oauth: Disallow synchronous DNS in libcurl

There is concern that a blocking DNS lookup in libpq could stall a
backend process (say, via FDW). Since there's currently no strong
evidence that synchronous DNS is a popular option, disallow it entirely
rather than warning at configure time. We can revisit if anyone
complains.

Per query from Andres Freund.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://postgr.es/m/p4bd7mn6dxr2zdak74abocyltpfdxif4pxqzixqpxpetjwt34h%40qc6jgfmoddvq

config/programs.m4

@@ -316,7 +316,7 @@ AC_DEFUN([PGAC_CHECK_LIBCURL],
               [Define to 1 if curl_global_init() is guaranteed to be thread-safe.])
   fi
 
-  # Warn if a thread-friendly DNS resolver isn't built.
+  # Fail if a thread-friendly DNS resolver isn't built.
   AC_CACHE_CHECK([for curl support for asynchronous DNS], [pgac_cv__libcurl_async_dns],
   [AC_RUN_IFELSE([AC_LANG_PROGRAM([
 #include <curl/curl.h>
@@ -332,10 +332,10 @@ AC_DEFUN([PGAC_CHECK_LIBCURL],
   [pgac_cv__libcurl_async_dns=yes],
   [pgac_cv__libcurl_async_dns=no],
   [pgac_cv__libcurl_async_dns=unknown])])
-  if test x"$pgac_cv__libcurl_async_dns" != xyes ; then
-    AC_MSG_WARN([
+  if test x"$pgac_cv__libcurl_async_dns" = xno ; then
+    AC_MSG_ERROR([
 *** The installed version of libcurl does not support asynchronous DNS
-*** lookups. Connection timeouts will not be honored during DNS resolution,
-*** which may lead to hangs in client programs.])
+*** lookups. Rebuild libcurl with the AsynchDNS feature enabled in order
+*** to use it with libpq.])
   fi
 ])# PGAC_CHECK_LIBCURL

configure

@@ -12493,7 +12493,7 @@ $as_echo "#define HAVE_THREADSAFE_CURL_GLOBAL_INIT 1" >>confdefs.h
 
   fi
 
-  # Warn if a thread-friendly DNS resolver isn't built.
+  # Fail if a thread-friendly DNS resolver isn't built.
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking for curl support for asynchronous DNS" >&5
 $as_echo_n "checking for curl support for asynchronous DNS... " >&6; }
 if ${pgac_cv__libcurl_async_dns+:} false; then :
@@ -12535,15 +12535,11 @@ fi
 fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv__libcurl_async_dns" >&5
 $as_echo "$pgac_cv__libcurl_async_dns" >&6; }
-  if test x"$pgac_cv__libcurl_async_dns" != xyes ; then
-    { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING:
-*** The installed version of libcurl does not support asynchronous DNS
-*** lookups. Connection timeouts will not be honored during DNS resolution,
-*** which may lead to hangs in client programs." >&5
-$as_echo "$as_me: WARNING:
+  if test x"$pgac_cv__libcurl_async_dns" = xno ; then
+    as_fn_error $? "
 *** The installed version of libcurl does not support asynchronous DNS
-*** lookups. Connection timeouts will not be honored during DNS resolution,
-*** which may lead to hangs in client programs." >&2;}
+*** lookups. Rebuild libcurl with the AsynchDNS feature enabled in order
+*** to use it with libpq." "$LINENO" 5
   fi
 
 fi

meson.build

@@ -909,9 +909,7 @@ if not libcurlopt.disabled()
       cdata.set('HAVE_THREADSAFE_CURL_GLOBAL_INIT', 1)
     endif
 
-    # Warn if a thread-friendly DNS resolver isn't built.
-    libcurl_async_dns = false
-
+    # Fail if a thread-friendly DNS resolver isn't built.
     if not meson.is_cross_build()
       r = cc.run('''
         #include <curl/curl.h>
@@ -931,16 +929,12 @@ if not libcurlopt.disabled()
       )
 
       assert(r.compiled())
-      if r.returncode() == 0
-        libcurl_async_dns = true
-      endif
-    endif
-
-    if not libcurl_async_dns
-      warning('''
+      if r.returncode() != 0
+        error('''
 *** The installed version of libcurl does not support asynchronous DNS
-*** lookups. Connection timeouts will not be honored during DNS resolution,
-*** which may lead to hangs in client programs.''')
+*** lookups. Rebuild libcurl with the AsynchDNS feature enabled in order
+*** to use it with libpq.''')
+      endif
     endif
   endif