Add passwordcheck.min_password_length.

This new parameter can be used to change the minimum allowed
password length (in bytes).  Note that it has no effect if a user
supplies a pre-encrypted password.

Author: Emanuele Musella, Maurizio Boriani
Reviewed-by: Tomas Vondra, Bertrand Drouvot, Japin Li
Discussion: https://postgr.es/m/CA%2BugDNyYtHOtWCqVD3YkSVYDWD_1fO8Jm_ahsDGA5dXhbDPwrQ%40mail.gmail.com

contrib/passwordcheck/expected/passwordcheck.out

@@ -6,6 +6,10 @@ ALTER USER regress_passwordcheck_user1 PASSWORD 'a_nice_long_password';
 -- error: too short
 ALTER USER regress_passwordcheck_user1 PASSWORD 'tooshrt';
 ERROR:  password is too short
+DETAIL:  password must be at least "passwordcheck.min_password_length" (8) bytes long
+-- ok
+SET passwordcheck.min_password_length = 6;
+ALTER USER regress_passwordcheck_user1 PASSWORD 'v_shrt';
 -- error: contains user name
 ALTER USER regress_passwordcheck_user1 PASSWORD 'xyzregress_passwordcheck_user1';
 ERROR:  password must not contain user name

contrib/passwordcheck/expected/passwordcheck_1.out

@@ -6,6 +6,10 @@ ALTER USER regress_passwordcheck_user1 PASSWORD 'a_nice_long_password';
 -- error: too short
 ALTER USER regress_passwordcheck_user1 PASSWORD 'tooshrt';
 ERROR:  password is too short
+DETAIL:  password must be at least "passwordcheck.min_password_length" (8) bytes long
+-- ok
+SET passwordcheck.min_password_length = 6;
+ALTER USER regress_passwordcheck_user1 PASSWORD 'v_shrt';
 -- error: contains user name
 ALTER USER regress_passwordcheck_user1 PASSWORD 'xyzregress_passwordcheck_user1';
 ERROR:  password must not contain user name

contrib/passwordcheck/passwordcheck.c

@@ -15,6 +15,7 @@
 #include "postgres.h"
 
 #include <ctype.h>
+#include <limits.h>
 
 #ifdef USE_CRACKLIB
 #include <crack.h>
@@ -29,8 +30,8 @@ PG_MODULE_MAGIC;
 /* Saved hook value */
 static check_password_hook_type prev_check_password_hook = NULL;
 
-/* passwords shorter than this will be rejected */
-#define MIN_PWD_LENGTH 8
+/* GUC variables */
+static int	min_password_length = 8;
 
 /*
  * check_password
@@ -93,10 +94,12 @@ check_password(const char *username,
 #endif
 
 		/* enforce minimum length */
-		if (pwdlen < MIN_PWD_LENGTH)
+		if (pwdlen < min_password_length)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-					 errmsg("password is too short")));
+					 errmsg("password is too short"),
+					 errdetail("password must be at least \"passwordcheck.min_password_length\" (%d) bytes long",
+							   min_password_length)));
 
 		/* check if the password contains the username */
 		if (strstr(password, username))
@@ -142,6 +145,19 @@ check_password(const char *username,
 void
 _PG_init(void)
 {
+	/* Define custom GUC variables. */
+	DefineCustomIntVariable("passwordcheck.min_password_length",
+							"Minimum allowed password length.",
+							NULL,
+							&min_password_length,
+							8,
+							0, INT_MAX,
+							PGC_SUSET,
+							GUC_UNIT_BYTE,
+							NULL, NULL, NULL);
+
+	MarkGUCPrefixReserved("passwordcheck");
+
 	/* activate password checks when the module is loaded */
 	prev_check_password_hook = check_password_hook;
 	check_password_hook = check_password;

contrib/passwordcheck/sql/passwordcheck.sql

@@ -9,6 +9,10 @@ ALTER USER regress_passwordcheck_user1 PASSWORD 'a_nice_long_password';
 -- error: too short
 ALTER USER regress_passwordcheck_user1 PASSWORD 'tooshrt';
 
+-- ok
+SET passwordcheck.min_password_length = 6;
+ALTER USER regress_passwordcheck_user1 PASSWORD 'v_shrt';
+
 -- error: contains user name
 ALTER USER regress_passwordcheck_user1 PASSWORD 'xyzregress_passwordcheck_user1';
 

doc/src/sgml/passwordcheck.sgml

@@ -59,4 +59,41 @@
   </para>
  </caution>
 
+ <sect2 id="passwordcheck-configuration-parameters">
+  <title>Configuration Parameters</title>
+
+  <variablelist>
+   <varlistentry>
+    <term>
+     <varname>passwordcheck.min_password_length</varname> (<type>integer</type>)
+     <indexterm>
+      <primary><varname>passwordcheck.min_password_length</varname> configuration parameter</primary>
+     </indexterm>
+    </term>
+    <listitem>
+     <para>
+      The minimum acceptable password length in bytes.  The default is 8.  Only
+      superusers can change this setting.
+     </para>
+     <note>
+      <para>
+       This parameter has no effect if a user supplies a pre-encrypted
+       password.
+      </para>
+     </note>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+
+  <para>
+   In ordinary usage, this parameter is set in
+   <filename>postgresql.conf</filename>, but superusers can alter it on-the-fly
+   within their own sessions.  Typical usage might be:
+  </para>
+
+<programlisting>
+# postgresql.conf
+passwordcheck.min_password_length = 12
+</programlisting>
+ </sect2>
 </sect1>