[v3 PATCH] Fix Cygwin bootstrap failure: ECANCELED in system_error

[Roger Sayle]

The following patch fixes the current libstdc++ bootstrap failure on 
i686-pc-cygwin.  The issue is that the new system_error header file 
unconditionally uses the value ECANCELED.  Alas not all almost POSIX systems 
provide this value in sys/errno.h.

The fix is similar to Ben Kosnik's previous fix for FreeBSD, and my previous 
fix for Darwin, to add a suitable configure check, and then guard the usage 
of ECANCEL and std::operation_canceled by _GLIBCXX_HAVE_ECANCELED.

The following patch has been tested on both i686-pc-cygwin and 
x86_64-unknown-linux-gnu with a full "make bootstrap", where it allows the 
former to complete without problems.  I then ran a top-level "make -k check" 
on the later to confirm that there are no new libstdc++ testsuite failures.

Ok for mainline?

2007-09-09  Roger Sayle  <roger@eyesopen.com>

        * acinclude.m4 (GLIBCXX_CHECK_SYSTEM_ERROR): Add ECANCELED for 
cygwin.
        * configure: Regenerate.
        * config/os/generic/error_constants.h: Guard.
        * testsuite/19_diagnostics/headers/system_error/
        types_std_c++0x.cc: Same.


I'm unsure as to the standard wording for system_error, but I was wondering 
if rather than removing the symbols operation_canceled and bad_message from 
the std namespace on affected platforms, we retain them but assign them a 
unique value.  This then preserves source code compatability of C++ user 
code, it's just that the OS won't ever generate an errno value of 
operation_canceled.  I was giving some thought to how one could assign these 
unique values when I discovered that E-values aren't required to be unique. 
Several OSes #define one value to another.  i.e. #define EAGAIN EWOULDBLOCK. 
If this is permitted, it means that these constants can't be used in switch 
statements, because they are not guaranteed unique.  This would simplify 
things to the point that on affected platforms we could simply have 
"operation_canceled = 0", "bad_message = 0" etc...  Thoughts?


Roger
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