mod(options): build options to log to stderr and/or a file

NOTE: file logging will not work correctly yet due to opening files
appending using the std is currently not implemented (as of zig version
0.13.0)!

src/zlog.zig

@@ -15,26 +15,52 @@ fn logFn(
 ) void {
     // TODO: provide build time configuration to allow tweaking corresponding output
     // - change output file for writing messages to (default `stderr`)
+    // write into own file for each level?
     const level_txt = comptime message_level.asText();
     const prefix = if (scope == .default) ": " else "(" ++ @tagName(scope) ++ "): ";
-    const stderr = std.io.getStdErr().writer();
-    var bw = std.io.bufferedWriter(stderr);
-    const writer = bw.writer();
+    const fmt = level_txt ++ prefix ++ format ++ "\n";
+    if (comptime build_options.file.len != 0) blk: {
+        // NOTE: with zig 0.13.0 there is currently no way to open files to append (except to use libc or talk directly to posix, which this lib should not have to do)
+        const file = std.fs.openFileAbsolute(build_options.file, .{
+            .mode = .read_write,
+        }) catch std.fs.createFileAbsolute(build_options.file, .{
+            .truncate = false,
+        }) catch break :blk;
+        defer file.close();
 
-    std.debug.lockStdErr();
-    defer std.debug.unlockStdErr();
-    nosuspend {
-        if (build_options.timestamp) {
-            const curtime = c_time.time(null);
-            const tm = c_time.localtime(&curtime);
+        var buffer = std.io.bufferedWriter(file.writer());
+        defer buffer.flush() catch {};
 
-            var buf: [32]u8 = undefined;
-            _ = c_time.strftime(@ptrCast(&buf), 32, "%F %R", tm);
-            writer.print("[{s}] ", .{buf}) catch return;
-        }
-        writer.print(level_txt ++ prefix ++ format ++ "\n", args) catch return;
-        bw.flush() catch return;
+        const writer = buffer.writer();
+        log_writing(writer, fmt, args);
     }
+
+    if (comptime build_options.stderr) {
+        var buffer = std.io.bufferedWriter(std.io.getStdErr().writer());
+        defer buffer.flush() catch {};
+
+        std.debug.lockStdErr();
+        defer std.debug.unlockStdErr();
+
+        const writer = buffer.writer();
+        log_writing(writer, fmt, args);
+    }
+}
+
+fn log_writing(writer: anytype, comptime fmt: []const u8, args: anytype) void {
+    nosuspend {
+        if (build_options.timestamp) log_timestamp(writer);
+        writer.print(fmt, args) catch return;
+    }
+}
+
+fn log_timestamp(writer: anytype) void {
+    const curtime = c_time.time(null);
+    const tm = c_time.localtime(&curtime);
+
+    var buffer: [32]u8 = undefined;
+    _ = c_time.strftime(@ptrCast(&buffer), 32, "%F %R", tm);
+    writer.print("[{s}] ", .{buffer}) catch return;
 }
 
 pub fn pretty_format(object: anytype, comptime fmt: []const u8, options: std.fmt.FormatOptions, writer: anytype) !void {