Fix up usage of krb_server_keyfile GUC parameter.

secure_open_gssapi() installed the krb_server_keyfile setting as
KRB5_KTNAME unconditionally, so long as it's not empty.  However,
pg_GSS_recvauth() only installed it if KRB5_KTNAME wasn't set already,
leading to a troubling inconsistency: in theory, clients could see
different sets of server principal names depending on whether they
use GSSAPI encryption.  Always using krb_server_keyfile seems like
the right thing, so make both places do that.  Also fix up
secure_open_gssapi()'s lack of a check for setenv() failure ---
it's unlikely, surely, but security-critical actions are no place
to be sloppy.

Also improve the associated documentation.

This patch does nothing about secure_open_gssapi()'s use of setenv(),
and indeed causes pg_GSS_recvauth() to use it too.  That's nominally
against project portability rules, but since this code is only built
with --with-gssapi, I do not feel a need to do something about this
in the back branches.  A fix will be forthcoming for HEAD though.

Back-patch to v12 where GSSAPI encryption was introduced.  The
dubious behavior in pg_GSS_recvauth() goes back further, but it
didn't have anything to be inconsistent with, so let it be.

Discussion: https://postgr.es/m/2187460.1609263156@sss.pgh.pa.us

doc/src/sgml/client-auth.sgml

@@ -1265,11 +1265,7 @@ omicron         bryanh                  guest1
 
    <para>
     The location of the server's keytab file is specified by the <xref
-    linkend="guc-krb-server-keyfile"/> configuration
-    parameter. The default is
-    <filename>FILE:/usr/local/pgsql/etc/krb5.keytab</filename>
-    (where the directory part is whatever was specified
-    as <varname>sysconfdir</varname> at build time).
+    linkend="guc-krb-server-keyfile"/> configuration parameter.
     For security reasons, it is recommended to use a separate keytab
     just for the <productname>PostgreSQL</productname> server rather
     than allowing the server to read the system keytab file.

doc/src/sgml/config.sgml

@@ -1057,10 +1057,16 @@ include_dir 'conf.d'
       </term>
       <listitem>
        <para>
-        Sets the location of the Kerberos server key file. See
-        <xref linkend="gssapi-auth"/>
-        for details. This parameter can only be set in the
+        Sets the location of the server's Kerberos key file.  The default is
+        <filename>FILE:/usr/local/pgsql/etc/krb5.keytab</filename>
+        (where the directory part is whatever was specified
+        as <varname>sysconfdir</varname> at build time; use
+        <literal>pg_config --sysconfdir</literal> to determine that).
+        If this parameter is set to an empty string, it is ignored and a
+        system-dependent default is used.
+        This parameter can only be set in the
         <filename>postgresql.conf</filename> file or on the server command line.
+        See <xref linkend="gssapi-auth"/> for more information.
        </para>
       </listitem>
      </varlistentry>

src/backend/libpq/auth.c

@@ -1054,29 +1054,18 @@ pg_GSS_recvauth(Port *port)
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("GSSAPI is not supported in protocol version 2")));
 
-	if (pg_krb_server_keyfile && strlen(pg_krb_server_keyfile) > 0)
+	/*
+	 * Use the configured keytab, if there is one.  Unfortunately, Heimdal
+	 * doesn't support the cred store extensions, so use the env var.
+	 */
+	if (pg_krb_server_keyfile != NULL && pg_krb_server_keyfile[0] != '\0')
 	{
-		/*
-		 * Set default Kerberos keytab file for the Krb5 mechanism.
-		 *
-		 * setenv("KRB5_KTNAME", pg_krb_server_keyfile, 0); except setenv()
-		 * not always available.
-		 */
-		if (getenv("KRB5_KTNAME") == NULL)
+		if (setenv("KRB5_KTNAME", pg_krb_server_keyfile, 1) != 0)
 		{
-			size_t		kt_len = strlen(pg_krb_server_keyfile) + 14;
-			char	   *kt_path = malloc(kt_len);
-
-			if (!kt_path ||
-				snprintf(kt_path, kt_len, "KRB5_KTNAME=%s",
-						 pg_krb_server_keyfile) != kt_len - 2 ||
-				putenv(kt_path) != 0)
-			{
-				ereport(LOG,
-						(errcode(ERRCODE_OUT_OF_MEMORY),
-						 errmsg("out of memory")));
-				return STATUS_ERROR;
-			}
+			/* The only likely failure cause is OOM, so use that errcode */
+			ereport(FATAL,
+					(errcode(ERRCODE_OUT_OF_MEMORY),
+					 errmsg("could not set environment: %m")));
 		}
 	}
 

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

@@ -525,8 +525,16 @@ secure_open_gssapi(Port *port)
 	 * Use the configured keytab, if there is one.  Unfortunately, Heimdal
 	 * doesn't support the cred store extensions, so use the env var.
 	 */
-	if (pg_krb_server_keyfile != NULL && strlen(pg_krb_server_keyfile) > 0)
-		setenv("KRB5_KTNAME", pg_krb_server_keyfile, 1);
+	if (pg_krb_server_keyfile != NULL && pg_krb_server_keyfile[0] != '\0')
+	{
+		if (setenv("KRB5_KTNAME", pg_krb_server_keyfile, 1) != 0)
+		{
+			/* The only likely failure cause is OOM, so use that errcode */
+			ereport(FATAL,
+					(errcode(ERRCODE_OUT_OF_MEMORY),
+					 errmsg("could not set environment: %m")));
+		}
+	}
 
 	while (true)
 	{

src/backend/utils/misc/postgresql.conf.sample

@@ -92,7 +92,7 @@
 #db_user_namespace = off
 
 # GSSAPI using Kerberos
-#krb_server_keyfile = ''
+#krb_server_keyfile = 'FILE:${sysconfdir}/krb5.keytab'
 #krb_caseins_users = off
 
 # - SSL -