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 <[email protected]>
Discussion: https://postgr.es/m/p4bd7mn6dxr2zdak74abocyltpfdxif4pxqzixqpxpetjwt34h%40qc6jgfmoddvq

3 files changed, 16 insertions, 26 deletions

diff --git a/config/programs.m4 b/config/programs.m4
index 061b13376ac..0a07feb37cc 100644
--- a/config/programs.m4
+++ b/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
diff --git a/configure b/configure
index 93fddd69981..559f535f5cd 100755
--- a/configure
+++ b/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
diff --git a/meson.build b/meson.build
index 13c13748e5d..b6daa5b7040 100644
--- a/meson.build
+++ b/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