Intercept assignment to $! to translate WSAExxx values to Exxx values on Windows

Since perl previously assigned WSAExxx values to $! on Windows it is quite
possible that (Perl-level) user code may also manually make similar assignments,
which will now cause breakage if the value put in $! is subsequently compared to
Errno/POSIX constants because the latter are now the corresponding Exxx values
where possible.

An example of this is in Net::Ping::tcp_connect(), which does the following to
fetch a socket-level error code:

unpack("i", getsockopt($self->{"fh"}, SOL_SOCKET, SO_ERROR))

and assigns the result (a WSAExxx value such as 10061) to $! and then goes wrong
in the subsequent test (in ping_tcp()) for $! == ECONNREFUSED (which is now 107
rather than 10061 if perl is built with VC10 or higher).

To avoid this we now intercept assignment to $! and convert any WSAExxx values
to Exxx values first.  This causes a minor oddity in that this:

perl -le "$! = 10061; print 0+$!"

will now output 107 (for VC10+ perls) but this is surely preferable to the
alternative breakage described above.

1 files changed, 13 insertions, 0 deletions

diff --git a/win32/win32.c b/win32/win32.c
index 1a64f9f2cb..0e8a89e6d6 100644
--- a/win32/win32.c
+++ b/win32/win32.c
@@ -2522,6 +2522,19 @@ win32_flock(int fd, int oper)
 
 #undef LK_LEN
 
+/* Get the errno value corresponding to the given err. This function is not
+ * intended to handle conversion of general GetLastError() codes. It only exists
+ * to translate Windows sockets error codes from WSAGetLastError(). Such codes
+ * used to be assigned to errno/$! in earlier versions of perl; this function is
+ * used to catch any old Perl code which is still trying to assign such values
+ * to $! and convert them to errno values instead.
+ */
+int
+win32_get_errno(int err)
+{
+    return convert_wsa_error_to_errno(err);
+}
+
 /*
  *  redirected io subsystem for all XS modules
  *