From: Junio C Hamano Date: Wed, 04 Mar 2026 22:01:18 GMT Subject: Re: [PATCH v2 05/10] compat/posix: introduce writev(3p) wrapper Message-ID: In-Reply-To: <20260303-pks-upload-pack-write-contention-v2-5-7321830f08fe@pks.im> Patrick Steinhardt writes: > In a subsequent commit we're going to add the first caller to > writev(3p). Introduce a compatibility wrapper for this syscall that we > can use on systems that don't have this syscall. > > The syscall exists on modern Unixes like Linux and macOS, and seemingly > even for NonStop according to [1]. It doesn't seem to exist on Windows > though. > > [1]: http://nonstoptools.com/manuals/OSS-SystemCalls.pdf > [2]: https://www.gnu.org/software/gnulib/manual/html_node/writev.html > > Signed-off-by: Patrick Steinhardt > --- > Makefile | 4 ++++ > compat/posix.h | 14 ++++++++++++++ > compat/writev.c | 29 +++++++++++++++++++++++++++++ > config.mak.uname | 2 ++ > meson.build | 1 + > 5 files changed, 50 insertions(+) > > diff --git a/Makefile b/Makefile > index 7f37ad8f58..cb95ff2daf 100644 > --- a/Makefile > +++ b/Makefile > @@ -2021,6 +2021,10 @@ 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 > diff --git a/compat/posix.h b/compat/posix.h > index 245386fa4a..3c611d2736 100644 > --- a/compat/posix.h > +++ b/compat/posix.h > @@ -137,6 +137,9 @@ > #include > #include > #include > +#ifndef NO_WRITEV > +#include > +#endif > #include > #ifndef NO_SYS_SELECT_H > #include > @@ -323,6 +326,17 @@ 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); > diff --git a/compat/writev.c b/compat/writev.c > new file mode 100644 > index 0000000000..b77e534d5d > --- /dev/null > +++ b/compat/writev.c > @@ -0,0 +1,29 @@ > +#include "../git-compat-util.h" > +#include "../wrapper.h" > + > +ssize_t git_writev(int fd, const struct iovec *iov, int iovcnt) > +{ > + size_t total_written = 0; > + > + for (int i = 0; i < iovcnt; i++) { > + const char *bytes = iov[i].iov_base; > + size_t iovec_written = 0; > + > + while (iovec_written < iov[i].iov_len) { > + ssize_t bytes_written = xwrite(fd, bytes + iovec_written, > + iov[i].iov_len - iovec_written); > + if (bytes_written < 0) { > + if (total_written) > + goto out; > + return bytes_written; > + } > + if (!bytes_written) > + goto out; > + iovec_written += bytes_written; > + total_written += bytes_written; > + } > + } > + > +out: > + return cast_size_t_to_ssize_t(total_written); > +} Because we do not check the accumulation of bytes_written in the two accumulator variables inside the inner loop, it is very possible for total_written to wraparound size_t and end up below the largest value possible to be stored in ssize_t type. IOW, the cast_size_t_to_ssize_t() introduced in the previous step is pointless, isn't it? According to [*1*], the real ssize_t writev(int fd, const struct iovec *iov, int iovcnt) is supposed to report error with errno set to EINVAL when the sum of iov_len member of the iov[] array elements exceed half of the maximum size_t. EINVAL The sum of the iov_len values overflows an ssize_t value. So instead of dying with cast_size_t_to_ssize_t(), we probably would want the check done in a more stupid and straight-forward way? Adding up iov[i].iov_len while the addition would not wraparound in each and every step, and return error with EINVAL before attempting even a single call to xwrite(), and then have the above double loop that does not care about integer wraparound at all, and return total_written with simple cast to (ssize_t)? [Reference] *1* https://pubs.opengroup.org/onlinepubs/9799919799/functions/writev.html > diff --git a/config.mak.uname b/config.mak.uname > index 5feb582558..ccb3f71881 100644 > --- a/config.mak.uname > +++ b/config.mak.uname > @@ -459,6 +459,7 @@ ifeq ($(uname_S),Windows) > SANE_TOOL_PATH ?= $(msvc_bin_dir_msys) > HAVE_ALLOCA_H = YesPlease > NO_PREAD = YesPlease > + NO_WRITEV = YesPlease > NEEDS_CRYPTO_WITH_SSL = YesPlease > NO_LIBGEN_H = YesPlease > NO_POLL = YesPlease > @@ -674,6 +675,7 @@ ifeq ($(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 > diff --git a/meson.build b/meson.build > index 762e2d0fc0..63514b6b84 100644 > --- a/meson.build > +++ b/meson.build > @@ -1409,6 +1409,7 @@ checkfuncs = { > 'initgroups' : [], > 'strtoumax' : ['strtoumax.c', 'strtoimax.c'], > 'pread' : ['pread.c'], > + 'writev' : ['writev.c'], > } > > if host_machine.system() == 'windows'