Avoid misleading psql password prompt when username is multiply specified.

When a password is needed, cases such as
	psql -d "postgresql://alice@localhost/testdb" -U bob
would incorrectly prompt for "Password for user bob: ", when actually the
connection will be attempted with username alice.  The priority order of
which name to use isn't that important here, but the misleading prompt is.

When we are prompting for a password after initial connection failure,
we can fix this reliably by looking at PQuser(conn) to see how libpq
interpreted the connection arguments.  But when we're doing a forced
password prompt because of a -W switch, we can't use that solution.
Fortunately, because the main use of -W is for noninteractive situations,
it's less critical to produce a helpful prompt in such cases.  I made
the startup prompt for -W just say "Password: " all the time, rather
than expending extra code on trying to identify which username to use.
In the case of a \c command (after -W has been given), there's already
logic in do_connect that determines whether the "dbname" is a connstring
or URI, so we can avoid lobotomizing the prompt except in cases that
are actually dubious.  (We could do similarly in startup.c if anyone
complains, but for now it seems not worthwhile, especially since that
would still be only a partial solution.)

Per bug #15025 from Akos Vandra.  Although this is arguably a bug fix,
it doesn't seem worth back-patching.  The case where it matters seems
like a very corner-case usage, and someone might complain that we'd
changed the behavior of -W in a minor release.

Discussion: https://postgr.es/m/20180123130013.7407.24749@wrigleys.postgresql.org

src/bin/psql/command.c

@@ -2829,7 +2829,7 @@ prompt_for_password(const char *username)
 {
 	char		buf[100];
 
-	if (username == NULL)
+	if (username == NULL || username[0] == '\0')
 		simple_prompt("Password: ", buf, sizeof(buf), false);
 	else
 	{
@@ -2960,7 +2960,14 @@ do_connect(enum trivalue reuse_previous_specification,
 	 */
 	if (pset.getPassword == TRI_YES)
 	{
-		password = prompt_for_password(user);
+		/*
+		 * If a connstring or URI is provided, we can't be sure we know which
+		 * username will be used, since we haven't parsed that argument yet.
+		 * Don't risk issuing a misleading prompt.  As in startup.c, it does
+		 * not seem worth working harder, since this getPassword option is
+		 * normally only used in noninteractive cases.
+		 */
+		password = prompt_for_password(has_connection_string ? NULL : user);
 	}
 	else if (o_conn && keep_password)
 	{
@@ -3026,8 +3033,12 @@ do_connect(enum trivalue reuse_previous_specification,
 		 */
 		if (!password && PQconnectionNeedsPassword(n_conn) && pset.getPassword != TRI_NO)
 		{
+			/*
+			 * Prompt for password using the username we actually connected
+			 * with --- it might've come out of "dbname" rather than "user".
+			 */
+			password = prompt_for_password(PQuser(n_conn));
 			PQfinish(n_conn);
-			password = prompt_for_password(user);
 			continue;
 		}
 

src/bin/psql/startup.c

@@ -101,7 +101,6 @@ main(int argc, char *argv[])
 	int			successResult;
 	bool		have_password = false;
 	char		password[100];
-	char	   *password_prompt = NULL;
 	bool		new_pass;
 
 	set_pglocale_pgservice(argv[0], PG_TEXTDOMAIN("psql"));
@@ -205,15 +204,14 @@ main(int argc, char *argv[])
 		pset.popt.topt.recordSep.separator_zero = false;
 	}
 
-	if (options.username == NULL)
-		password_prompt = pg_strdup(_("Password: "));
-	else
-		password_prompt = psprintf(_("Password for user %s: "),
-								   options.username);
-
 	if (pset.getPassword == TRI_YES)
 	{
-		simple_prompt(password_prompt, password, sizeof(password), false);
+		/*
+		 * We can't be sure yet of the username that will be used, so don't
+		 * offer a potentially wrong one.  Typical uses of this option are
+		 * noninteractive anyway.
+		 */
+		simple_prompt("Password: ", password, sizeof(password), false);
 		have_password = true;
 	}
 
@@ -252,15 +250,28 @@ main(int argc, char *argv[])
 			!have_password &&
 			pset.getPassword != TRI_NO)
 		{
+			/*
+			 * Before closing the old PGconn, extract the user name that was
+			 * actually connected with --- it might've come out of a URI or
+			 * connstring "database name" rather than options.username.
+			 */
+			const char *realusername = PQuser(pset.db);
+			char	   *password_prompt;
+
+			if (realusername && realusername[0])
+				password_prompt = psprintf(_("Password for user %s: "),
+										   realusername);
+			else
+				password_prompt = pg_strdup(_("Password: "));
 			PQfinish(pset.db);
+
 			simple_prompt(password_prompt, password, sizeof(password), false);
+			free(password_prompt);
 			have_password = true;
 			new_pass = true;
 		}
 	} while (new_pass);
 
-	free(password_prompt);
-
 	if (PQstatus(pset.db) == CONNECTION_BAD)
 	{
 		fprintf(stderr, "%s: %s", pset.progname, PQerrorMessage(pset.db));