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The following bug has been logged on the website:

Bug reference:      17083
Logged by:          Adrian Ho
Email address:      ml+postgresql@03s.net
PostgreSQL version: 13.3
Operating system:   Ubuntu 20.04.2 LTS
Description:        

OpenLDAP 2.5 merged the `ldap_r` and `ldap` libraries, so all per-handle
routines are now thread-safe by default. However, the removal of `ldap_r`
results in the following `configure` error:
```
checking for ldap_bind in -lldap... yes
checking for ldap_simple_bind in -lldap_r... no
configure: error: library 'ldap_r' is required for LDAP
```
This patch therefore fixes and clarifies the LDAP library detection
logic, by first selecting on thread-safety requirements, then looking
for the appropriate libraries. The underlying assumption is that for a
pre-2.5 setup, both `ldap` and `ldap_r` libraries are installed, so it
should only fall back to (thread-safe) `ldap` in a 2.5.x installation.

diff --git a/configure.in b/configure.in
index 14a6be6..02ad260 100644
--- a/configure.in
+++ b/configure.in
@@ -1241,17 +1241,19 @@ fi
 if test "$with_ldap" = yes ; then
   _LIBS="$LIBS"
   if test "$PORTNAME" != "win32"; then
-    AC_CHECK_LIB(ldap, ldap_bind, [],
-		 [AC_MSG_ERROR([library 'ldap' is required for LDAP])],
-		 [$EXTRA_LDAP_LIBS])
-    LDAP_LIBS_BE="-lldap $EXTRA_LDAP_LIBS"
     if test "$enable_thread_safety" = yes; then
       # on some platforms ldap_r fails to link without PTHREAD_LIBS
-      AC_CHECK_LIB(ldap_r, ldap_simple_bind, [],
-		   [AC_MSG_ERROR([library 'ldap_r' is required for LDAP])],
+      # OpenLDAP 2.5 merged ldap_r with ldap
+      AC_SEARCH_LIBS(ldap_simple_bind, [ldap_r ldap], [],
+		   [AC_MSG_ERROR([not found in any LDAP library])],
 		   [$PTHREAD_CFLAGS $PTHREAD_LIBS $EXTRA_LDAP_LIBS])
-      LDAP_LIBS_FE="-lldap_r $EXTRA_LDAP_LIBS"
+      LDAP_LIBS_BE="-lldap $EXTRA_LDAP_LIBS"
+      LDAP_LIBS_FE="${ac_lib:+-l}$ac_lib $EXTRA_LDAP_LIBS"
     else
+      AC_CHECK_LIB(ldap, ldap_bind, [],
+		   [AC_MSG_ERROR([library 'ldap' is required for LDAP])],
+		   [$EXTRA_LDAP_LIBS])
+      LDAP_LIBS_BE="-lldap $EXTRA_LDAP_LIBS"
       LDAP_LIBS_FE="-lldap $EXTRA_LDAP_LIBS"
     fi
     AC_CHECK_FUNCS([ldap_initialize])