feat(lib/logging): add slog filter handler, move slog init logic here

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/logging/filter.go

@@ -0,0 +1,67 @@
+package logging
+
+import (
+	"context"
+	"log/slog"
+)
+
+// Filterer is the shape of any type that can perform log filtering. This takes
+// the context of the log filtering call and the log record to be filtered.
+type Filterer interface {
+	Filter(ctx context.Context, r slog.Record) bool
+}
+
+// FilterFunc lets you make inline log filters with plain functions.
+type FilterFunc func(ctx context.Context, r *slog.Record) bool
+
+// Filter implements Filterer for FilterFunc.
+func (ff FilterFunc) Filter(ctx context.Context, r *slog.Record) bool {
+	return ff(ctx, r)
+}
+
+// FilterHandler wraps a slog Handler with one or more filters, enabling administrators
+// to customize the logging subsystem of Anubis.
+type FilterHandler struct {
+	next    slog.Handler
+	filters []Filterer
+}
+
+// NewFilterHandler creates a new filtering handler with the given base handler and filters.
+func NewFilterHandler(handler slog.Handler, filters ...Filterer) *FilterHandler {
+	return &FilterHandler{
+		next:    handler,
+		filters: filters,
+	}
+}
+
+// Enabled passes through to the upstream slog Handler.
+func (h *FilterHandler) Enabled(ctx context.Context, level slog.Level) bool {
+	return h.next.Enabled(ctx, level)
+}
+
+// Handle implements slog.Handler and applies all filters before delegating to the base handler.
+func (h *FilterHandler) Handle(ctx context.Context, r slog.Record) error {
+	// Apply all filters - if any filter returns false, skip the log
+	for _, filter := range h.filters {
+		if !filter.Filter(ctx, r) {
+			return nil // Skip this log record
+		}
+	}
+	return h.next.Handle(ctx, r)
+}
+
+// WithAttrs implements slog.Handler.
+func (h *FilterHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	return &FilterHandler{
+		next:    h.next.WithAttrs(attrs),
+		filters: h.filters,
+	}
+}
+
+// WithGroup implements slog.Handler.
+func (h *FilterHandler) WithGroup(name string) slog.Handler {
+	return &FilterHandler{
+		next:    h.next.WithGroup(name),
+		filters: h.filters,
+	}
+}

lib/logging/logging.go

@@ -0,0 +1,24 @@
+package logging
+
+import (
+	"fmt"
+	"log/slog"
+	"os"
+)
+
+func Init(level string) slog.Handler {
+	var programLevel slog.Level
+	if err := (&programLevel).UnmarshalText([]byte(level)); err != nil {
+		fmt.Fprintf(os.Stderr, "invalid log level %s: %v, using info\n", level, err)
+		programLevel = slog.LevelInfo
+	}
+
+	leveler := &slog.LevelVar{}
+	leveler.Set(programLevel)
+
+	h := slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{
+		AddSource: true,
+		Level:     leveler,
+	})
+	return h
+}