oauth: Move the builtin flow into a separate module

The additional packaging footprint of the OAuth Curl dependency, as well
as the existence of libcurl in the address space even if OAuth isn't
ever used by a client, has raised some concerns. Split off this
dependency into a separate loadable module called libpq-oauth.

When configured using --with-libcurl, libpq.so searches for this new
module via dlopen(). End users may choose not to install the libpq-oauth
module, in which case the default flow is disabled.

For static applications using libpq.a, the libpq-oauth staticlib is a
mandatory link-time dependency for --with-libcurl builds. libpq.pc has
been updated accordingly.

The default flow relies on some libpq internals. Some of these can be
safely duplicated (such as the SIGPIPE handlers), but others need to be
shared between libpq and libpq-oauth for thread-safety. To avoid
exporting these internals to all libpq clients forever, these
dependencies are instead injected from the libpq side via an
initialization function. This also lets libpq communicate the offsets of
PGconn struct members to libpq-oauth, so that we can function without
crashing if the module on the search path came from a different build of
Postgres. (A minor-version upgrade could swap the libpq-oauth module out
from under a long-running libpq client before it does its first load of
the OAuth flow.)

This ABI is considered "private". The module has no SONAME or version
symlinks, and it's named libpq-oauth-<major>.so to avoid mixing and
matching across Postgres versions. (Future improvements may promote this
"OAuth flow plugin" to a first-class concept, at which point we would
need a public API to replace this anyway.)

Additionally, NLS support for error messages in b3f0be788a was
incomplete, because the new error macros weren't being scanned by
xgettext. Fix that now.

Per request from Tom Lane and Bruce Momjian. Based on an initial patch
by Daniel Gustafsson, who also contributed docs changes. The "bare"
dlopen() concept came from Thomas Munro. Many people reviewed the design
and implementation; thank you!

Co-authored-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Christoph Berg <myon@debian.org>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Wolfgang Walther <walther@technowledgy.de>
Discussion: https://postgr.es/m/641687.1742360249%40sss.pgh.pa.us

configure

@@ -655,6 +655,7 @@ UUID_LIBS
 LDAP_LIBS_BE
 LDAP_LIBS_FE
 with_ssl
+LIBCURL_LDLIBS
 PTHREAD_CFLAGS
 PTHREAD_LIBS
 PTHREAD_CC
@@ -711,6 +712,8 @@ with_libxml
 LIBNUMA_LIBS
 LIBNUMA_CFLAGS
 with_libnuma
+LIBCURL_LDFLAGS
+LIBCURL_CPPFLAGS
 LIBCURL_LIBS
 LIBCURL_CFLAGS
 with_libcurl
@@ -9053,19 +9056,27 @@ $as_echo "yes" >&6; }
 
 fi
 
-  # We only care about -I, -D, and -L switches;
-  # note that -lcurl will be added by PGAC_CHECK_LIBCURL below.
+  # Curl's flags are kept separate from the standard CPPFLAGS/LDFLAGS. We use
+  # them only for libpq-oauth.
+  LIBCURL_CPPFLAGS=
+  LIBCURL_LDFLAGS=
+
+  # We only care about -I, -D, and -L switches. Note that -lcurl will be added
+  # to LIBCURL_LDLIBS by PGAC_CHECK_LIBCURL, below.
   for pgac_option in $LIBCURL_CFLAGS; do
     case $pgac_option in
-      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+      -I*|-D*) LIBCURL_CPPFLAGS="$LIBCURL_CPPFLAGS $pgac_option";;
     esac
   done
   for pgac_option in $LIBCURL_LIBS; do
     case $pgac_option in
-      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+      -L*) LIBCURL_LDFLAGS="$LIBCURL_LDFLAGS $pgac_option";;
     esac
   done
 
+
+
+
   # OAuth requires python for testing
   if test "$with_python" != yes; then
     { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: *** OAuth support tests require --with-python to run" >&5
@@ -12704,9 +12715,6 @@ fi
 
 fi
 
-# XXX libcurl must link after libgssapi_krb5 on FreeBSD to avoid segfaults
-# during gss_acquire_cred(). This is possibly related to Curl's Heimdal
-# dependency on that platform?
 if test "$with_libcurl" = yes ; then
 
   ac_fn_c_check_header_mongrel "$LINENO" "curl/curl.h" "ac_cv_header_curl_curl_h" "$ac_includes_default"
@@ -12754,17 +12762,26 @@ fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_lib_curl_curl_multi_init" >&5
 $as_echo "$ac_cv_lib_curl_curl_multi_init" >&6; }
 if test "x$ac_cv_lib_curl_curl_multi_init" = xyes; then :
-  cat >>confdefs.h <<_ACEOF
-#define HAVE_LIBCURL 1
-_ACEOF
 
-  LIBS="-lcurl $LIBS"
+
+$as_echo "#define HAVE_LIBCURL 1" >>confdefs.h
+
+				 LIBCURL_LDLIBS=-lcurl
+
 
 else
   as_fn_error $? "library 'curl' does not provide curl_multi_init" "$LINENO" 5
 fi
 
 
+  pgac_save_CPPFLAGS=$CPPFLAGS
+  pgac_save_LDFLAGS=$LDFLAGS
+  pgac_save_LIBS=$LIBS
+
+  CPPFLAGS="$LIBCURL_CPPFLAGS $CPPFLAGS"
+  LDFLAGS="$LIBCURL_LDFLAGS $LDFLAGS"
+  LIBS="$LIBCURL_LDLIBS $LIBS"
+
   # Check to see whether the current platform supports threadsafe Curl
   # initialization.
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking for curl_global_init thread safety" >&5
@@ -12868,6 +12885,10 @@ $as_echo "$pgac_cv__libcurl_async_dns" >&6; }
 *** to use it with libpq." "$LINENO" 5
   fi
 
+  CPPFLAGS=$pgac_save_CPPFLAGS
+  LDFLAGS=$pgac_save_LDFLAGS
+  LIBS=$pgac_save_LIBS
+
 fi
 
 if test "$with_gssapi" = yes ; then
@@ -14516,6 +14537,13 @@ done
 
 fi
 
+if test "$with_libcurl" = yes ; then
+  # Error out early if this platform can't support libpq-oauth.
+  if test "$ac_cv_header_sys_event_h" != yes -a "$ac_cv_header_sys_epoll_h" != yes; then
+    as_fn_error $? "client OAuth is not supported on this platform" "$LINENO" 5
+  fi
+fi
+
 ##
 ## Types, structures, compiler characteristics
 ##