configure: check for __builtin_available() availability (#1788)

This change does two things:
1. It un-breaks the build in Xcode 9.0. (Xcode 9.0 is currently
   failing trying to compile connectx() in lib/connect.c.)
2. It finally weak-links the connectx() function, and falls back on
   connect() when run on older operating systems.

3 files changed, 46 insertions, 10 deletions

diff --git a/acinclude.m4 b/acinclude.m4
index 539322870..dd5d896bc 100644..100755
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -3243,3 +3243,29 @@ AC_DEFUN([CURL_MAC_CFLAGS], [
   fi
 
 ])
+
+
+dnl CURL_SUPPORTS_BUILTIN_AVAILABLE
+dnl
+dnl Check to see if the compiler supports __builtin_available. This built-in
+dnl compiler function first appeared in Apple LLVM 9.0.0. It's so new that, at
+dnl the time this macro was written, the function was not yet documented. Its
+dnl purpose is to return true if the code is running under a certain OS version
+dnl or later.
+
+AC_DEFUN([CURL_SUPPORTS_BUILTIN_AVAILABLE], [
+  AC_MSG_CHECKING([to see if the compiler supports __builtin_available()])
+  AC_COMPILE_IFELSE([
+    AC_LANG_PROGRAM([[
+#include <stdlib.h>
+    ]],[[
+      if (__builtin_available(macOS 10.8, iOS 5.0, *)) {}
+    ]])
+  ],[
+    AC_MSG_RESULT([yes])
+    AC_DEFINE_UNQUOTED(HAVE_BUILTIN_AVAILABLE, 1,
+        [Define to 1 if you have the __builtin_available function.])
+  ],[
+    AC_MSG_RESULT([no])
+  ])
+])
diff --git a/configure.ac b/configure.ac
index edebfc77b..605abc3fd 100644..100755
--- a/configure.ac
+++ b/configure.ac
@@ -354,6 +354,7 @@ esac
 CURL_CHECK_WIN32_LARGEFILE
 
 CURL_MAC_CFLAGS
+CURL_SUPPORTS_BUILTIN_AVAILABLE
 
 dnl ************************************************************
 dnl switch off particular protocols
diff --git a/lib/connect.c b/lib/connect.c
index 8d5d32ba6..927ff7cec 100644..100755
--- a/lib/connect.c
+++ b/lib/connect.c
@@ -1059,16 +1059,25 @@ static CURLcode singleipconnect(struct connectdata *conn,
   if(!isconnected && (conn->socktype == SOCK_STREAM)) {
     if(conn->bits.tcp_fastopen) {
 #if defined(CONNECT_DATA_IDEMPOTENT) /* OS X */
-      sa_endpoints_t endpoints;
-      endpoints.sae_srcif = 0;
-      endpoints.sae_srcaddr = NULL;
-      endpoints.sae_srcaddrlen = 0;
-      endpoints.sae_dstaddr = &addr.sa_addr;
-      endpoints.sae_dstaddrlen = addr.addrlen;
-
-      rc = connectx(sockfd, &endpoints, SAE_ASSOCID_ANY,
-                    CONNECT_RESUME_ON_READ_WRITE | CONNECT_DATA_IDEMPOTENT,
-                    NULL, 0, NULL, NULL);
+#ifdef HAVE_BUILTIN_AVAILABLE
+      if(__builtin_available(macOS 10.11, iOS 9.0, tvOS 9.0, watchOS 2.0, *)) {
+#endif /* HAVE_BUILTIN_AVAILABLE */
+        sa_endpoints_t endpoints;
+        endpoints.sae_srcif = 0;
+        endpoints.sae_srcaddr = NULL;
+        endpoints.sae_srcaddrlen = 0;
+        endpoints.sae_dstaddr = &addr.sa_addr;
+        endpoints.sae_dstaddrlen = addr.addrlen;
+
+        rc = connectx(sockfd, &endpoints, SAE_ASSOCID_ANY,
+                      CONNECT_RESUME_ON_READ_WRITE | CONNECT_DATA_IDEMPOTENT,
+                      NULL, 0, NULL, NULL);
+#ifdef HAVE_BUILTIN_AVAILABLE
+      }
+      else {
+        rc = connect(sockfd, &addr.sa_addr, addr.addrlen);
+      }
+#endif /* HAVE_BUILTIN_AVAILABLE */
 #elif defined(MSG_FASTOPEN) /* Linux */
       if(conn->given->flags & PROTOPT_SSL)
         rc = connect(sockfd, &addr.sa_addr, addr.addrlen);