[Bug stdio/16605] New: Invalid caching of file position in stdio streams

["bugdal at aerifal dot cx" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16605

            Bug ID: 16605
           Summary: Invalid caching of file position in stdio streams
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: bugdal at aerifal dot cx

Test case:

#include <stdio.h>
#include <unistd.h>

int main()
{
    FILE *f = tmpfile();
    printf("%ld\n", ftell(f));
    write(fileno(f), "abc", 3);
    printf("%ld\n", ftell(f));
}

Expected output:
0
3

glibc output:
0
0

The issue is that glibc is wrongly caching the file position when it does not
have any guarantee that the stdio FILE stream is or will remain the "active
handle" for the open file description (see POSIX XSH 2.5.1 Interaction of File
Descriptors and Standard I/O Streams). While the exact semantics for active
handle are complex (read the cited text), the basic situation is that you can
only guarantee that the FILE remains the active handle once you've started
reading from or writing to it, and up until an operation that allows the
application to switch to a different active handle (a successful seek, explicit
flush, newline-caused flush on line-buffered stream, reaching eof reading,
etc.).

Note that the test case above only shows the issue for writing, but as far as I
can tell it applies equally to reading, and applies to all possible open modes
(read/write/update/append/etc.). I used tmpfile() for convenience but the same
should work with fopen or fdopen.

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