Added thread safety

main.cpp

@@ -1,5 +1,3 @@
-// Start using chrono when C++20 becomes available
-// #include <chrono>
 #include <filesystem>
 #include <iostream>
 #include <list>
@@ -7,6 +5,7 @@
 #include <memory>
 #include <random>
 #include <set>
+#include <shared_mutex>
 #include <string>
 #include <utility>
 #include <vector>
@@ -15,7 +14,6 @@
 #include <stdint.h>
 #include <time.h>
 
-// Stop using these when C++20 becomes available
 #include <fcntl.h>
 #include <sys/stat.h>
 
@@ -34,7 +32,21 @@ constexpr int API_KEY_LENGHT = 16;
 constexpr char DEFAULT_IMAGE_URL[] = "https://trmnl.com/images/setup/setup-logo.bmp";
 constexpr char DEFAULT_IMAGE_FNAME[] = "2024-09-20T00:00:00";
 
-string generate_api_key(int len = API_KEY_LENGHT)
+// Generates a random api key with specified length
+string generate_api_key(int len = API_KEY_LENGHT);
+
+// Reloads devices from specified filename
+// If forced is false - checks last modification time to determine whether to actually do it
+bool reload_container(TRMNLContainer& container, shared_mutex& mut_container, const string& filename, bool forced = false);
+
+// Returns the last modification time down to seconds in ISO format - 'YYYY-MM-DDThh:mm:ss'
+string get_timestamp_from_filename(const string& filepath);
+
+// First string is image filename for URL
+// Second string is timestamp for TRMNL filename
+pair<string, string> find_image_for_friendly(const string& folder_images, const string& friendly_id);
+
+string generate_api_key(int len)
 {
 	string result = "";
 	std::random_device rand;
@@ -82,14 +94,32 @@ string generate_api_key(int len = API_KEY_LENGHT)
 	return result;
 }
 
-bool reload_container(TRMNLContainer& container, const string& filename)
+bool reload_container(TRMNLContainer& container, shared_mutex& mut_container, const string& filename, bool forced)
 {
+	static mutex mut_mod_time;
+	static string last_mod_time = "";
 	json j;
-	bool ok = read_file_json(j, filename, &cout);
+	bool ok;
+	lock_guard guard(mut_mod_time);
+
+	// Check if file mod time has changed since last reload
+	if (!forced)
+	{
+		if (last_mod_time == get_timestamp_from_filename(filename))
+		{
+			return true;
+		}
+	}
+
+	ok = read_file_json(j, filename, &cout);
 	if (!ok)
 	{
 		return false;
 	}
+
+	// Lock Write (exclusive) Mutex & automatically unlock on scope exit
+	unique_lock l(mut_container);
+
 	container.clear();
 	if (j.is_array())
 	{
@@ -98,6 +128,7 @@ bool reload_container(TRMNLContainer& container, const string& filename)
 			container.add_device(j[i]);
 		}
 	}
+	last_mod_time = get_timestamp_from_filename(filename);
 
 	return true;
 }
@@ -127,31 +158,6 @@ string get_timestamp_from_filename(const string& filepath)
 	return result;
 }
 
-// Use this when C++20 becomes available
-#if 0
-string get_timestamp_from_file_time(const fs::file_time_type& ftime)
-{
-	char result[128] = "";
-	time_t file_time = std::chrono::system_clock::to_time_t(std::chrono::file_clock::to_sys(ftime));
-
-	tm* file_tm;
-	file_tm = gmtime(&file_time);
-
-	snprintf(result, sizeof(result) - 1, "%04d-%02d-%02dT%02d:%02d:%02d",
-		file_tm->tm_year + 1900,
-		file_tm->tm_mon + 1,
-		file_tm->tm_mday,
-		file_tm->tm_hour,
-		file_tm->tm_min,
-		file_tm->tm_sec
-	);
-
-	return result;
-}
-#endif
-
-// First string is image filename for URL
-// Second string is timestamp for TRMNL filename
 pair<string, string> find_image_for_friendly(const string& folder_images, const string& friendly_id)
 {
 	pair<string, string> result = {"", ""};
@@ -169,8 +175,6 @@ pair<string, string> find_image_for_friendly(const string& folder_images, const
 		{
 			result.first = entry.path().filename();
 			result.second = get_timestamp_from_filename(entry.path());
-			// Change to this when C++20 becomes available
-			// result.second = get_timestamp_from_file_time(entry.last_write_time());
 		}
 	}
 
@@ -199,6 +203,7 @@ int main(int argc, char **argv)
 	json cfg;
 	json devs;
 	TRMNLContainer container;
+	shared_mutex mut_container;
 
 	if (argc > 2)
 	{
@@ -264,14 +269,15 @@ int main(int argc, char **argv)
 		server = make_shared<httplib::Server>();
 	}
 
-	ok = reload_container(container, devices_filename);
+	ok = reload_container(container, mut_container, devices_filename);
 	if (!ok)
 	{
 		cout << "Could not read devices file" << endl;
 		return -1;
 	}
 
-	auto setup_handler = [&container, &devices_filename](const httplib::Request& req, httplib::Response& res)
+	auto setup_handler = [&container, &mut_container, &devices_filename]
+		(const httplib::Request& req, httplib::Response& res)
 	{
 		json response;
 
@@ -291,8 +297,10 @@ int main(int argc, char **argv)
 
 		// Refresh data from file
 		// Someone might have put a new device in
-		reload_container(container, devices_filename);
+		reload_container(container, mut_container, devices_filename);
 
+		// Write Lock - setup does not happen often enough, but 2 setups can lead to race condition on swapping read/write locks
+		unique_lock l(mut_container);
 		string id = req.get_header_value("ID");
 		TRMNL* trmnl = container.get_device_by_id(id);
 
@@ -341,7 +349,7 @@ int main(int argc, char **argv)
 		}
 	};
 
-	auto display_handler = [&container, &devices_filename, &folder_images, &base_url, refresh_min, refresh_offset]
+	auto display_handler = [&container, &mut_container, &devices_filename, &folder_images, &base_url, refresh_min, refresh_offset]
 		(const httplib::Request& req, httplib::Response& res)
 	{
 		json response;
@@ -361,8 +369,10 @@ int main(int argc, char **argv)
 
 		// Refresh data from file
 		// Someone might have put a new device in
-		reload_container(container, devices_filename);
+		reload_container(container, mut_container, devices_filename);
 
+		// Lock for reading
+		shared_lock l(mut_container);
 		string id = req.get_header_value("ID");
 		string api_key = req.get_header_value("Access-Token");
 		TRMNL* trmnl = container.get_device_by_id(id);