[newlib-cygwin/main] Cygwin: select: workaround FD_WRITE network event handling
Corinna Vinschen
corinna@sourceware.org
Tue Aug 1 12:23:30 GMT 2023
https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=dedbbd74d0a8f3b7dfae6188321703a47bb8a2b3
commit dedbbd74d0a8f3b7dfae6188321703a47bb8a2b3
Author: Corinna Vinschen <corinna@vinschen.de>
AuthorDate: Tue Aug 1 14:22:55 2023 +0200
Commit: Corinna Vinschen <corinna@vinschen.de>
CommitDate: Tue Aug 1 14:22:55 2023 +0200
Cygwin: select: workaround FD_WRITE network event handling
The FD_WRITE event is a false friend. It indicates ready to write
even if the next send fails with WSAEWOULDBLOCK. *After* the fact,
FD_WRITE will be cleared until sending is again possible, but that
is too late for a select/write loop.
Workaround that by using the WinSock select function when peeking
at a socket and FD_WRITE gets indicated. WinSock select fortunately
indicates writability correctly.
Fixes: 70e476d27be8 ("(peek_socket): Use event handling for peeking socket.")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
Diff:
---
winsup/cygwin/autoload.cc | 1 +
winsup/cygwin/select.cc | 30 +++++++++++++++++++++++++++---
2 files changed, 28 insertions(+), 3 deletions(-)
diff --git a/winsup/cygwin/autoload.cc b/winsup/cygwin/autoload.cc
index c9ad92530c83..67149f828266 100644
--- a/winsup/cygwin/autoload.cc
+++ b/winsup/cygwin/autoload.cc
@@ -645,6 +645,7 @@ LoadDLLfunc (getsockname, ws2_32)
LoadDLLfunc (getsockopt, ws2_32)
LoadDLLfunc (ioctlsocket, ws2_32)
LoadDLLfunc (listen, ws2_32)
+LoadDLLfunc_pfx_only (select, ws2_32)
LoadDLLfunc (setsockopt, ws2_32)
LoadDLLfunc (shutdown, ws2_32)
LoadDLLfunc (socket, ws2_32)
diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
index bad4c37f3b2e..7b9473849ec2 100644
--- a/winsup/cygwin/select.cc
+++ b/winsup/cygwin/select.cc
@@ -1757,6 +1757,15 @@ fhandler_base::select_except (select_stuff *ss)
return s;
}
+struct wfd_set
+{
+ u_int fd_count;
+ SOCKET fd_array[1];
+};
+
+/* autoload exposes the Winsock select function only with a _win32_ prefix. */
+extern "C" int _win32_select(int, wfd_set *, wfd_set *, wfd_set *, TIMEVAL *);
+
static int
peek_socket (select_record *me, bool)
{
@@ -1772,9 +1781,24 @@ peek_socket (select_record *me, bool)
if (me->read_selected)
me->read_ready |= ret || !!(events & (FD_READ | FD_ACCEPT | FD_CLOSE));
if (me->write_selected)
- /* Don't check for FD_CLOSE here. Only an error case (ret == -1)
- will set ready for writing. */
- me->write_ready |= ret || !!(events & (FD_WRITE | FD_CONNECT));
+ {
+ /* The FD_WRITE event is a false friend. It indicates ready to write
+ even if the next send fails with WSAEWOULDBLOCK. *After* the fact,
+ FD_WRITE will be cleared until sending is again possible.
+ For the time being, workaround that here by using the WinSock
+ select function. It indicates writability correctly. */
+ if (events & FD_WRITE)
+ {
+ wfd_set w = { 1, { fh->get_socket () } };
+ TIMEVAL t = { 0 };
+
+ if (_win32_select (0, NULL, &w, NULL, &t) == 0)
+ events &= ~FD_WRITE;
+ }
+ /* Don't check for FD_CLOSE here. Only an error case (ret == -1)
+ will set ready for writing. */
+ me->write_ready |= ret || !!(events & (FD_WRITE | FD_CONNECT));
+ }
if (me->except_selected)
me->except_ready |= !!(events & FD_OOB);
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