[PATCH] prevent retries on fclose/fflush after write errors

[Denys Vlasenko]

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.

-- 
vda

diff -d -urpN uClibc.0/libc/stdio/_wcommit.c uClibc.1/libc/stdio/_wcommit.c
--- uClibc.0/libc/stdio/_wcommit.c	2011-02-07 00:04:34.000000000 +0100
+++ uClibc.1/libc/stdio/_wcommit.c	2011-02-07 00:55:24.000000000 +0100
@@ -20,7 +20,18 @@ size_t attribute_hidden __stdio_wcommit(
 
 	__STDIO_STREAM_VALIDATE(stream);
 
-	if ((bufsize = __STDIO_STREAM_BUFFER_WUSED(stream)) != 0) {
+	/* Note: we do not write anything if write error has been detected.
+	 * Otherwise, stdio user has no way to prevent retries after
+	 * failed write - and some users do want to not have any retries!
+	 * IOW: if write errored out, neither fflush nor fclose should
+	 * try to write buffered data.
+	 * clearerr may be used to enable retry if needed.
+	 */
+
+	bufsize = __STDIO_STREAM_BUFFER_WUSED(stream);
+	if (bufsize != 0
+	 && !(stream->__modeflags & __FLAG_ERROR)
+	) {
 		stream->__bufpos = stream->__bufstart;
 		__stdio_WRITE(stream, stream->__bufstart, bufsize);
 	}