Add a structured logger

In debug mode, it has a more user-friendly output:

level | the log messge | ms since last message | key=value key=value

In release mode, it logs using logfmt, which is supported by most log
ingestion frameworks.

Not being used anywhere right now, keeping this PR small with no impact on
existing code.

src/log.zig

@@ -0,0 +1,377 @@
+const std = @import("std");
+const builtin = @import("builtin");
+
+const Allocator = std.mem.Allocator;
+
+// synchronizes writes to the output
+// in debug mode, also synchronizes the timestamp counter for a more human-
+// readable time display
+var mutex: std.Thread.Mutex = .{};
+
+const LogLevel: Log.Level = blk: {
+    const root = @import("root");
+    break :blk if (@hasDecl(root, "LogLevel")) root.LogLevel else .info;
+};
+
+pub const Log = LogT(std.fs.File, builtin.mode == .Debug);
+
+// Generic so that we can test it against an ArrayList
+fn LogT(comptime Out: type, comptime enhanced_readability: bool) type {
+    return struct {
+        out: Out,
+        inject: ?[]const u8,
+        allocator: Allocator,
+        buffer: std.ArrayListUnmanaged(u8),
+
+        const Self = @This();
+
+        pub const Level = enum {
+            debug,
+            info,
+            warn,
+            @"error",
+            fatal,
+        };
+
+        //
+        pub fn init(allocator: Allocator) !Self {
+            return initTo(allocator, std.io.getStdErr());
+        }
+
+        // Used for tests
+        fn initTo(allocator: Allocator, out: Out) !Self {
+            var buffer: std.ArrayListUnmanaged(u8) = .{};
+            try buffer.ensureTotalCapacity(allocator, 2048);
+
+            return .{
+                .out = out,
+                .inject = null,
+                .buffer = buffer,
+                .allocator = allocator,
+            };
+        }
+
+        pub fn deinit(self: *Self) void {
+            self.buffer.deinit(self.allocator);
+        }
+
+        pub fn enabled(comptime level: Level) bool {
+            return @intFromEnum(level) >= @intFromEnum(LogLevel);
+        }
+
+        pub fn debug(self: *Self, comptime msg: []const u8, data: anytype) void {
+            self.log(.debug, msg, data);
+        }
+
+        pub fn info(self: *Self, comptime msg: []const u8, data: anytype) void {
+            self.log(.info, msg, data);
+        }
+
+        pub fn warn(self: *Self, comptime msg: []const u8, data: anytype) void {
+            self.log(.warn, msg, data);
+        }
+
+        pub fn err(self: *Self, comptime msg: []const u8, data: anytype) void {
+            self.log(.@"error", msg, data);
+        }
+
+        pub fn fatal(self: *Self, comptime msg: []const u8, data: anytype) void {
+            self.log(.fatal, msg, data);
+        }
+
+        fn log(self: *Self, comptime level: Level, comptime msg: []const u8, data: anytype) void {
+            if (comptime enabled(level) == false) {
+                return;
+            }
+            defer self.buffer.clearRetainingCapacity();
+            self._log(level, msg, data) catch |e| {
+                std.debug.print("log error: {}  ({s} - {s})\n", .{ e, @tagName(level), msg });
+            };
+        }
+
+        fn _log(self: *Self, comptime level: Level, comptime msg: []const u8, data: anytype) !void {
+            const allocator = self.allocator;
+
+            // We use *AssumeCapacity here because we expect buffer to have
+            // a reasonable default size. We expect time + level + msg + inject
+            // to fit in the initial buffer;
+            var buffer = &self.buffer;
+
+            comptime {
+                if (msg.len > 512) {
+                    @compileError("log msg cannot be greater than 512 characters: '" ++ msg ++ "'");
+                }
+                for (msg) |b| {
+                    switch (b) {
+                        'A'...'Z', 'a'...'z', ' ', '0'...'9', '_', '-', '.', '{', '}' => {},
+                        else => @compileError("log msg contains an invalid character '" ++ msg ++ "'"),
+                    }
+                }
+            }
+
+            std.debug.assert(buffer.capacity >= 1024);
+
+            if (comptime enhanced_readability) {
+                // used when developing, and we prefer readability over having
+                // the output in logfmt
+
+                // write the level this way so that the column lines up.
+                switch (level) {
+                    .info => buffer.appendSliceAssumeCapacity("info  | "),
+                    .debug => buffer.appendSliceAssumeCapacity("debug | "),
+                    .warn => buffer.appendSliceAssumeCapacity("\x1b[33m warn\x1b[0m  | "),
+                    .@"error" => buffer.appendSliceAssumeCapacity("\x1b[31m error\x1b[0m | "),
+                    .fatal => buffer.appendSliceAssumeCapacity("\x1b[41m fatal\x1b[0m | "),
+                }
+
+                buffer.appendSliceAssumeCapacity(msg ++ " | ");
+                const since_last_log = msSinceLastLog();
+
+                if (since_last_log > 1000) {
+                    buffer.appendSliceAssumeCapacity("\x1b[35m");
+                }
+                try std.fmt.format(buffer.writer(allocator), "{d}\x1b[0m |", .{since_last_log});
+            } else {
+                buffer.appendSliceAssumeCapacity("_time=");
+                try std.fmt.format(buffer.writer(allocator), "{d}", .{getTime()});
+
+                const level_and_msg = comptime blk: {
+                    // only wrap msg in quotes if it contains a space
+                    const lm = " _level=" ++ @tagName(level) ++ " _msg=";
+                    if (std.mem.indexOfScalar(u8, msg, ' ') == null) {
+                        break :blk lm ++ msg;
+                    }
+                    break :blk lm ++ "\"" ++ msg ++ "\"";
+                };
+                buffer.appendSliceAssumeCapacity(level_and_msg);
+            }
+
+            if (self.inject) |inject| {
+                buffer.appendAssumeCapacity(' ');
+                buffer.appendSliceAssumeCapacity(inject);
+            }
+
+            inline for (@typeInfo(@TypeOf(data)).Struct.fields) |f| {
+                // + 2 for the leading space and the equal sign
+                // + 5 to save space for null/false/true common values
+                const key_len = f.name.len + 7;
+                try buffer.ensureUnusedCapacity(allocator, key_len);
+                buffer.appendAssumeCapacity(' ');
+                buffer.appendSliceAssumeCapacity(f.name);
+                buffer.appendAssumeCapacity('=');
+                try writeValue(allocator, buffer, @field(data, f.name));
+            }
+
+            if (comptime enhanced_readability) {
+                // reset any color
+                try buffer.appendSlice(allocator, "\x1b[0m");
+            }
+
+            try buffer.append(allocator, '\n');
+
+            mutex.lock();
+            defer mutex.unlock();
+            try self.out.writeAll(self.buffer.items);
+        }
+    };
+}
+
+fn writeValue(allocator: Allocator, buffer: *std.ArrayListUnmanaged(u8), value: anytype) !void {
+    const T = @TypeOf(value);
+    switch (@typeInfo(T)) {
+        .Optional => {
+            if (value) |v| {
+                return writeValue(allocator, buffer, v);
+            }
+            // in _log, we reserved space for a value of up to 5 bytes.
+            return buffer.appendSliceAssumeCapacity("null");
+        },
+        .ComptimeInt, .Int, .ComptimeFloat, .Float => {
+            return std.fmt.format(buffer.writer(allocator), "{d}", .{value});
+        },
+        .Bool => {
+            // in _log, we reserved space for a value of up to 5 bytes.
+            return buffer.appendSliceAssumeCapacity(if (value) "true" else "false");
+        },
+        .ErrorSet => return buffer.appendSlice(allocator, @errorName(value)),
+        .Enum => return buffer.appendSlice(allocator, @tagName(value)),
+        .Array => return writeValue(allocator, buffer, &value),
+        .Pointer => |ptr| switch (ptr.size) {
+            .Slice => switch (ptr.child) {
+                u8 => return writeString(allocator, buffer, value),
+                else => {},
+            },
+            .One => switch (@typeInfo(ptr.child)) {
+                .Array => |arr| if (arr.child == u8) {
+                    return writeString(allocator, buffer, value);
+                },
+                else => return false,
+            },
+            else => {},
+        },
+        else => {},
+    }
+    @compileError("cannot log a: " ++ @typeName(T));
+}
+
+fn writeString(allocator: Allocator, buffer: *std.ArrayListUnmanaged(u8), value: []const u8) !void {
+    var space_count: usize = 0;
+    var escape_count: usize = 0;
+    var binary_count: usize = 0;
+
+    for (value) |b| {
+        switch (b) {
+            '\r', '\n', '"' => escape_count += 1,
+            ' ' => space_count += 1,
+            '\t', '!', '#'...'~' => {}, // printable characters
+            else => binary_count += 1,
+        }
+    }
+
+    if (binary_count > 0) {
+        // TODO: use a different encoding if the ratio of binary data / printable
+        // is low
+        // TODO: Zig 0.14 adds an encodeWriter
+        return buffer.appendSlice(allocator, "\"<binary data> (will be supported once we move to Zig 0.14\"");
+        // return std.base64.standard_no_pad.Encoder.encodeWriter(buffer.writer(allocator), value);
+    }
+
+    if (escape_count == 0) {
+        if (space_count == 0) {
+            return buffer.appendSlice(allocator, value);
+        }
+        try buffer.ensureUnusedCapacity(allocator, 2 + value.len);
+        buffer.appendAssumeCapacity('"');
+        buffer.appendSliceAssumeCapacity(value);
+        buffer.appendAssumeCapacity('"');
+        return;
+    }
+
+    // + 2 for the quotes
+    // + escape_count because every character that needs escaping is + 1
+    try buffer.ensureUnusedCapacity(allocator, 2 + value.len + escape_count);
+
+    buffer.appendAssumeCapacity('"');
+
+    var rest = value;
+    while (rest.len > 0) {
+        const pos = std.mem.indexOfAny(u8, rest, "\r\n\"") orelse {
+            buffer.appendSliceAssumeCapacity(rest);
+            break;
+        };
+        buffer.appendSliceAssumeCapacity(rest[0..pos]);
+        buffer.appendAssumeCapacity('\\');
+        switch (rest[pos]) {
+            '"' => buffer.appendAssumeCapacity('"'),
+            '\r' => buffer.appendAssumeCapacity('r'),
+            '\n' => buffer.appendAssumeCapacity('n'),
+            else => unreachable,
+        }
+        rest = rest[pos + 1 ..];
+    }
+
+    buffer.appendAssumeCapacity('"');
+}
+
+fn getTime() i64 {
+    if (comptime @import("builtin").is_test) {
+        return 1739795092929;
+    }
+    return std.time.milliTimestamp();
+}
+
+var last_log_for_debug: i64 = 0;
+fn msSinceLastLog() i64 {
+    if (comptime builtin.mode != .Debug) {
+        @compileError("Log's enhanced_readability is not safe to use in non-Debug mode");
+    }
+    const now = getTime();
+
+    mutex.lock();
+    defer mutex.unlock();
+    defer last_log_for_debug = now;
+    if (last_log_for_debug == 0) {
+        return 0;
+    }
+    return now - last_log_for_debug;
+}
+
+const testing = std.testing;
+const TestLogger = LogT(std.ArrayListUnmanaged(u8).Writer, false);
+
+test "log: data" {
+    var buf: std.ArrayListUnmanaged(u8) = .{};
+    defer buf.deinit(testing.allocator);
+
+    var log = try TestLogger.initTo(testing.allocator, buf.writer(testing.allocator));
+    defer log.deinit();
+
+    {
+        log.err("nope", .{});
+        try testing.expectEqualStrings("_time=1739795092929 _level=error _msg=nope\n", buf.items);
+    }
+
+    {
+        buf.clearRetainingCapacity();
+        const string = try testing.allocator.dupe(u8, "spice_must_flow");
+        defer testing.allocator.free(string);
+
+        log.warn("a msg", .{
+            .cint = 5,
+            .cfloat = 3.43,
+            .int = @as(i16, -49),
+            .float = @as(f32, 0.0003232),
+            .bt = true,
+            .bf = false,
+            .nn = @as(?i32, 33),
+            .n = @as(?i32, null),
+            .lit = "over9000!",
+            .slice = string,
+            .err = error.Nope,
+            .level = Log.Level.warn,
+        });
+
+        try testing.expectEqualStrings("_time=1739795092929 _level=warn _msg=\"a msg\" " ++
+            "cint=5 cfloat=3.43 int=-49 float=0.0003232 bt=true bf=false " ++
+            "nn=33 n=null lit=over9000! slice=spice_must_flow " ++
+            "err=Nope level=warn\n", buf.items);
+    }
+}
+
+test "log: string escape" {
+    var buf: std.ArrayListUnmanaged(u8) = .{};
+    defer buf.deinit(testing.allocator);
+
+    var log = try TestLogger.initTo(testing.allocator, buf.writer(testing.allocator));
+    defer log.deinit();
+
+    const prefix = "_time=1739795092929 _level=error _msg=test ";
+    {
+        log.err("test", .{ .string = "hello world" });
+        try testing.expectEqualStrings(prefix ++ "string=\"hello world\"\n", buf.items);
+    }
+
+    {
+        buf.clearRetainingCapacity();
+        log.err("test", .{ .string = "\n \thi  \" \" " });
+        try testing.expectEqualStrings(prefix ++ "string=\"\\n \thi  \\\" \\\" \"\n", buf.items);
+    }
+
+    // TODO: Zig 0.14
+    // {
+    //     log.err("test", .{.string = [_]u8{0, 244, 55, 77}});
+    //     try testing.expectEqualStrings(prefix ++ "string=\"\\n \\thi  \\\" \\\" \"\n", buf.items);
+    // }
+}
+
+test "log: with inject" {
+    var buf: std.ArrayListUnmanaged(u8) = .{};
+    defer buf.deinit(testing.allocator);
+
+    var log = try TestLogger.initTo(testing.allocator, buf.writer(testing.allocator));
+    defer log.deinit();
+
+    log.inject = "conn_id=339494";
+    log.fatal("hit", .{ .over = 9000 });
+    try testing.expectEqualStrings("_time=1739795092929 _level=fatal _msg=hit conn_id=339494 over=9000\n", buf.items);
+}

src/unit_tests.zig

@@ -381,4 +381,5 @@ test {
     std.testing.refAllDecls(@import("iterator/iterator.zig"));
     std.testing.refAllDecls(@import("server.zig"));
     std.testing.refAllDecls(@import("cdp/cdp.zig"));
+    std.testing.refAllDecls(@import("log.zig"));
 }