vaultik/internal/log/tty_handler.go

package log //nolint:revive,nolintlint // stdlib log unused here; see #76

import (
	"context"
	"fmt"
	"io"
	"log/slog"
	"strings"
	"sync"
	"time"
)

// groupSeparator joins an open group path to an attribute key. This
// format has no nesting, so a group becomes a dotted key prefix:
// slog.New(h).WithGroup("db").With("rows", 3) renders "db.rows=3".
const groupSeparator = "."

// bytesAttrKey is the attribute key whose int64 value is rendered as a
// human-readable byte count rather than a bare number.
const bytesAttrKey = "bytes"

// ANSI color codes
const (
	colorReset  = "\033[0m"
	colorRed    = "\033[31m"
	colorYellow = "\033[33m"
	colorBlue   = "\033[34m"
	colorGray   = "\033[90m"
	colorGreen  = "\033[32m"
	colorCyan   = "\033[36m"
	colorBold   = "\033[1m"
)

// TTYHandler is a custom slog handler for TTY output with colors.
//
// A handler and the handlers derived from it via WithAttrs/WithGroup
// all write to the same stream, so they share one mutex; that is why mu
// is a pointer. A value mutex would give every derived handler its own
// lock and stop serializing writes to the stream they have in common.
type TTYHandler struct {
	opts slog.HandlerOptions
	mu   *sync.Mutex
	out  io.Writer

	// attrs are the attributes accumulated through WithAttrs, emitted
	// ahead of each record's own attributes. Their keys already carry
	// the group path that was open when they were added, so no
	// qualification happens at write time.
	attrs []slog.Attr

	// groups is the group path opened by WithGroup, applied as a key
	// prefix to attributes that arrive later — both on a record and
	// through a further WithAttrs.
	groups []string
}

// NewTTYHandler creates a new TTY handler with colored output.
func NewTTYHandler(out io.Writer, opts *slog.HandlerOptions) *TTYHandler {
	if opts == nil {
		opts = &slog.HandlerOptions{}
	}

	return &TTYHandler{
		out:  out,
		opts: *opts,
		mu:   &sync.Mutex{},
	}
}

// Enabled reports whether the handler handles records at the given level.
func (h *TTYHandler) Enabled(_ context.Context, level slog.Level) bool {
	return level >= h.opts.Level.Level()
}

// Handle writes the log record to the output with color formatting.
func (h *TTYHandler) Handle(_ context.Context, r slog.Record) error {
	h.mu.Lock()
	defer h.mu.Unlock()

	// Format timestamp
	timestamp := r.Time.Format("15:04:05")

	// Level and color
	level := r.Level.String()

	var levelColor string

	switch r.Level {
	case slog.LevelDebug:
		levelColor = colorGray
		level = "DEBUG"
	case slog.LevelInfo:
		levelColor = colorGreen
		level = "INFO "
	case slog.LevelWarn:
		levelColor = colorYellow
		level = "WARN "
	case slog.LevelError:
		levelColor = colorRed
		level = "ERROR"
	default:
		levelColor = colorReset
	}

	// Print main message
	_, _ = fmt.Fprintf(h.out, "%s%s%s %s%s%s %s%s%s",
		colorGray, timestamp, colorReset,
		levelColor, level, colorReset,
		colorBold, r.Message, colorReset)

	// Attributes carried by the handler come first, then the record's
	// own. Handler attributes were qualified when they were added; the
	// record's are qualified now, against whatever group path is open.
	for _, a := range h.attrs {
		h.writeAttr(a)
	}

	prefix := strings.Join(h.groups, groupSeparator)

	r.Attrs(func(a slog.Attr) bool {
		for _, flat := range appendAttr(nil, prefix, a) {
			h.writeAttr(flat)
		}

		return true
	})

	_, _ = fmt.Fprintln(h.out)

	return nil
}

// appendAttr flattens a into dst, folding prefix into its key and
// expanding group values into further dotted keys. Following the
// slog.Handler contract: an empty Attr is dropped, a group with no
// attributes is dropped, and a group with an empty key is inlined into
// its parent rather than contributing a level.
func appendAttr(dst []slog.Attr, prefix string, a slog.Attr) []slog.Attr {
	a.Value = a.Value.Resolve()

	if a.Equal(slog.Attr{}) {
		return dst
	}

	key := a.Key

	switch {
	case prefix == "":
		// key stands alone.
	case key == "":
		key = prefix
	default:
		key = prefix + groupSeparator + key
	}

	if a.Value.Kind() != slog.KindGroup {
		return append(dst, slog.Attr{Key: key, Value: a.Value})
	}

	for _, member := range a.Value.Group() {
		dst = appendAttr(dst, key, member)
	}

	return dst
}

// WithAttrs returns a new handler that emits attrs on every record it
// handles, in addition to whatever the handler already carried. Keys
// are qualified by the group path open at the time of the call, so
// WithGroup("db").WithAttrs(rows=3) later renders "db.rows=3".
//
// The receiver is not modified.
func (h *TTYHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	if len(attrs) == 0 {
		return h
	}

	prefix := strings.Join(h.groups, groupSeparator)
	next := h.clone()

	for _, a := range attrs {
		next.attrs = appendAttr(next.attrs, prefix, a)
	}

	return next
}

// WithGroup returns a new handler that qualifies every subsequent
// attribute key with name. This format is a single line with nowhere to
// nest, so grouping is rendered as a dotted key prefix: after
// WithGroup("db"), an attribute "rows" is emitted as "db.rows".
//
// An empty name returns the receiver unchanged, per the slog.Handler
// contract. The receiver is not modified.
func (h *TTYHandler) WithGroup(name string) slog.Handler {
	if name == "" {
		return h
	}

	next := h.clone()
	next.groups = append(next.groups, name)

	return next
}

// clone returns a copy of h that shares its output stream and mutex but
// owns its attribute and group slices.
//
// The slices are copied rather than resliced on purpose. slog permits
// one handler to be derived from concurrently, and two derivations that
// appended into a shared backing array would each overwrite the other's
// attribute — a data race with a silent wrong-output failure mode.
func (h *TTYHandler) clone() *TTYHandler {
	next := &TTYHandler{
		opts:   h.opts,
		mu:     h.mu,
		out:    h.out,
		attrs:  make([]slog.Attr, len(h.attrs), len(h.attrs)+1),
		groups: make([]string, len(h.groups), len(h.groups)+1),
	}

	copy(next.attrs, h.attrs)
	copy(next.groups, h.groups)

	return next
}

// writeAttr renders one already-flattened, already-qualified attribute
// as " key=value". Callers hold h.mu.
func (h *TTYHandler) writeAttr(a slog.Attr) {
	value := a.Value.String()
	// Special handling for certain attribute types
	switch a.Value.Kind() {
	case slog.KindDuration:
		if d, ok := a.Value.Any().(time.Duration); ok {
			value = formatDuration(d)
		}
	case slog.KindInt64:
		if a.Key == bytesAttrKey {
			value = formatBytes(a.Value.Int64())
		}
	case slog.KindAny, slog.KindBool, slog.KindFloat64, slog.KindString,
		slog.KindTime, slog.KindUint64, slog.KindGroup, slog.KindLogValuer:
		// Plain string form above is already correct for these kinds.
	default:
		// Future kinds also use the plain string form.
	}

	_, _ = fmt.Fprintf(h.out, " %s%s%s=%s%s%s",
		colorCyan, a.Key, colorReset,
		colorBlue, value, colorReset)
}

// formatDuration formats a duration in a human-readable way
func formatDuration(d time.Duration) string {
	switch {
	case d < time.Millisecond:
		return fmt.Sprintf("%dµs", d.Microseconds())
	case d < time.Second:
		return fmt.Sprintf("%dms", d.Milliseconds())
	case d < time.Minute:
		return fmt.Sprintf("%.1fs", d.Seconds())
	default:
		return d.String()
	}
}

// formatBytes formats bytes in a human-readable way
func formatBytes(b int64) string {
	const unit = 1024
	if b < unit {
		return fmt.Sprintf("%d B", b)
	}

	div, exp := int64(unit), 0
	for n := b / unit; n >= unit; n /= unit {
		div *= unit
		exp++
	}

	return fmt.Sprintf("%.1f %cB", float64(b)/float64(div), "KMGTPE"[exp])
}