writev: retract the topic until we have a better emulation

The emulation layer we added for writev(3p) tries to be too faithful
to the spec that on systems with SSIZE_MAX set to lower than 64kB to
fit a single sideband packet would fail just like the real system
writev(), which makes our use of writev() for sideband messages
unworkable.

Let's revert them and reboot the effort after the release.  The
reverted commits are:

    $ git log -Swritev --oneline 8023abc^..v2.52.0-rc1
    89152af cmake: use writev(3p) wrapper as needed
    26986f4 sideband: use writev(3p) to send pktlines
    1970fce wrapper: introduce writev(3p) wrappers
    3b9b2c2 compat/posix: introduce writev(3p) wrapper

8023abc is the merge of ps/upload-pack-buffer-more-writes topic to
the mainline.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

Makefile

@@ -2029,10 +2029,6 @@ ifdef NO_PREAD
 	COMPAT_CFLAGS += -DNO_PREAD
 	COMPAT_OBJS += compat/pread.o
 endif
-ifdef NO_WRITEV
-	COMPAT_CFLAGS += -DNO_WRITEV
-	COMPAT_OBJS += compat/writev.o
-endif
 ifdef NO_FAST_WORKING_DIRECTORY
 	BASIC_CFLAGS += -DNO_FAST_WORKING_DIRECTORY
 endif

compat/posix.h

@@ -137,9 +137,6 @@
 #include <sys/socket.h>
 #include <sys/ioctl.h>
 #include <sys/statvfs.h>
-#ifndef NO_WRITEV
-#include <sys/uio.h>
-#endif
 #include <termios.h>
 #ifndef NO_SYS_SELECT_H
 #include <sys/select.h>
@@ -326,17 +323,6 @@ int git_lstat(const char *, struct stat *);
 ssize_t git_pread(int fd, void *buf, size_t count, off_t offset);
 #endif
 
-#ifdef NO_WRITEV
-#define writev git_writev
-#define iovec git_iovec
-struct git_iovec {
-	void *iov_base;
-	size_t iov_len;
-};
-
-ssize_t git_writev(int fd, const struct iovec *iov, int iovcnt);
-#endif
-
 #ifdef NO_SETENV
 #define setenv gitsetenv
 int gitsetenv(const char *, const char *, int);

compat/writev.c

@@ -459,7 +459,6 @@ ifeq ($(uname_S),Windows)
 	SANE_TOOL_PATH ?= $(msvc_bin_dir_msys)
 	HAVE_ALLOCA_H = YesPlease
 	NO_PREAD = YesPlease
-	NO_WRITEV = YesPlease
 	NEEDS_CRYPTO_WITH_SSL = YesPlease
 	NO_LIBGEN_H = YesPlease
 	NO_POLL = YesPlease
@@ -675,7 +674,6 @@ ifeq ($(uname_S),MINGW)
 	pathsep = ;
 	HAVE_ALLOCA_H = YesPlease
 	NO_PREAD = YesPlease
-	NO_WRITEV = YesPlease
 	NEEDS_CRYPTO_WITH_SSL = YesPlease
 	NO_LIBGEN_H = YesPlease
 	NO_POLL = YesPlease

config.mak.uname

@@ -376,7 +376,7 @@ endif()
 #function checks
 set(function_checks
 	strcasestr memmem strlcpy strtoimax strtoumax strtoull
-	setenv mkdtemp poll pread memmem writev)
+	setenv mkdtemp poll pread memmem)
 
 #unsetenv,hstrerror are incompatible with windows build
 if(NOT WIN32)
@@ -421,10 +421,6 @@ if(NOT HAVE_MEMMEM)
 	list(APPEND compat_SOURCES compat/memmem.c)
 endif()
 
-if(NOT HAVE_WRITEV)
-	list(APPEND compat_SOURCES compat/writev.c)
-endif()
-
 if(NOT WIN32)
 	if(NOT HAVE_UNSETENV)
 		list(APPEND compat_SOURCES compat/unsetenv.c)

contrib/buildsystems/CMakeLists.txt

@@ -1429,7 +1429,6 @@ checkfuncs = {
   'initgroups' : [],
   'strtoumax' : ['strtoumax.c', 'strtoimax.c'],
   'pread' : ['pread.c'],
-  'writev' : ['writev.c'],
 }
 
 if host_machine.system() == 'windows'

meson.build

@@ -264,7 +264,6 @@ void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_ma
 	const char *p = data;
 
 	while (sz) {
-		struct iovec iov[2];
 		unsigned n;
 		char hdr[5];
 
@@ -274,19 +273,12 @@ void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_ma
 		if (0 <= band) {
 			xsnprintf(hdr, sizeof(hdr), "%04x", n + 5);
 			hdr[4] = band;
-			iov[0].iov_base = hdr;
-			iov[0].iov_len = 5;
+			write_or_die(fd, hdr, 5);
 		} else {
 			xsnprintf(hdr, sizeof(hdr), "%04x", n + 4);
-			iov[0].iov_base = hdr;
-			iov[0].iov_len = 4;
+			write_or_die(fd, hdr, 4);
 		}
-
-		iov[1].iov_base = (void *) p;
-		iov[1].iov_len = n;
-
-		writev_or_die(fd, iov, ARRAY_SIZE(iov));
-
+		write_or_die(fd, p, n);
 		p += n;
 		sz -= n;
 	}

sideband.c

@@ -323,47 +323,6 @@ ssize_t write_in_full(int fd, const void *buf, size_t count)
 	return total;
 }
 
-ssize_t writev_in_full(int fd, struct iovec *iov, int iovcnt)
-{
-	ssize_t total_written = 0;
-
-	while (iovcnt) {
-		ssize_t bytes_written = writev(fd, iov, iovcnt);
-		if (bytes_written < 0) {
-			if (errno == EINTR || errno == EAGAIN)
-				continue;
-			return -1;
-		}
-		if (!bytes_written) {
-			errno = ENOSPC;
-			return -1;
-		}
-
-		total_written += bytes_written;
-
-		/*
-		 * We first need to discard any iovec entities that have been
-		 * fully written.
-		 */
-		while (iovcnt && (size_t)bytes_written >= iov->iov_len) {
-			bytes_written -= iov->iov_len;
-			iov++;
-			iovcnt--;
-		}
-
-		/*
-		 * Finally, we need to adjust the last iovec in case we have
-		 * performed a partial write.
-		 */
-		if (iovcnt && bytes_written) {
-			iov->iov_base = (char *) iov->iov_base + bytes_written;
-			iov->iov_len -= bytes_written;
-		}
-	}
-
-	return total_written;
-}
-
 ssize_t pread_in_full(int fd, void *buf, size_t count, off_t offset)
 {
 	char *p = buf;

wrapper.c

@@ -47,15 +47,6 @@ ssize_t read_in_full(int fd, void *buf, size_t count);
 ssize_t write_in_full(int fd, const void *buf, size_t count);
 ssize_t pread_in_full(int fd, void *buf, size_t count, off_t offset);
 
-/*
- * Try to write all iovecs. Returns -1 in case an error occurred with a proper
- * errno set, the number of bytes written otherwise.
- *
- * Note that the iovec will be modified as a result of this call to adjust for
- * partial writes!
- */
-ssize_t writev_in_full(int fd, struct iovec *iov, int iovcnt);
-
 static inline ssize_t write_str_in_full(int fd, const char *str)
 {
 	return write_in_full(fd, str, strlen(str));

wrapper.h

@@ -96,14 +96,6 @@ void write_or_die(int fd, const void *buf, size_t count)
 	}
 }
 
-void writev_or_die(int fd, struct iovec *iov, int iovlen)
-{
-	if (writev_in_full(fd, iov, iovlen) < 0) {
-		check_pipe(errno);
-		die_errno("writev error");
-	}
-}
-
 void fwrite_or_die(FILE *f, const void *buf, size_t count)
 {
 	if (fwrite(buf, 1, count, f) != count)