enhanced logger: add fixed-width [LEVEL] prefixes, introduce fatalLog for .fatal messages, improve alignment and error handling, made fatal noreturn

src/logging/log.zig

@@ -10,6 +10,34 @@ pub const Level = enum {
     warn,
     @"error",
     fatal,
+
+    /// Returns the fixed-width, uppercase label used as the log-line
+    /// prefix for `level` (e.g. `"INFO "` for `.info`), derived from
+    /// the enum's own field name via `@tagName`.
+    /// All labels are padded to the width of the longest level name,
+    /// so prefixes line up in a fixed-width terminal/file.
+    fn prefix(level: Level) []const u8
+    {
+        return switch (level)
+        {
+            inline else => |l| comptime comptimeTag(@tagName(l)),
+        };
+    }
+
+    fn comptimeTag(comptime name: []const u8) []const u8
+    {
+        const inner_width = 5;
+        const total_width = inner_width + 2; // account for "[" and "]"
+
+        comptime var buf: [total_width]u8 = .{' '} ** total_width;
+
+        buf[0] = '[';
+        inline for (name, 0..) |c, i| buf[i + 1] = std.ascii.toUpper(c);
+        buf[name.len + 1] = ']';
+
+        const result = buf;
+        return &result;
+    }
 };
 
 /// Configuration passed to `Logger.init`.
@@ -148,26 +176,54 @@ pub const Logger = struct {
     {
         if (@intFromEnum(level) < @intFromEnum(logger.level)) return;
 
+        if (level == .fatal) return logger.fatalLog(fmt, args);
+
         try logger.state.mutex.lock(logger.io);
         defer logger.state.mutex.unlock(logger.io);
 
-        try logger.state.writer.interface.print(fmt, args);
+        try logger.state.writer.interface.print(
+            "{s} " ++ fmt,
+            .{Level.prefix(level)} ++ args
+        );
         try logger.state.writer.interface.writeByte('\n');
 
         switch (level)
         {
             .warn, .@"error" => { try logger.state.writer.interface.flush(); },
-            .fatal           => { std.debug.panic(fmt, args); },
+            else             => return,
-            else             => return
         }
     }
 
+    /// Best-effort write-then-panic path for `.fatal` messages.
+    ///
+    /// Ignores lock/write/flush failures rather than propagating them,
+    /// a failure to persist the fatal message must never prevent the
+    /// panic itself. Marked cold since this is checked on every `log`
+    /// call but taken essentially never.
+    fn fatalLog(logger: *Self, comptime fmt: []const u8, args: anytype) noreturn
+    {
+        @branchHint(.cold);
+
+        logger.state.mutex.lock(logger.io)             catch {};
+
+        logger.state.writer.interface.print(
+            "{s} " ++ fmt,
+            .{Level.prefix(.fatal)} ++ args
+        ) catch {};
+
+        logger.state.writer.interface.writeByte('\n')  catch {};
+        logger.state.writer.interface.flush()          catch {};
+
+        std.debug.panic(fmt, args);
+    }
+
     //
 
     pub fn    debug(logger: *Self, comptime fmt: []const u8, args: anytype) void     { logger.log(fmt, args, .debug)    catch |e| if (logger.on_error) |h| h(e); }
     pub fn     info(logger: *Self, comptime fmt: []const u8, args: anytype) void     { logger.log(fmt, args, .info)     catch |e| if (logger.on_error) |h| h(e); }
     pub fn     warn(logger: *Self, comptime fmt: []const u8, args: anytype) void     { logger.log(fmt, args, .warn)     catch |e| if (logger.on_error) |h| h(e); }
     pub fn @"error"(logger: *Self, comptime fmt: []const u8, args: anytype) void     { logger.log(fmt, args, .@"error") catch |e| if (logger.on_error) |h| h(e); }
+    pub fn    fatal(logger: *Self, comptime fmt: []const u8, args: anytype) noreturn { logger.log(fmt, args, .fatal)    catch unreachable; unreachable; }
 
     /// Forces any buffered output to be written to the underlying
     /// writer immediately.
@@ -240,7 +296,7 @@ test "filtering, buffering and automatic flushing"
 
     {
         const contents = try dir.readFile(io, fname, &read_buf);
-        try std.testing.expectEqualStrings("info=1\n", contents);
+        try std.testing.expectEqualStrings("[INFO]  info=1\n", contents);
     }
     try file.setLength(io, 0);
     try log.state.writer.seekTo(0);
@@ -251,7 +307,7 @@ test "filtering, buffering and automatic flushing"
 
     {
         const contents = try dir.readFile(io, fname, &read_buf);
-        try std.testing.expectEqualStrings("warn=1\n", contents);
+        try std.testing.expectEqualStrings("[WARN]  warn=1\n", contents);
     }
     try file.setLength(io, 0);
     try log.state.writer.seekTo(0);
@@ -263,7 +319,7 @@ test "filtering, buffering and automatic flushing"
     {
         const contents = try dir.readFile(io, fname, &read_buf);
         // Note: This doesn't expect a newline, as it only drains the actual overflow, not the newline written into the buffer afterwards!
-        try std.testing.expectEqualStrings("info=" ++ ([_]u8{'x'} ** 300), contents);
+        try std.testing.expectEqualStrings("[INFO]  info=" ++ ([_]u8{'x'} ** 300), contents);
     }
     try file.setLength(io, 0);
     try log.state.writer.seekTo(0);