vacuumdb: don't prompt for passwords over and over

Having the script prompt for passwords over and over was a preexisting
problem when it processed multiple databases or when it processed
multiple analyze stages, but the parallel mode introduced in commit
a179232047 made it worse.

Fix the annoyance by keeping a copy of the password used by the first
connection that requires one.  Since users can (currently) only have a
single password, there's no need for more complex arrangements (such as
remembering one password per database).

Per bug #13741 reported by Eric Brown.  Patch authored and
cross-reviewed by Haribabu Kommi and Michael Paquier, slightly tweaked
by Álvaro Herrera.

Discussion: http://www.postgresql.org/message-id/20151027193919.931.54948@wrigleys.postgresql.org
Backpatch to 9.5, where parallel vacuumdb was introduced.

src/bin/scripts/vacuumdb.c

@@ -43,7 +43,8 @@ static void vacuum_one_database(const char *dbname, vacuumingOptions *vacopts,
 					const char *host, const char *port,
 					const char *username, enum trivalue prompt_password,
 					int concurrentCons,
-					const char *progname, bool echo, bool quiet);
+					const char *progname, bool echo, bool quiet,
+					char **password);
 
 static void vacuum_all_databases(vacuumingOptions *vacopts,
 					 bool analyze_in_stages,
@@ -275,6 +276,8 @@ main(int argc, char *argv[])
 	}
 	else
 	{
+		char *password = NULL;
+
 		if (dbname == NULL)
 		{
 			if (getenv("PGDATABASE"))
@@ -296,7 +299,8 @@ main(int argc, char *argv[])
 									&tables,
 									host, port, username, prompt_password,
 									concurrentCons,
-									progname, echo, quiet);
+									progname, echo, quiet,
+									&password);
 			}
 		}
 		else
@@ -305,7 +309,10 @@ main(int argc, char *argv[])
 								&tables,
 								host, port, username, prompt_password,
 								concurrentCons,
-								progname, echo, quiet);
+								progname, echo, quiet,
+								&password);
+
+		pg_free(password);
 	}
 
 	exit(0);
@@ -323,15 +330,21 @@ main(int argc, char *argv[])
  * If concurrentCons is > 1, multiple connections are used to vacuum tables
  * in parallel.  In this case and if the table list is empty, we first obtain
  * a list of tables from the database.
+ *
+ * 'password' is both an input and output parameter.  If one is not passed,
+ * then whatever is used in a connection is returned so that caller can
+ * reuse it in future connections.
  */
 static void
 vacuum_one_database(const char *dbname, vacuumingOptions *vacopts,
 					int stage,
 					SimpleStringList *tables,
 					const char *host, const char *port,
-					const char *username, enum trivalue prompt_password,
+					const char *username,
+					enum trivalue prompt_password,
 					int concurrentCons,
-					const char *progname, bool echo, bool quiet)
+					const char *progname, bool echo, bool quiet,
+					char **password)
 {
 	PQExpBufferData sql;
 	PGconn	   *conn;
@@ -365,8 +378,15 @@ vacuum_one_database(const char *dbname, vacuumingOptions *vacopts,
 		fflush(stdout);
 	}
 
-	conn = connectDatabase(dbname, host, port, username, prompt_password,
-						   progname, false);
+	conn = connectDatabase(dbname, host, port, username, *password,
+						   prompt_password, progname, false);
+
+	/*
+	 * If no password was not specified by caller and the connection required
+	 * one, remember it; this suppresses further password prompts.
+	 */
+	if (PQconnectionUsedPassword(conn) && *password == NULL)
+		*password = pg_strdup(PQpass(conn));
 
 	initPQExpBuffer(&sql);
 
@@ -424,10 +444,20 @@ vacuum_one_database(const char *dbname, vacuumingOptions *vacopts,
 	init_slot(slots, conn);
 	if (parallel)
 	{
+		const char *pqpass;
+
+		/*
+		 * If a password was supplied for the initial connection, use it for
+		 * subsequent ones too.  (Note that since we're connecting to the same
+		 * database with the same user, there's no need to update the stored
+		 * password any further.)
+		 */
+		pqpass = PQpass(conn);
+
 		for (i = 1; i < concurrentCons; i++)
 		{
-			conn = connectDatabase(dbname, host, port, username, prompt_password,
-								   progname, false);
+			conn = connectDatabase(dbname, host, port, username, pqpass,
+								   prompt_password, progname, false);
 			init_slot(slots + i, conn);
 		}
 	}
@@ -542,12 +572,23 @@ vacuum_all_databases(vacuumingOptions *vacopts,
 	PGresult   *result;
 	int			stage;
 	int			i;
+	char	   *password = NULL;
 
 	conn = connectMaintenanceDatabase(maintenance_db, host, port,
 									  username, prompt_password, progname);
+
 	result = executeQuery(conn,
 			"SELECT datname FROM pg_database WHERE datallowconn ORDER BY 1;",
 						  progname, echo);
+
+	/*
+	 * Remember the password for further connections.  If no password was
+	 * required for the maintenance db connection, this gets updated for the
+	 * first connection that does.
+	 */
+	if (PQconnectionUsedPassword(conn))
+		password = pg_strdup(PQpass(conn));
+
 	PQfinish(conn);
 
 	if (analyze_in_stages)
@@ -572,7 +613,8 @@ vacuum_all_databases(vacuumingOptions *vacopts,
 									NULL,
 									host, port, username, prompt_password,
 									concurrentCons,
-									progname, echo, quiet);
+									progname, echo, quiet,
+									&password);
 			}
 		}
 	}
@@ -588,11 +630,13 @@ vacuum_all_databases(vacuumingOptions *vacopts,
 								NULL,
 								host, port, username, prompt_password,
 								concurrentCons,
-								progname, echo, quiet);
+								progname, echo, quiet,
+								&password);
 		}
 	}
 
 	PQclear(result);
+	pg_free(password);
 }
 
 /*