8,19 @@
  * IDENTIFICATION
  *	  src/port/pwritev.c
  *
- * Note that this implementation changes the current file position, unlike
- * the POSIX-like function, so we use the name pg_pwritev().
- *
  *-------------------------------------------------------------------------
  */
 
 
 #include "postgres.h"
 
-#ifdef WIN32
-#include <windows.h>
-#else
 #include <unistd.h>
-#endif
 
 #include "port/pg_iovec.h"
 
 ssize_t
-pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 {
-#ifdef HAVE_WRITEV
-	if (iovcnt == 1)
-		return pwrite(fd, iov[0].iov_base, iov[0].iov_len, offset);
-	if (lseek(fd, offset, SEEK_SET) < 0)
-		return -1;
-	return writev(fd, iov, iovcnt);
-#else
 	ssize_t		sum = 0;
 	ssize_t		part;
 
@@ -54,5 +40,4 @@ pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 			return sum;
 	}
 	return sum;
-#endif
 }

src/tools/msvc/Solution.pm

@@ -332,7 +332,6 @@ sub GenerateFiles
 		HAVE_READLINE_H             => undef,
 		HAVE_READLINE_HISTORY_H     => undef,
 		HAVE_READLINE_READLINE_H    => undef,
-		HAVE_READV                  => undef,
 		HAVE_RL_COMPLETION_MATCHES  => undef,
 		HAVE_RL_COMPLETION_SUPPRESS_QUOTE        => undef,
 		HAVE_RL_FILENAME_COMPLETION_FUNCTION     => undef,
@@ -408,7 +407,6 @@ sub GenerateFiles
 		HAVE_WINLDAP_H                           => undef,
 		HAVE_WCSTOMBS_L                          => 1,
 		HAVE_VISIBILITY_ATTRIBUTE                => undef,
-		HAVE_WRITEV                              => undef,
 		HAVE_X509_GET_SIGNATURE_NID              => 1,
 		HAVE_X86_64_POPCNTQ                      => undef,
 		HAVE__BOOL                               => undef,