thcrap: Log: Show an error message if thcrap_log.txt can't be written…

… to. [V]

The challenge here was avoiding a second message box if the user decides
to continue and uses vpatch. As it turns out though, this is once again
the best kind of reference counting where we only care about 0 or ≥1, so
we only need to *create* a 1-byte file mapping and not actually write a
reference counter to it.

thcrap/src/log.cpp

@@ -14,6 +14,9 @@
 // -------
 static FILE *log_file = NULL;
 static int console_open = 0;
+// For checking nested thcrap instances that access the same log file.
+// We only want to print an error message for the first instance.
+static HANDLE log_filemapping = INVALID_HANDLE_VALUE;
 static const char LOG[] = "thcrap_log.txt";
 static const char LOG_ROTATED[] = "thcrap_log.%d.txt";
 static const int ROTATIONS = 1; // Number of backups to keep
@@ -194,6 +197,36 @@ void log_init(int console)
 		VLA_FREE(line);
 	}
 #endif
+	size_t cur_dir_len = GetCurrentDirectoryU(0, nullptr);
+	size_t full_fn_len = cur_dir_len + sizeof(LOG);
+	VLA(char, full_fn, full_fn_len);
+	defer(VLA_FREE(full_fn));
+	GetCurrentDirectoryU(cur_dir_len, full_fn);
+	full_fn[cur_dir_len - 1] = '/';
+	str_slash_normalize(full_fn); // Necessary!
+	memcpy(full_fn + cur_dir_len, LOG, sizeof(LOG));
+
+	log_filemapping = CreateFileMappingU(
+		INVALID_HANDLE_VALUE, nullptr, PAGE_READWRITE, 0, 1, full_fn
+	);
+	if(!log_file && GetLastError() != ERROR_ALREADY_EXISTS) {
+		auto ret = log_mboxf(nullptr, MB_OKCANCEL | MB_ICONHAND,
+			"Error creating %s: %s\n"
+			"\n"
+			"Logging will be unavailable. "
+			"Further writes to this directory are likely to fail as well. "
+			"Moving %s to a different directory will probably fix this.\n"
+			"\n"
+			"Continue?",
+			full_fn, strerror(errno), PROJECT_NAME_SHORT()
+		);
+		if(ret == IDCANCEL) {
+			auto pExitProcess = ((void (__stdcall*)(UINT))detour_top(
+				"kernel32.dll", "ExitProcess", (FARPROC)thcrap_ExitProcess
+			));
+			pExitProcess(-1);
+		}
+	}
 }
 
 void log_exit(void)
@@ -202,6 +235,7 @@ void log_exit(void)
 		FreeConsole();
 	}
 	if(log_file) {
+		CloseHandle(log_filemapping);
 		fclose(log_file);
 		log_file = NULL;
 	}