quic: use send/recvmmsg when available

[icing]

Enable use of send/recvmmsg on platforms that support it. Add checks for sendmmsg in configure and CmakeLists.txt. Will have effect for ngtcp2 and quiche HTTP/3.

One quirk: we need to define _GNU_SOURCE in vquic.c so that sys/socket.h offers the functions.

Performance: scorecard testing show only a little improvement in download numbers. Mostly, curl is only receiving 1-2 packets at a time. This might point to Caddy's inability to send packets fast enough or may be the side-effect of other system configurations.

[icing]

@vszakats cmake does not detect sendmmsg because the _GNU_SOURCE is not defined when testing and sys/socket.h then does not expose it. Any idea how best to progress here?

[vszakats]

Wouldn't we need to define _GNU_SOURCE globally for this? Depending on what it changes, setting it for a single file, or for the duration of an #include, may lead to issues, esp thinking of unity builds (where an earlier file might include sys/socket.h without _GNU_SOURCE being set).

If so, it would likely be better to define it globally (for platforms that support it or might need it) with add_definitions("-D_GNU_SOURCE"). This should make the feature test work as it is. (Also something along these lines with ./configure)

[bagder]

Wouldn't we need to define _GNU_SOURCE globally for this?

I suppose we need to, at least when building with a backend for which send/recvmmsg can be useful.

[icing]

So, should I try for a define in curl_setup.h when we are on linux?

[vszakats]

So, should I try for a define in curl_setup.h when we are on linux?

I think setting it on the build level has some advantages over curl_setup.h, e.g. it guarantees they are matching with detection results. E.g. something like:

diff --git a/CMake/Platforms/WindowsCache.cmake b/CMake/Platforms/WindowsCache.cmake
index 317f21c875..57b2b40bd4 100644
--- a/CMake/Platforms/WindowsCache.cmake
+++ b/CMake/Platforms/WindowsCache.cmake
@@ -88,6 +88,7 @@ set(HAVE_FREEADDRINFO 1)
 set(HAVE_FCHMOD 0)
 set(HAVE_SOCKETPAIR 0)
 set(HAVE_SENDMSG 0)
+set(HAVE_SENDMMSG 0)
 set(HAVE_ALARM 0)
 set(HAVE_FCNTL 0)
 set(HAVE_GETPPID 0)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index e84c3af325..d8eb3d0d77 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -192,6 +192,10 @@ cmake_dependent_option(ENABLE_THREADED_RESOLVER "Enable threaded DNS lookup"
 
 include(PickyWarnings)
 
+if(CMAKE_SYSTEM_NAME MATCHES "Linux")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_GNU_SOURCE")  # Required for sendmmsg()
+endif()
+
 option(ENABLE_DEBUG "Enable curl debug features" OFF)
 option(ENABLE_CURLDEBUG "Enable TrackMemory feature" ${ENABLE_DEBUG})
 
@@ -1477,6 +1481,7 @@ check_symbol_exists("socketpair"      "${CURL_INCLUDES}" HAVE_SOCKETPAIR)
 check_symbol_exists("recv"            "${CURL_INCLUDES}" HAVE_RECV)
 check_symbol_exists("send"            "${CURL_INCLUDES}" HAVE_SEND)
 check_symbol_exists("sendmsg"         "${CURL_INCLUDES}" HAVE_SENDMSG)
+check_symbol_exists("sendmmsg"        "sys/socket.h" HAVE_SENDMMSG)
 check_symbol_exists("select"          "${CURL_INCLUDES}" HAVE_SELECT)
 check_symbol_exists("strdup"          "${CURL_INCLUDES};string.h" HAVE_STRDUP)
 check_symbol_exists("strtok_r"        "${CURL_INCLUDES};string.h" HAVE_STRTOK_R)

(add_definitions() doesn't apply to detections, as it turns out.)

And something similar in configure, via CPPFLAGS:

case $host in
  *-*-linux*)
    CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE";;
esac

# then detection as normal

[icing]

Thanks, @vszakats, incorporated that into the latest push.

Hmm, cmake does not detect it and we have other fallouts...

[vszakats]

Right, this needs to be added too to make the setting propagate:

--- a/lib/curl_config.h.cmake
+++ b/lib/curl_config.h.cmake
@@ -455,6 +455,9 @@
 /* Define to 1 if you have the sendmsg function. */
 #cmakedefine HAVE_SENDMSG 1
 
+/* Define to 1 if you have the sendmmsg function. */
+#cmakedefine HAVE_SENDMMSG 1
+
 /* Define to 1 if you have the 'fsetxattr' function. */
 #cmakedefine HAVE_FSETXATTR 1

It should fix the cmake-vs-autotools check and enable sendmmsg in CMake builds.

As for the curl-for-win fallout, I think we're seeing the effect of _GNU_SOURCE. It's triggering the declaration of calloc() in system headers, which collides with curl's PP override for that symbol. The solution is probably to copy (or move?) this:

#if defined(USE_THREADS_POSIX) && defined(HAVE_PTHREAD_H)
#  include <pthread.h>
#endif

to somewhere in curl_setup.h to always have it included before curl_memory.h and memdebug.h.

[bagder]

That scan-build warning needs to be fixed!

[icing]

That scan-build warning needs to be fixed!

I do not understand the error in tool_operate.c:173. I clear the struct explicitly now, but it seems unnecessary.