Permission: refactor getPermissionFromPassword() to return std::optional

This replaces the output parameter (which is bad API design).  As a
side effect, it fixes the bad [[gnu::pure]] attribute added by commit
a636d2127 which caused optimizing compilers to miscompile calls to
that function.  "Pure" functions can be assumed to have no output
arguments, so the compiler can assume the function doesn't modify
them.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1282

NEWS

@@ -1,5 +1,6 @@
 ver 0.23.1 (not yet released)
 * fix libfmt linker problems
+* fix broken password authentication
 
 ver 0.23 (2021/10/14)
 * protocol

src/Permission.cxx

@@ -161,15 +161,14 @@ GetPermissionsFromAddress(SocketAddress address) noexcept
 
 #endif
 
-int
-getPermissionFromPassword(const char *password, unsigned *permission) noexcept
+std::optional<unsigned>
+GetPermissionFromPassword(const char *password) noexcept
 {
 	auto i = permission_passwords.find(password);
 	if (i == permission_passwords.end())
-		return -1;
+		return std::nullopt;
 
-	*permission = i->second;
-	return 0;
+	return i->second;
 }
 
 unsigned

src/Permission.hxx

@@ -22,6 +22,8 @@
 
 #include "config.h"
 
+#include <optional>
+
 struct ConfigData;
 class SocketAddress;
 
@@ -32,9 +34,13 @@ static constexpr unsigned PERMISSION_CONTROL = 4;
 static constexpr unsigned PERMISSION_ADMIN = 8;
 static constexpr unsigned PERMISSION_PLAYER = 16;
 
+/**
+ * @return the permissions for the given password or std::nullopt if
+ * the password is not accepted
+ */
 [[gnu::pure]]
-int
-getPermissionFromPassword(const char *password, unsigned *permission) noexcept;
+std::optional<unsigned>
+GetPermissionFromPassword(const char *password) noexcept;
 
 [[gnu::const]]
 unsigned

src/command/ClientCommands.cxx

@@ -58,13 +58,13 @@ handle_binary_limit(Client &client, Request args,
 CommandResult
 handle_password(Client &client, Request args, Response &r)
 {
-	unsigned permission = 0;
-	if (getPermissionFromPassword(args.front(), &permission) < 0) {
+	const auto permission = GetPermissionFromPassword(args.front());
+	if (!permission) {
 		r.Error(ACK_ERROR_PASSWORD, "incorrect password");
 		return CommandResult::ERROR;
 	}
 
-	client.SetPermission(permission);
+	client.SetPermission(*permission);
 
 	return CommandResult::OK;
 }