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

From: Patrick Steinhardt <[email-masked]>
Date: Thu, 9 Apr 2026 13:29:16 +0200

On Thu, Apr 09, 2026 at 10:48:02AM +0100, Phillip Wood wrote:
> 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.

Ah, thanks for the pointer. I've adapted the patch a bit to the below
one. Again, I've tested it with `#define MAX_IO_SIZE 100` to verify that
it works as expected.

I guess I'll send a polished version to the mailing list in a bit.

Patrick

diff --git a/wrapper.c b/wrapper.c
index be8fa575e6..d989c78b4b 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -323,21 +323,60 @@ 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;
+	size_t total_length;
+	int i;
+
+	/*
+	 * We need to make sure that writev(3p) call does not write more than
+	 * `MAX_IO_SIZE` many bytes. If we do exceed that limit, we only pass
+	 * those iovecs to writev(3p) that sum up to less than the limit.
+	 *
+	 * If on the other hand the first iovec entry already exceeds this
+	 * limit we'll instead use xwrite() to write it, which knows to handle
+	 * `MAX_IO_SIZE` for us.
+	 */
+	for (i = 0, total_length = 0; i < iovcnt; i++) {
+		if (unsigned_add_overflows(total_length, iov[i].iov_len))
+			break;
+
+		total_length += iov[i].iov_len;
+		if (total_length > MAX_IO_SIZE)
+			break;
+	}
+
+	if (i < iovcnt) {
+		/*
+		 * The first entry exceeds MAX_IO_SIZE, so we pass it to
+		 * xwrite, which knows to handle this 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);