spawn/IProtocol: add EXEC_PATH

The `exec_path` field never worked because it was never copied to the
server.

src/spawn/Client.cxx

@@ -277,6 +277,8 @@ Serialize(SpawnSerializer &s, const PreparedChildProcess &p)
 
 	s.WriteOptionalString(SpawnExecCommand::HOOK_INFO, p.hook_info);
 
+	s.WriteOptionalString(SpawnExecCommand::EXEC_PATH, p.exec_path);
+
 	for (const char *i : p.args)
 		s.WriteString(SpawnExecCommand::ARG, i);
 

src/spawn/IProtocol.hxx

@@ -21,6 +21,7 @@ enum class SpawnRequestCommand : uint8_t {
 };
 
 enum class SpawnExecCommand : uint8_t {
+	EXEC_PATH,
 	ARG,
 	SETENV,
 	UMASK,

src/spawn/Server.cxx

@@ -511,6 +511,10 @@ SpawnServerConnection::HandleExecMessage(SpawnPayload payload,
 	while (!payload.empty()) {
 		const SpawnExecCommand cmd = (SpawnExecCommand)payload.ReadByte();
 		switch (cmd) {
+		case SpawnExecCommand::EXEC_PATH:
+			p.exec_path = payload.ReadString();
+			break;
+
 		case SpawnExecCommand::ARG:
 			if (p.args.size() >= 16384)
 				throw MalformedSpawnPayloadError();