Add new function, PQchangePassword(), to libpq

Essentially this moves the non-interactive part of psql's "\password"
command into an exported client function. The password is not sent to the
server in cleartext because it is "encrypted" (in the case of scram and md5
it is actually hashed, but we have called these encrypted passwords for a
long time now) on the client side. This is good because it ensures the
cleartext password is never known by the server, and therefore won't end up
in logs, pg_stat displays, etc.

In other words, it exists for the same reason as PQencryptPasswordConn(), but
is more convenient as it both builds and runs the "ALTER USER" command for
you. PQchangePassword() uses PQencryptPasswordConn() to do the password
encryption. PQencryptPasswordConn() is passed a NULL for the algorithm
argument, hence encryption is done according to the server's
password_encryption setting.

Also modify the psql client to use the new function. That provides a builtin
test case. Ultimately drivers built on top of libpq should expose this
function and its use should be generally encouraged over doing ALTER USER
directly for password changes.

Author: Joe Conway
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/flat/b75955f7-e8cc-4bbd-817f-ef536bacbe93%40joeconway.com

doc/src/sgml/libpq.sgml

@@ -7116,6 +7116,45 @@ char *PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user,
     </listitem>
    </varlistentry>
 
+   <varlistentry id="libpq-PQchangePassword">
+    <term><function>PQchangePassword</function><indexterm><primary>PQchangePassword</primary></indexterm></term>
+
+    <listitem>
+     <para>
+      Changes a <productname>PostgreSQL</productname> password.
+<synopsis>
+PGresult *PQchangePassword(PGconn *conn, const char *user, const char *passwd);
+</synopsis>
+      This function uses <function>PQencryptPasswordConn</function>
+      to build and execute the command <literal>ALTER USER ... PASSWORD
+      '...'</literal>, thereby changing the user's password. It exists for
+      the same reason as <function>PQencryptPasswordConn</function>, but
+      is more convenient as it both builds and runs the command for you.
+      <xref linkend="libpq-PQencryptPasswordConn"/> is passed a
+      <symbol>NULL</symbol> for the algorithm argument, hence encryption is
+      done according to the server's <xref linkend="guc-password-encryption"/>
+      setting.
+     </para>
+
+     <para>
+      The <parameter>user</parameter> and <parameter>passwd</parameter> arguments
+      are the SQL name of the target user, and the new cleartext password.
+     </para>
+
+     <para>
+      Returns a <structname>PGresult</structname> pointer representing
+      the result of the <literal>ALTER USER</literal> command, or
+      a null pointer if the routine failed before issuing any command.
+      The <xref linkend="libpq-PQresultStatus"/> function should be called
+      to check the return value for any errors (including the value of a null
+      pointer, in which case it will return
+      <symbol>PGRES_FATAL_ERROR</symbol>). Use
+      <xref linkend="libpq-PQerrorMessage"/> to get more information about
+      such errors.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry id="libpq-PQencryptPassword">
     <term><function>PQencryptPassword</function><indexterm><primary>PQencryptPassword</primary></indexterm></term>
 

src/bin/psql/command.c

@@ -2158,29 +2158,15 @@ exec_command_password(PsqlScanState scan_state, bool active_branch)
 		}
 		else
 		{
-			char	   *encrypted_password;
+			PGresult   *res = PQchangePassword(pset.db, user, pw1);
 
-			encrypted_password = PQencryptPasswordConn(pset.db, pw1, user, NULL);
-
-			if (!encrypted_password)
+			if (PQresultStatus(res) != PGRES_COMMAND_OK)
 			{
 				pg_log_info("%s", PQerrorMessage(pset.db));
 				success = false;
 			}
-			else
-			{
-				PGresult   *res;
 
-				printfPQExpBuffer(&buf, "ALTER USER %s PASSWORD ",
-								  fmtId(user));
-				appendStringLiteralConn(&buf, encrypted_password, pset.db);
-				res = PSQLexec(buf.data);
-				if (!res)
-					success = false;
-				else
-					PQclear(res);
-				PQfreemem(encrypted_password);
-			}
+			PQclear(res);
 		}
 
 		free(user);

src/interfaces/libpq/exports.txt

@@ -191,3 +191,4 @@ PQclosePrepared           188
 PQclosePortal             189
 PQsendClosePrepared       190
 PQsendClosePortal         191
+PQchangePassword          192

src/interfaces/libpq/fe-auth.c

@@ -1372,3 +1372,84 @@ PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user,
 
 	return crypt_pwd;
 }
+
+/*
+ * PQchangePassword -- exported routine to change a password
+ *
+ * This is intended to be used by client applications that wish to
+ * change the password for a user. The password is not sent in
+ * cleartext because it is encrypted on the client side. This is
+ * good because it ensures the cleartext password is never known by
+ * the server, and therefore won't end up in logs, pg_stat displays,
+ * etc. The password encryption is performed by PQencryptPasswordConn(),
+ * which is passed a NULL for the algorithm argument. Hence encryption
+ * is done according to the server's password_encryption
+ * setting. We export the function so that clients won't be dependent
+ * on the implementation specific details with respect to how the
+ * server changes passwords.
+ *
+ * Arguments are a connection object, the SQL name of the target user,
+ * and the cleartext password.
+ *
+ * Return value is the PGresult of the executed ALTER USER statement
+ * or NULL if we never get there. The caller is responsible to PQclear()
+ * the returned PGresult.
+ *
+ * PQresultStatus() should be called to check the return value for errors,
+ * and PQerrorMessage() used to get more information about such errors.
+ */
+PGresult *
+PQchangePassword(PGconn *conn, const char *user, const char *passwd)
+{
+	char	   *encrypted_password = PQencryptPasswordConn(conn, passwd,
+														   user, NULL);
+
+	if (!encrypted_password)
+	{
+		/* PQencryptPasswordConn() already registered the error */
+		return NULL;
+	}
+	else
+	{
+		char	   *fmtpw = PQescapeLiteral(conn, encrypted_password,
+											strlen(encrypted_password));
+
+		/* no longer needed, so clean up now */
+		PQfreemem(encrypted_password);
+
+		if (!fmtpw)
+		{
+			/* PQescapeLiteral() already registered the error */
+			return NULL;
+		}
+		else
+		{
+			char	   *fmtuser = PQescapeIdentifier(conn, user, strlen(user));
+
+			if (!fmtuser)
+			{
+				/* PQescapeIdentifier() already registered the error */
+				PQfreemem(fmtpw);
+				return NULL;
+			}
+			else
+			{
+				PQExpBufferData buf;
+				PGresult   *res;
+
+				initPQExpBuffer(&buf);
+				printfPQExpBuffer(&buf, "ALTER USER %s PASSWORD %s",
+								  fmtuser, fmtpw);
+
+				res = PQexec(conn, buf.data);
+
+				/* clean up */
+				termPQExpBuffer(&buf);
+				PQfreemem(fmtuser);
+				PQfreemem(fmtpw);
+
+				return res;
+			}
+		}
+	}
+}

src/interfaces/libpq/libpq-fe.h

@@ -659,6 +659,7 @@ extern int	PQenv2encoding(void);
 
 extern char *PQencryptPassword(const char *passwd, const char *user);
 extern char *PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user, const char *algorithm);
+extern PGresult *PQchangePassword(PGconn *conn, const char *user, const char *passwd);
 
 /* === in encnames.c === */