Explicitly require MIT Kerberos for GSSAPI

WHen building with GSSAPI support, explicitly require MIT Kerberos and
check for gssapi_ext.h in configure.ac and meson.build.  Also add
documentation explicitly stating that we now require MIT Kerberos when
building with GSSAPI support.

Reveiwed by: Johnathan Katz
Discussion: https://postgr.es/m/abcc73d0-acf7-6896-e0dc-f5bc12a61bb1@postgresql.org

configure

@@ -14104,6 +14104,33 @@ done
 
 fi
 
+done
+
+  for ac_header in gssapi/gssapi_ext.h
+do :
+  ac_fn_c_check_header_mongrel "$LINENO" "gssapi/gssapi_ext.h" "ac_cv_header_gssapi_gssapi_ext_h" "$ac_includes_default"
+if test "x$ac_cv_header_gssapi_gssapi_ext_h" = xyes; then :
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_GSSAPI_GSSAPI_EXT_H 1
+_ACEOF
+
+else
+  for ac_header in gssapi_ext.h
+do :
+  ac_fn_c_check_header_mongrel "$LINENO" "gssapi_ext.h" "ac_cv_header_gssapi_ext_h" "$ac_includes_default"
+if test "x$ac_cv_header_gssapi_ext_h" = xyes; then :
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_GSSAPI_EXT_H 1
+_ACEOF
+
+else
+  as_fn_error $? "gssapi_ext.h header file is required for GSSAPI" "$LINENO" 5
+fi
+
+done
+
+fi
+
 done
 
 fi

configure.ac

@@ -1562,6 +1562,8 @@ fi
 if test "$with_gssapi" = yes ; then
   AC_CHECK_HEADERS(gssapi/gssapi.h, [],
 	[AC_CHECK_HEADERS(gssapi.h, [], [AC_MSG_ERROR([gssapi.h header file is required for GSSAPI])])])
+  AC_CHECK_HEADERS(gssapi/gssapi_ext.h, [],
+	[AC_CHECK_HEADERS(gssapi_ext.h, [], [AC_MSG_ERROR([gssapi_ext.h header file is required for GSSAPI])])])
 fi
 
 PGAC_PATH_PROGS(OPENSSL, openssl)

doc/src/sgml/client-auth.sgml

@@ -1426,7 +1426,7 @@ omicron         bryanh                  guest1
     The keytab file is generated using the Kerberos software; see the
     Kerberos documentation for details. The following example shows
     doing this using the <application>kadmin</application> tool of
-    MIT-compatible Kerberos 5 implementations:
+    MIT Kerberos:
 <screen>
 <prompt>kadmin% </prompt><userinput>addprinc -randkey postgres/server.my.domain.org</userinput>
 <prompt>kadmin% </prompt><userinput>ktadd -k krb5.keytab postgres/server.my.domain.org</userinput>

doc/src/sgml/installation.sgml

@@ -252,9 +252,9 @@ documentation.  See standalone-profile.xsl for details.
 
     <listitem>
      <para>
-      You need <application>Kerberos</application>, <productname>OpenLDAP</productname>,
-      and/or <application>PAM</application>, if you want to support authentication
-      using those services.
+      You need <application>MIT Kerberos</application> (for GSSAPI),
+      <productname>OpenLDAP</productname>, and/or <application>PAM</application>,
+      if you want to support authentication using those services.
      </para>
     </listitem>
 
@@ -1048,9 +1048,9 @@ build-postgresql:
        <term><option>--with-gssapi</option></term>
        <listitem>
         <para>
-         Build with support for GSSAPI authentication. On many systems, the
-         GSSAPI system (usually a part of the Kerberos installation) is not
-         installed in a location
+         Build with support for GSSAPI authentication. MIT Kerberos is required
+         to be installed for GSSAPI.  On many systems, the GSSAPI system (a part
+         of the MIT Kerberos installation) is not installed in a location
          that is searched by default (e.g., <filename>/usr/include</filename>,
          <filename>/usr/lib</filename>), so you must use the options
          <option>--with-includes</option> and <option>--with-libraries</option> in
@@ -2497,10 +2497,11 @@ ninja install
       <term><option>-Dgssapi={ auto | enabled | disabled }</option></term>
       <listitem>
        <para>
-        Build with support for GSSAPI authentication. On many systems, the
-        GSSAPI system (usually a part of the Kerberos installation) is not
-        installed in a location that is searched by default (e.g.,
-        <filename>/usr/include</filename>, <filename>/usr/lib</filename>).  In
+        Build with support for GSSAPI authentication. MIT Kerberos is required
+        to be installed for GSSAPI.  On many systems, the GSSAPI system (a part
+        of the MIT Kerberos installation) is not installed in a location
+        that is searched by default (e.g., <filename>/usr/include</filename>,
+        <filename>/usr/lib</filename>).  In
         those cases, PostgreSQL will query <command>pkg-config</command> to
         detect the required compiler and linker options.  Defaults to auto.
         <filename>meson configure</filename> will check for the required

meson.build

@@ -623,6 +623,16 @@ if not gssapiopt.disabled()
     have_gssapi = false
   endif
 
+  if not have_gssapi
+  elif cc.check_header('gssapi/gssapi_ext.h', dependencies: gssapi, required: false,
+      args: test_c_args, include_directories: postgres_inc)
+    cdata.set('HAVE_GSSAPI_GSSAPI_EXT_H', 1)
+  elif cc.check_header('gssapi_ext.h', args: test_c_args, dependencies: gssapi, required: gssapiopt)
+    cdata.set('HAVE_GSSAPI_EXT_H', 1)
+  else
+    have_gssapi = false
+  endif
+
   if not have_gssapi
   elif cc.has_function('gss_init_sec_context', dependencies: gssapi,
       args: test_c_args, include_directories: postgres_inc)

src/backend/libpq/auth.c

@@ -922,8 +922,9 @@ pg_GSS_recvauth(Port *port)
 	gss_cred_id_t delegated_creds;
 
 	/*
-	 * Use the configured keytab, if there is one.  Unfortunately, Heimdal
-	 * doesn't support the cred store extensions, so use the env var.
+	 * Use the configured keytab, if there is one.  As we now require MIT
+	 * Kerberos, we might consider using the credential store extensions in
+	 * the future instead of the environment variable.
 	 */
 	if (pg_krb_server_keyfile != NULL && pg_krb_server_keyfile[0] != '\0')
 	{

src/backend/libpq/be-secure-gssapi.c

@@ -526,8 +526,9 @@ secure_open_gssapi(Port *port)
 	PqGSSRecvLength = PqGSSResultLength = PqGSSResultNext = 0;
 
 	/*
-	 * Use the configured keytab, if there is one.  Unfortunately, Heimdal
-	 * doesn't support the cred store extensions, so use the env var.
+	 * Use the configured keytab, if there is one.  As we now require MIT
+	 * Kerberos, we might consider using the credential store extensions in the
+	 * future instead of the environment variable.
 	 */
 	if (pg_krb_server_keyfile != NULL && pg_krb_server_keyfile[0] != '\0')
 	{