Refactor local time getter functions (#12221)

This commit introduces mt_localtime() in src/gettime.h, a wrapper
around the OS-specific thread-safe versions of localtime()
(resp. localtime_s on Windows and localtime_r in other systems).

Per the Open Group recommendation,
«portable applications should call tzset() explicitly before using
ctime_r() or localtime_r() because setting timezone information is
optional for those functions», so we also do a one-shot
call of tzset() (_tzset() on Windows to avoid warning C4996).

The function is used to replace the localtime() calls in
getTimestamp() and makeScreenshot().

(The only reminaing call to localtime() in the tree now is the one in
the local copy of the Lua source code.)

src/client/client.cpp

@@ -1810,11 +1810,10 @@ void Client::makeScreenshot()
 	if (!raw_image)
 		return;
 
-	time_t t = time(NULL);
-	struct tm *tm = localtime(&t);
+	const struct tm tm = mt_localtime();
 
 	char timetstamp_c[64];
-	strftime(timetstamp_c, sizeof(timetstamp_c), "%Y%m%d_%H%M%S", tm);
+	strftime(timetstamp_c, sizeof(timetstamp_c), "%Y%m%d_%H%M%S", &tm);
 
 	std::string screenshot_dir;
 

src/gettime.h

@@ -21,6 +21,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 #include <ctime>
 #include <string>
+#include <mutex>
 
 enum TimePrecision
 {
@@ -30,13 +31,34 @@ enum TimePrecision
 	PRECISION_NANO
 };
 
-inline std::string getTimestamp()
+inline struct tm mt_localtime()
 {
+	// initialize the time zone on first invocation
+	static std::once_flag tz_init;
+	std::call_once(tz_init, [] {
+#ifdef _WIN32
+		_tzset();
+#else
+		tzset();
+#endif
+		});
+
+	struct tm ret;
 	time_t t = time(NULL);
-	// This is not really thread-safe but it won't break anything
-	// except its own output, so just go with it.
-	struct tm *tm = localtime(&t);
+	// TODO we should check if the function returns NULL, which would mean error
+#ifdef _WIN32
+	localtime_s(&ret, &t);
+#else
+	localtime_r(&t, &ret);
+#endif
+	return ret;
+}
+
+
+inline std::string getTimestamp()
+{
+	const struct tm tm = mt_localtime();
 	char cs[20]; // YYYY-MM-DD HH:MM:SS + '\0'
-	strftime(cs, 20, "%Y-%m-%d %H:%M:%S", tm);
+	strftime(cs, 20, "%Y-%m-%d %H:%M:%S", &tm);
 	return cs;
 }