stdio: prevent retries on fclose/fflush after write errors

Some test results:

The longer patch posted at Sun 14:46:24 +0100 made my target system
unbootable.  I did not attempt to troubleshoot it, as we are focusing
our efforts on the shorter patch now.

The shorter patch posted at Mon 01:50:27 +0100 is a good start, but it
didn't completely fix the problem for me.  I am posting an updated
version with a few changes at the end of this message; the patched
uClibc 0.9.32.1 tree passes both of my test cases.

My changes:

1) Need to break out of the loop on "hard" errors.  Otherwise the
library call never returns:

open("/dev/null", O_RDONLY)             = 4
dup2(4, 1)                              = 1
write(1, "hello world\n", 12)           = -1 EBADF (Bad file descriptor)
write(1, "hello world\n", 12)           = -1 EBADF (Bad file descriptor)
write(1, "hello world\n", 12)           = -1 EBADF (Bad file descriptor)
write(1, "hello world\n", 12)           = -1 EBADF (Bad file descriptor)
...

2) Move all of the error handling logic back into the "else" clause.  In
particular, I believe we do not want to be checking errno unless
__WRITE() had indicated a failure, since the value may be undefined:

        if (errno == EINTR
         || errno == EAGAIN
         /* do we have other "soft" errors? */
        ) {

3) Whitespace/indentation consistency.

-- 8< --

From: Denys Vlasenko <vda.linux@googlemail.com>

Currently, uclibc retains buffered data on stdio write errors,
and subsequent fclose and fflush will try to write it out again
(in most cases, in vain).

Which results in something like this:

On Wednesday 26 January 2011 13:21, Baruch Siach wrote:
> Hi busybox list,
>
> I'm running the following command under strace (thanks Rob):
>
> echo 56 > /sys/class/gpio/export
>
> and I see the following output:
>
> write(1, "56\n", 3)                     = -1 EBUSY (Device or resource busy)
> write(1, "5", 1)                        = 1
>
> The first EBUSY is OK, since GPIO 56 is already requested. But the second
> write() attempt seems strange, and leads to an unwanted outcome. GPIO 5 gets
> exported.

This patch prevents that.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

1 files changed, 22 insertions, 6 deletions

diff --git a/libc/stdio/_WRITE.c b/libc/stdio/_WRITE.c
index 4011b7180..6af5da8e4 100644
--- a/libc/stdio/_WRITE.c
+++ b/libc/stdio/_WRITE.c
@@ -57,14 +57,30 @@ size_t attribute_hidden __stdio_WRITE(register FILE *stream,
 #endif
 			todo -= rv;
 			buf += rv;
-		} else
-#ifdef __UCLIBC_MJN3_ONLY__
-#warning EINTR?
-#endif
-/* 		if (errno != EINTR) */
-		{
+		} else {
+
 			__STDIO_STREAM_SET_ERROR(stream);
 
+			/* We buffer data on "transient" errors, but discard it
+			 * on "hard" ones. Example of a hard error:
+			 *
+			 * close(fileno(stdout));
+			 * printf("Hi there 1\n"); // EBADF
+			 * dup2(good_fd, fileno(stdout));
+			 * printf("Hi there 2\n"); // buffers new data
+			 *
+			 * This program should not print "Hi there 1" to good_fd.
+			 * The rationale is that the caller of writing operation
+			 * should check for error and act on it.
+			 * If he didn't, then future users of the stream
+			 * have no idea what to do.
+			 * It's least confusing to at least not burden them with
+			 * some hidden buffered crap in the buffer.
+			 */
+			if (errno != EINTR && errno != EAGAIN) {
+				/* do we have other "soft" errors? */
+				break;
+			}
 #ifdef __STDIO_BUFFERS
 			stodo = __STDIO_STREAM_BUFFER_SIZE(stream);
 			if (stodo != 0) {