Bug#658639: kfreebsd-9: 'write' system call violates POSIX standard

[Nicolas Bourdaud <nicolas.bourdaud@gmail.com>]

Source: kfreebsd-9
Severity: important
Tags: upstream

When a write() cannot transfer as many bytes as requested (because of a file
limit), it fails instead of transferring as many bytes as there is room to
write.

This is a violation of the POSIX standard:
http://pubs.opengroup.org/onlinepubs/007904975/functions/write.html

fsize-lim.c (attached) illustrates the problem. With a freebsd kernel, the
output is:
failed when adding 27 bytes after 59994 bytes (error: File too large)

The expected output (like with a linux kernel) should be:
added 6 bytes instead of 27 bytes after 59994 bytes
failed when adding 27 bytes after 60000 bytes (error: File too large)


-- System Information:
Debian Release: wheezy/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: kfreebsd-amd64 (x86_64)

Kernel: kFreeBSD 9.0-1-amd64
Locale: LANG=C, LC_CTYPE=C (charmap=ANSI_X3.4-1968)
Shell: /bin/sh linked to /bin/dash

#include <unistd.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <signal.h>
#include <errno.h>
#include <string.h>

#define TARGETSIZE	80000
#define LIMSIZE		60000
#define PATTSIZE	27


int main(void)
{
	struct rlimit lim;
	int fd;
	ssize_t retc;
	size_t count = 0;
	const char pattern[PATTSIZE] = "Hello world!";
	
	signal(SIGXFSZ, SIG_IGN);
	lim.rlim_cur = LIMSIZE; 
	setrlimit(RLIMIT_FSIZE, &lim);

	fd = open("result.txt", O_WRONLY|O_CREAT|O_TRUNC, S_IRUSR|S_IWUSR);

	while (count < TARGETSIZE) {
		retc = write(fd, pattern, PATTSIZE);

		if (retc < PATTSIZE && retc > 0)
			fprintf(stderr,
			        "added %zi bytes instead of %u bytes after %zu bytes\n",
				retc, PATTSIZE, count);
		else if (retc < 0) {
			fprintf(stderr,
			        "failed when adding %u bytes after %zu bytes (error: %s)\n",
				PATTSIZE, count, strerror(errno));
			break;
		}
		count += retc;
	}

	close(fd);

	return 0;
}