Introduce proper error handling for file streams

src/filesys.h

@@ -23,6 +23,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <string>
 #include <string_view>
 #include <vector>
+#include <fstream>
 
 #ifdef _WIN32
 #define DIR_DELIM "\\"
@@ -148,14 +149,74 @@ std::string AbsolutePath(const std::string &path);
 // delimiter is found.
 const char *GetFilenameFromPath(const char *path);
 
+// Replace the content of a file on disk in a way that is safe from
+// corruption/truncation on a crash.
+// logs and returns false on error
 bool safeWriteToFile(const std::string &path, std::string_view content);
 
 #ifndef SERVER
 bool extractZipFile(irr::io::IFileSystem *fs, const char *filename, const std::string &destination);
 #endif
 
-bool ReadFile(const std::string &path, std::string &out);
+bool ReadFile(const std::string &path, std::string &out, bool log_error = false);
 
 bool Rename(const std::string &from, const std::string &to);
 
+/**
+ * Open a file buffer with error handling, commonly used with `std::fstream.rdbuf()`.
+ *
+ * @param stream stream references, must not already be open
+ * @param filename filename to open
+ * @param mode mode bits (used as-is)
+ * @param log_error log failure to errorstream?
+ * @param log_warn log failure to warningstream?
+ * @return true if success
+*/
+bool OpenStream(std::filebuf &stream, const char *filename,
+	std::ios::openmode mode, bool log_error, bool log_warn);
+
 } // namespace fs
+
+// outside of namespace for convenience:
+
+/**
+ * Helper function to open an output file stream (= writing).
+ *
+ * For compatibility with fopen() binary mode and truncate are always set.
+ * Use `fs::OpenStream` for more control.
+ * @param name file name
+ * @param log if true, failure to open the file will be logged to errorstream
+ * @param mode additional mode bits (e.g. std::ios::app)
+ * @return file stream, will be !good in case of error
+*/
+inline std::ofstream open_ofstream(const char *name, bool log,
+	std::ios::openmode mode = std::ios::openmode())
+{
+	mode |= std::ios::out | std::ios::binary;
+	if (!(mode & std::ios::app))
+		mode |= std::ios::trunc;
+	std::ofstream ofs;
+	if (!fs::OpenStream(*ofs.rdbuf(), name, mode, log, false))
+		ofs.setstate(std::ios::failbit);
+	return ofs;
+}
+
+/**
+ * Helper function to open an input file stream (= reading).
+ *
+ * For compatibility with fopen() binary mode is always set.
+ * Use `fs::OpenStream` for more control.
+ * @param name file name
+ * @param log if true, failure to open the file will be logged to errorstream
+ * @param mode additional mode bits (e.g. std::ios::ate)
+ * @return file stream, will be !good in case of error
+*/
+inline std::ifstream open_ifstream(const char *name, bool log,
+	std::ios::openmode mode = std::ios::openmode())
+{
+	mode |= std::ios::in | std::ios::binary;
+	std::ifstream ifs;
+	if (!fs::OpenStream(*ifs.rdbuf(), name, mode, log, false))
+		ifs.setstate(std::ios::failbit);
+	return ifs;
+}