Fix #6544: Don't join AI company when loading network game in singleplayer

src/company_cmd.cpp

@@ -1198,3 +1198,31 @@ CommandCost CmdGiveMoney(DoCommandFlag flags, uint32 money, CompanyID dest_compa
 	/* Subtract money from local-company */
 	return amount;
 }
+
+/**
+ * Get the index of the first available company. It attempts,
+ *  from first to last, and as soon as the attempt succeeds,
+ *  to get the index of the company:
+ *  1st - get the first existing human company.
+ *  2nd - get the first non-existing company.
+ *  3rd - get COMPANY_FIRST.
+ * @return the index of the first available company.
+ */
+CompanyID GetFirstPlayableCompanyID()
+{
+	for (Company *c : Company::Iterate()) {
+		if (Company::IsHumanID(c->index)) {
+			return c->index;
+		}
+	}
+
+	if (Company::CanAllocateItem()) {
+		for (CompanyID c = COMPANY_FIRST; c < MAX_COMPANIES; c++) {
+			if (!Company::IsValidID(c)) {
+				return c;
+			}
+		}
+	}
+
+	return COMPANY_FIRST;
+}

src/company_func.h

@@ -56,5 +56,6 @@ static inline bool IsInteractiveCompany(CompanyID company)
 }
 
 int CompanyServiceInterval(const Company *c, VehicleType type);
+CompanyID GetFirstPlayableCompanyID();
 
 #endif /* COMPANY_FUNC_H */

src/openttd.cpp

@@ -835,9 +835,9 @@ void HandleExitGameRequest()
  */
 static void OnStartGame(bool dedicated_server)
 {
-	/* Update the local company for a loaded game. It is either always
-	 * company #1 (eg 0) or in the case of a dedicated server a spectator */
-	SetLocalCompany(dedicated_server ? COMPANY_SPECTATOR : COMPANY_FIRST);
+	/* Update the local company for a loaded game. It is either the first available company
+	 * or in the case of a dedicated server, a spectator */
+	SetLocalCompany(dedicated_server ? COMPANY_SPECTATOR : GetFirstPlayableCompanyID());
 
 	/* Update the static game info to set the values from the new game. */
 	NetworkServerUpdateGameInfo();

src/saveload/afterload.cpp

@@ -823,10 +823,12 @@ bool AfterLoadGame()
 	 *  a company does not exist yet. So create one here.
 	 * 1 exception: network-games. Those can have 0 companies
 	 *   But this exception is not true for non-dedicated network servers! */
-	if (!Company::IsValidID(COMPANY_FIRST) && (!_networking || (_networking && _network_server && !_network_dedicated))) {
-		DoStartupNewCompany(false);
-		Company *c = Company::Get(COMPANY_FIRST);
-		c->settings = _settings_client.company;
+	if (!_networking || (_networking && _network_server && !_network_dedicated)) {
+		CompanyID first_human_company = GetFirstPlayableCompanyID();
+		if (!Company::IsValidID(first_human_company)) {
+			Company *c = DoStartupNewCompany(false, first_human_company);
+			c->settings = _settings_client.company;
+		}
 	}
 
 	/* Fix the cache for cargo payments. */
@@ -1006,10 +1008,10 @@ bool AfterLoadGame()
 		/* When loading a game, _local_company is not yet set to the correct value.
 		 * However, in a dedicated server we are a spectator, so nothing needs to
 		 * happen. In case we are not a dedicated server, the local company always
-		 * becomes company 0, unless we are in the scenario editor where all the
-		 * companies are 'invalid'.
+		 * becomes the first available company, unless we are in the scenario editor
+		 * where all the companies are 'invalid'.
 		 */
-		Company *c = Company::GetIfValid(COMPANY_FIRST);
+		Company *c = Company::GetIfValid(GetFirstPlayableCompanyID());
 		if (!_network_dedicated && c != nullptr) {
 			c->settings = _settings_client.company;
 		}