build: add handler set implementations

This commit then also adds an implementation of the btclog.Handler
interface called `handlerSets` which basically lets us have a Handler
backed by many Handlers (we need one for the stdout and one for our log
file).

build/handler_sets.go

@@ -0,0 +1,177 @@
+package build
+
+import (
+	"context"
+	"log/slog"
+
+	btclogv1 "github.com/btcsuite/btclog"
+	"github.com/btcsuite/btclog/v2"
+)
+
+// handlerSet is an implementation of Handler that abstracts away multiple
+// Handlers.
+type handlerSet struct {
+	level btclogv1.Level
+	set   []btclog.Handler
+}
+
+// newHandlerSet constructs a new HandlerSet.
+func newHandlerSet(level btclogv1.Level, set ...btclog.Handler) *handlerSet {
+	h := &handlerSet{
+		set:   set,
+		level: level,
+	}
+	h.SetLevel(level)
+
+	return h
+}
+
+// Enabled reports whether the handler handles records at the given level.
+//
+// NOTE: this is part of the slog.Handler interface.
+func (h *handlerSet) Enabled(ctx context.Context, level slog.Level) bool {
+	for _, handler := range h.set {
+		if !handler.Enabled(ctx, level) {
+			return false
+		}
+	}
+
+	return true
+}
+
+// Handle handles the Record.
+//
+// NOTE: this is part of the slog.Handler interface.
+func (h *handlerSet) Handle(ctx context.Context, record slog.Record) error {
+	for _, handler := range h.set {
+		if err := handler.Handle(ctx, record); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// WithAttrs returns a new Handler whose attributes consist of both the
+// receiver's attributes and the arguments.
+//
+// NOTE: this is part of the slog.Handler interface.
+func (h *handlerSet) WithAttrs(attrs []slog.Attr) slog.Handler {
+	newSet := &reducedSet{set: make([]slog.Handler, len(h.set))}
+	for i, handler := range h.set {
+		newSet.set[i] = handler.WithAttrs(attrs)
+	}
+
+	return newSet
+}
+
+// WithGroup returns a new Handler with the given group appended to the
+// receiver's existing groups.
+//
+// NOTE: this is part of the slog.Handler interface.
+func (h *handlerSet) WithGroup(name string) slog.Handler {
+	newSet := &reducedSet{set: make([]slog.Handler, len(h.set))}
+	for i, handler := range h.set {
+		newSet.set[i] = handler.WithGroup(name)
+	}
+
+	return newSet
+}
+
+// SubSystem creates a new Handler with the given sub-system tag.
+//
+// NOTE: this is part of the Handler interface.
+func (h *handlerSet) SubSystem(tag string) btclog.Handler {
+	newSet := &handlerSet{set: make([]btclog.Handler, len(h.set))}
+	for i, handler := range h.set {
+		newSet.set[i] = handler.SubSystem(tag)
+	}
+
+	return newSet
+}
+
+// SetLevel changes the logging level of the Handler to the passed
+// level.
+//
+// NOTE: this is part of the btclog.Handler interface.
+func (h *handlerSet) SetLevel(level btclogv1.Level) {
+	for _, handler := range h.set {
+		handler.SetLevel(level)
+	}
+	h.level = level
+}
+
+// Level returns the current logging level of the Handler.
+//
+// NOTE: this is part of the btclog.Handler interface.
+func (h *handlerSet) Level() btclogv1.Level {
+	return h.level
+}
+
+// A compile-time check to ensure that handlerSet implements btclog.Handler.
+var _ btclog.Handler = (*handlerSet)(nil)
+
+// reducedSet is an implementation of the slog.Handler interface which is
+// itself backed by multiple slog.Handlers. This is used by the handlerSet
+// WithGroup and WithAttrs methods so that we can apply the WithGroup and
+// WithAttrs to the underlying handlers in the set. These calls, however,
+// produce slog.Handlers and not btclog.Handlers. So the reducedSet represents
+// the resulting set produced.
+type reducedSet struct {
+	set []slog.Handler
+}
+
+// Enabled reports whether the handler handles records at the given level.
+//
+// NOTE: this is part of the slog.Handler interface.
+func (r *reducedSet) Enabled(ctx context.Context, level slog.Level) bool {
+	for _, handler := range r.set {
+		if !handler.Enabled(ctx, level) {
+			return false
+		}
+	}
+
+	return true
+}
+
+// Handle handles the Record.
+//
+// NOTE: this is part of the slog.Handler interface.
+func (r *reducedSet) Handle(ctx context.Context, record slog.Record) error {
+	for _, handler := range r.set {
+		if err := handler.Handle(ctx, record); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// WithAttrs returns a new Handler whose attributes consist of both the
+// receiver's attributes and the arguments.
+//
+// NOTE: this is part of the slog.Handler interface.
+func (r *reducedSet) WithAttrs(attrs []slog.Attr) slog.Handler {
+	newSet := &reducedSet{set: make([]slog.Handler, len(r.set))}
+	for i, handler := range r.set {
+		newSet.set[i] = handler.WithAttrs(attrs)
+	}
+
+	return newSet
+}
+
+// WithGroup returns a new Handler with the given group appended to the
+// receiver's existing groups.
+//
+// NOTE: this is part of the slog.Handler interface.
+func (r *reducedSet) WithGroup(name string) slog.Handler {
+	newSet := &reducedSet{set: make([]slog.Handler, len(r.set))}
+	for i, handler := range r.set {
+		newSet.set[i] = handler.WithGroup(name)
+	}
+
+	return newSet
+}
+
+// A compile-time check to ensure that handlerSet implements slog.Handler.
+var _ slog.Handler = (*reducedSet)(nil)