Subject: Re: Git 2.54.0-rc1, subtests of t5310, t5326, t5327

From: Phillip Wood <[email-masked]>
Date: Thu, 9 Apr 2026 10:48:02 +0100

On 09/04/2026 09:17, Patrick Steinhardt wrote:
> 
> One question to Randall though: does MAX_IO_SIZE apply to the overall
> size of the iovec or to the individual iovec entries? 

In <[email-masked]> Randall says

     Specifying  the sum of the iov_len values in the iov array greater
     than the OSS I/O size limit for that open causes the  writev()
     function  to return  -1  and  set errno to [EINVAL].

So it is the overall size which fits with POSIX limiting to overall size 
to SSIZE_MAX.

Thanks

Phillip

> I think it should
> be the latter, but I cannot easily verify and couldn't find any docs
> around this. So could you please try the patch at the end of this mail
> to verify that it works on your system?
> 
> In any case, I've tested that my patch also works when defining
> MAX_IO_SIZE to 128 bytes on my system, which hopefully demonstrates that
> it works as expected:
> 
> diff --git a/git-compat-util.h b/git-compat-util.h
> index 4b4ea2498f..8e02b5f673 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -690,14 +690,8 @@ static inline uint64_t u64_add(uint64_t a, uint64_t b)
>    * to override this, if the definition of SSIZE_MAX given by the platform
>    * is broken.
>    */
> -#ifndef MAX_IO_SIZE
> -# define MAX_IO_SIZE_DEFAULT (8*1024*1024)
> -# if defined(SSIZE_MAX) && (SSIZE_MAX < MAX_IO_SIZE_DEFAULT)
> -#  define MAX_IO_SIZE SSIZE_MAX
> -# else
> -#  define MAX_IO_SIZE MAX_IO_SIZE_DEFAULT
> -# endif
> -#endif
> +#undef MAX_IO_SIZE
> +#define MAX_IO_SIZE 128
>   
>   #ifdef HAVE_ALLOCA_H
>   # include <alloca.h>
> 
> I'm happy to go either way, but think that we should definitely aim for
> the below patch eventually. Just let me know which way you prefer and
> I'm happy to polish up the patch.
> 
> Patrick
> 
> diff --git a/wrapper.c b/wrapper.c
> index be8fa575e6..645dbc5f20 100644
> --- a/wrapper.c
> +++ b/wrapper.c
> @@ -323,21 +323,50 @@ ssize_t write_in_full(int fd, const void *buf, size_t count)
>   	return total;
>   }
>   
> +ssize_t xwritev(int fd, struct iovec *iov, int iovcnt)
> +{
> +	ssize_t bytes_written;
> +	int i;
> +
> +	/*
> +	 * We need to make sure that no individual iovec entry exceeds
> +	 * `MAX_IO_SIZE`. If there's any entry that does exceed this limit
> +	 * we'll pass all entries up to it to `writev()`, and then process the
> +	 * exceeding entry via a call to `xwrite()`.
> +	 */
> +	for (i = 0; i < iovcnt; i++)
> +		if (iov[i].iov_len > MAX_IO_SIZE)
> +			break;
> +	if (i < iovcnt) {
> +		/*
> +		 * The first entry exceeds MAX_IO_SIZE, so we pass it to
> +		 * xwrite, which knows to handle his case.
> +		 */
> +		if (!i)
> +			return xwrite(fd, iov->iov_base, iov->iov_len);
> +		iovcnt = i;
> +	}
> +
> +	bytes_written = writev(fd, iov, iovcnt);
> +	if (!bytes_written) {
> +		errno = ENOSPC;
> +		return -1;
> +	}
> +
> +	return bytes_written;
> +}
> +
>   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) {
> +		ssize_t bytes_written = xwritev(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;
>   
> diff --git a/wrapper.h b/wrapper.h
> index 27519b32d1..a6287d7f4d 100644
> --- a/wrapper.h
> +++ b/wrapper.h
> @@ -16,6 +16,7 @@ void *xmmap_gently(void *start, size_t length, int prot, int flags, int fd, off_
>   int xopen(const char *path, int flags, ...);
>   ssize_t xread(int fd, void *buf, size_t len);
>   ssize_t xwrite(int fd, const void *buf, size_t len);
> +ssize_t xwritev(int fd, struct iovec *iov, int iovcnt);
>   ssize_t xpread(int fd, void *buf, size_t len, off_t offset);
>   int xdup(int fd);
>   FILE *xfopen(const char *path, const char *mode);
>