[PATCH v2 0/5] Reintroduce writev(3p)

Hi,

this patch series reintroduces the writev(3p) wrapper. This wrapper was
originally introduced as part of Git 2.54 [1], but was ejected due to
issues on NonStop [2].

This patch series here revives the effort with a couple of fixes on top:

  - It picks Dscho's fix for CMake [3].

  - It picks a fix for NonStop [4] and polishes it a bit.

  - It adapts one more site to demonstrate that its usefulness is not
    limited to a single callsite, only.

Furthermore, I have included benchmarks now that demonstrate the
benefits to make this series a bit more appealing. Ultimately, I'd be
fine if we say we rather don't want to go this way though. I merely
wanted to tie some loose ends that I left dangling.

That, and it's nice to not work on pluggable object databases once in a
while.

Changes in v2:
  - Adapt the writev compatibility shim to not do torn writes anymore
    across multiple iovecs. Instead, we now only write the first iovec,
    which should be fine as callers are expected to loop around writev
    anyway.
  - Link to v1: https://patch.msgid.link/[email protected]

Thanks!

Patrick

[1]: <20260227-pks-upload-pack-write-contention-v1-0-7166fe255704@pks.im>
[2]: <[email protected]>
[3]: <[email protected]>
[4]: <[email protected]>

---
Patrick Steinhardt (5):
      compat/posix: introduce writev(3p) wrapper
      wrapper: introduce writev(3p) wrappers
      wrapper: properly handle MAX_IO_SIZE in writev(3p)
      sideband: use writev(3p) to send pktlines
      fast-import: use writev(3p) to send cat-blob responses

 Makefile                            |  4 ++
 builtin/fast-import.c               | 18 +++++++--
 compat/posix.h                      | 14 +++++++
 compat/writev.c                     | 41 +++++++++++++++++++
 config.mak.uname                    |  2 +
 contrib/buildsystems/CMakeLists.txt |  6 ++-
 meson.build                         |  1 +
 sideband.c                          | 14 +++++--
 wrapper.c                           | 78 +++++++++++++++++++++++++++++++++++++
 wrapper.h                           | 10 +++++
 write-or-die.c                      |  8 ++++
 write-or-die.h                      |  1 +
 12 files changed, 190 insertions(+), 7 deletions(-)

Range-diff versus v1:

1:  69b8be6ec5 ! 1:  f519260452 compat/posix: introduce writev(3p) wrapper
    @@ compat/writev.c (new)
     +
     +ssize_t git_writev(int fd, const struct iovec *iov, int iovcnt)
     +{
    -+	size_t total_written = 0;
     +	size_t sum = 0;
     +
    ++	if (iovcnt <= 0) {
    ++		errno = EINVAL;
    ++		return -1;
    ++	}
    ++
     +	/*
     +	 * According to writev(3p), the syscall shall error with EINVAL in case
     +	 * the sum of `iov_len` overflows `ssize_t`.
     +	 */
     +	for (int i = 0; i < iovcnt; i++) {
     +		if (iov[i].iov_len > maximum_signed_value_of_type(ssize_t) ||
    ++		    unsigned_add_overflows(iov[i].iov_len, sum) ||
     +		    iov[i].iov_len + sum > maximum_signed_value_of_type(ssize_t)) {
     +			errno = EINVAL;
     +			return -1;
    @@ compat/writev.c (new)
     +		sum += iov[i].iov_len;
     +	}
     +
    ++	/*
    ++	 * We only ever write the first non-empty vector so that we can
    ++	 * guarantee the call to be non-interleaving as guaranteed by POSIX.
    ++	 * This works just fine as callers have to loop around writev anyway.
    ++	 */
     +	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;
    -+		}
    ++		if (!iov[i].iov_len)
    ++			continue;
    ++		return xwrite(fd, iov[i].iov_base, iov[i].iov_len);
     +	}
     +
    -+out:
    -+	return (ssize_t) total_written;
    ++	/* When all iovec members were zero we ought to return 0 according to POSIX. */
    ++	return 0;
     +}
     
      ## config.mak.uname ##
2:  f6013a18ba = 2:  41814b6668 wrapper: introduce writev(3p) wrappers
3:  3dc0eff00b = 3:  af2e351491 wrapper: properly handle MAX_IO_SIZE in writev(3p)
4:  95c872432f = 4:  d3ba9d73d2 sideband: use writev(3p) to send pktlines
5:  4c1efb5284 = 5:  148a2c8928 fast-import: use writev(3p) to send cat-blob responses

---
base-commit: 55526a18268bbc1ddaf8a6b7850c33d984eac9e9
change-id: 20260714-pks-reintroduce-writev-2d8f7e52eee9