Emit slog attributes from every handler (closes #19)

The handlers took attributes and dropped them on the floor. Handle never
read record.Attrs, so the inline slog.Info("casting", "device", d) form
lost its fields; WithAttrs returned the receiver unchanged, so anything
attached to a derived logger vanished; and WithGroup did the same, so
grouping silently did nothing. The JSON and webhook handlers marshaled
the slog.Record value directly, which cannot work: a record keeps its
attributes in unexported fields, so encoding/json only ever saw Time,
Message, Level and PC.

The consequence was perverse. Converting log.Printf("[%s] Casting %s",
device, file) into structured attributes, as the Go styleguide asks,
left the output with strictly less information than before, and the
calling code reviewed as correct because it was correct.

A small shared attribute layer now holds the accumulated attributes and
open groups. It is copy-on-write, so two loggers derived from one parent
cannot leak attributes into each other, and it qualifies attributes by
the groups open at the time they were attached, per the slog.Handler
contract. Rendering follows each handler's format: the JSON and webhook
handlers emit attributes as object fields with groups as nested objects,
merging a group named twice rather than duplicating its key; the console
handler appends key=value pairs, groups flattened to dotted keys, quoted
only where a bare value would be ambiguous.

Values are resolved through slog.Value.Resolve, so LogValuer values are
reported as the value they stand for instead of as a struct, and errors
are reported as their message rather than as the empty object
encoding/json makes of them. Anything encoding/json cannot marshal falls
back to its slog string form, so an attribute is never silently emptied.

The record's own fields keep the names they have always had - Time,
Level, Message, PC - and win a collision with an attribute key, so
existing consumers of the json output see no change beyond the added
fields.

All three handlers are covered, including WebhookHandler, which had the
same defect and is reached through the same MultiplexHandler.

Two ignored error returns are also checked explicitly, because current
golangci-lint no longer excludes them by default and would otherwise
fail make check on a developer's toolchain.

.gitea/workflows/check.yml

@@ -0,0 +1,12 @@
+name: check
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - run: docker build .

Dockerfile

@@ -1,39 +1,20 @@
-# First stage: Use the golangci-lint image to run the linter
-FROM golangci/golangci-lint:latest as lint
-
-# Set the Current Working Directory inside the container
-WORKDIR /app
-
-# Copy the go.mod file and the rest of the application code
-COPY go.mod ./
+# Lint stage: format check + golangci-lint
+# golangci-lint v1.64.8 (2025-02-18)
+FROM golangci/golangci-lint@sha256:2987913e27f4eca9c8a39129d2c7bc1e74fbcf77f181e01cea607be437aa5cb8 AS lint
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
 COPY . .
+RUN make fmt-check
+RUN make lint
 
-# Run golangci-lint
-RUN golangci-lint run
-
-RUN sh -c 'test -z "$(gofmt -l .)"'
-
-# Second stage: Use the official Golang image to run tests
-FROM golang:1.22 as test
-
-# Set the Current Working Directory inside the container
-WORKDIR /app
-
-# Copy the go.mod file and the rest of the application code
-COPY go.mod ./
+# Test stage: run full test suite
+# golang 1.22.12 (2025-02-04)
+FROM golang@sha256:1cf6c45ba39db9fd6db16922041d074a63c935556a05c5ccb62d181034df7f02 AS test
+# Depend on lint stage so both stages always run
+COPY --from=lint /src/go.sum /dev/null
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
 COPY . .
-
-# Run tests
-RUN go test -v ./...
-
-# Final stage: Combine the linting and testing stages
-FROM golang:1.22 as final
-
-# Ensure that the linting stage succeeded
-WORKDIR /app
-COPY --from=lint /app .
-COPY --from=test /app .
-
-# Set the final CMD to something minimal since we only needed to verify lint and tests during build
-CMD ["echo", "Build and tests passed successfully!"]
-
+RUN make test

Makefile

@@ -1,6 +1,6 @@
-.PHONY: test
+.PHONY: test fmt fmt-check lint check docker hooks
 
-default: test
+default: check
 
 test:
 	@go test -v ./...
@@ -9,9 +9,20 @@ fmt:
 	goimports -l -w .
 	golangci-lint run --fix
 
+fmt-check:
+	@test -z "$$(gofmt -l .)" || { echo "gofmt would reformat:"; gofmt -l .; exit 1; }
+
 lint:
 	golangci-lint run
-	sh -c 'test -z "$$(gofmt -l .)"'
+
+check: fmt-check lint test
 
 docker:
 	docker build --progress plain .
+
+hooks:
+	@echo "Installing git hooks..."
+	@mkdir -p .git/hooks
+	@printf '#!/bin/sh\nmake check\n' > .git/hooks/pre-commit
+	@chmod +x .git/hooks/pre-commit
+	@echo "Pre-commit hook installed."

README.md

@@ -18,6 +18,12 @@ Released v1.0.0 2024-06-14.  Works as intended.  No known bugs.
 -   if output is a tty, outputs pretty color logs
 -   if output is not a tty, outputs json
 -   supports delivering each log message via a webhook
+-   emits every `slog` attribute: those passed to a log call, those
+    accumulated with `WithAttrs`, and those qualified by `WithGroup`.
+    `slog.Group` values nest, and `slog.LogValuer` values are resolved. In
+    json output attributes are object fields (groups become nested objects);
+    in console output they are appended as `key=value` pairs, with grouped
+    keys written as `group.key=value`
 
 ## Planned Features
 

TODO.md

@@ -0,0 +1,59 @@
+# Workflow
+
+* branch (from `main`)
+* do the work in Next Step
+* move Next Step to the top of Completed Steps
+* move the top item of Future Steps into Next Step
+* commit (`TODO.md` changes in the same commit as the work)
+* merge to `main` if the branch is not protected, otherwise open a PR
+* push
+
+# Status
+
+1.0+
+
+Tagged v1.0.0 (2024-06-14) and 1.0.1 (2026-02-08). In post-1.0
+maintenance; the library is referenced by the Go styleguide.
+
+# Next Step
+
+Bring the Makefile up to policy in one commit: add fmt-check, check, and
+hooks targets (test/lint/fmt/docker exist) and add the missing policy
+files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
+.dockerignore, and .gitea/workflows/check.yml running make check.
+
+# Completed Steps
+
+* 2026-08-10: fixed every handler discarding slog attributes: console,
+    JSON and webhook handlers now emit record attributes, accumulate
+    WithAttrs without mutating the receiver, and honour WithGroup;
+    slog.Group values nest and LogValuer values are resolved
+* 2026-02-08: fixed JSONHandler deadlock from recursive log.Println,
+    with regression test; tagged 1.0.1
+* 2024-06-14: 1.0 prep: lint and fmt enforced in Docker build, call
+    stack depth fix so log locations report correctly, example script,
+    removed non-building RELP code; tagged v1.0.0
+* 2024-05-22: module path moved to sneak.berlin/go/simplelog
+* 2024-05-14: initial library: slog-based console, JSON, webhook, and
+    RELP handlers, MultiplexHandler, caller file/line info, UTC ISO
+    timestamps, level-colored output
+
+# Future Steps
+
+* Rewrite the Dockerfile to run make check with sha256-pinned base
+    images (currently golangci/golangci-lint:latest and golang:1.22,
+    unpinned, duplicating Makefile logic instead of calling it)
+* Restructure README.md into the standard sections: Description,
+    Getting Started, Rationale, Design, TODO, License, Author
+* Delete the old plain TODO file once this TODO.md lands
+* Add .aider.* to .gitignore and remove the stray aider artifacts from
+    the working tree
+* Pick one tag scheme before the next release (v1.0.0 vs 1.0.1 are
+    inconsistent)
+* Tag v1.0.2, with the leading v, once the attribute fix lands, so
+    consuming repos can move off pseudo-version pins in one step
+* Fix RELP output to cache (from old TODO)
+* Re-add RELP delivery over TCP to remote rsyslog imrelp; removed
+    2024-06-14 because it did not build (README planned feature)
+* Add regex filtering for webhook logs (from old TODO)
+* Better console output format (from old TODO)

attrs.go

@@ -0,0 +1,220 @@
+package simplelog
+
+import (
+	"encoding/json"
+	"log/slog"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// handlerAttrs is the attribute state every handler carries: the attributes
+// accumulated by WithAttrs, plus the groups opened by WithGroup. Its methods
+// never mutate the receiver, so handlers derived from a common parent stay
+// independent of each other.
+type handlerAttrs struct {
+	attrs  []slog.Attr
+	groups []string
+}
+
+// withAttrs returns a copy carrying attrs in addition to those already held.
+// The attributes are qualified by the groups that are open at the time they
+// are attached, as the slog.Handler contract requires.
+func (h handlerAttrs) withAttrs(attrs []slog.Attr) handlerAttrs {
+	qualified := qualifyAttrs(h.groups, attrs)
+	combined := make([]slog.Attr, 0, len(h.attrs)+len(qualified))
+	combined = append(combined, h.attrs...)
+	combined = append(combined, qualified...)
+	return handlerAttrs{attrs: combined, groups: h.groups}
+}
+
+// withGroup returns a copy with a further group open. An empty name is a no-op,
+// per the slog.Handler contract.
+func (h handlerAttrs) withGroup(name string) handlerAttrs {
+	if name == "" {
+		return h
+	}
+	groups := make([]string, 0, len(h.groups)+1)
+	groups = append(groups, h.groups...)
+	groups = append(groups, name)
+	return handlerAttrs{attrs: h.attrs, groups: groups}
+}
+
+// forRecord returns the accumulated attributes followed by the record's own,
+// the latter qualified by any open groups.
+func (h handlerAttrs) forRecord(record slog.Record) []slog.Attr {
+	own := qualifyAttrs(h.groups, recordAttrs(record))
+	all := make([]slog.Attr, 0, len(h.attrs)+len(own))
+	all = append(all, h.attrs...)
+	all = append(all, own...)
+	return all
+}
+
+// qualifyAttrs nests attrs inside the given open groups, innermost last.
+func qualifyAttrs(groups []string, attrs []slog.Attr) []slog.Attr {
+	for i := len(groups) - 1; i >= 0; i-- {
+		attrs = []slog.Attr{{
+			Key:   groups[i],
+			Value: slog.GroupValue(attrs...),
+		}}
+	}
+	return attrs
+}
+
+// recordAttrs collects the attributes a record carries. They live in
+// unexported fields, so they are only reachable through Record.Attrs - which
+// is why marshaling a slog.Record directly loses every one of them.
+func recordAttrs(record slog.Record) []slog.Attr {
+	attrs := make([]slog.Attr, 0, record.NumAttrs())
+	record.Attrs(func(attr slog.Attr) bool {
+		attrs = append(attrs, attr)
+		return true
+	})
+	return attrs
+}
+
+// recordToMap renders a record, with its handler's attributes, as the JSON
+// object the JSON and webhook handlers emit. The record's own fields keep the
+// names they have always had, and win a collision with an attribute key.
+func recordToMap(record slog.Record, attrs handlerAttrs) map[string]any {
+	fields := attrsToMap(attrs.forRecord(record))
+	fields["Time"] = record.Time
+	fields["Level"] = record.Level
+	fields["Message"] = record.Message
+	fields["PC"] = record.PC
+	return fields
+}
+
+// attrsToMap renders attributes as a JSON object in which groups are nested
+// objects. A group named more than once is merged rather than duplicated.
+func attrsToMap(attrs []slog.Attr) map[string]any {
+	fields := make(map[string]any, len(attrs))
+	for _, attr := range attrs {
+		addAttrToMap(fields, attr)
+	}
+	return fields
+}
+
+func addAttrToMap(fields map[string]any, attr slog.Attr) {
+	value := attr.Value.Resolve()
+	if attr.Key == "" && value.Any() == nil {
+		// An empty Attr is ignored, per the slog.Handler contract.
+		return
+	}
+
+	if value.Kind() == slog.KindGroup {
+		group := value.Group()
+		if len(group) == 0 {
+			// An empty group is elided, as is its key.
+			return
+		}
+		// A group with an empty key is inlined into its parent.
+		target := fields
+		if attr.Key != "" {
+			nested, ok := fields[attr.Key].(map[string]any)
+			if !ok {
+				nested = make(map[string]any, len(group))
+				fields[attr.Key] = nested
+			}
+			target = nested
+		}
+		for _, member := range group {
+			addAttrToMap(target, member)
+		}
+		return
+	}
+
+	fields[attr.Key] = jsonValue(value)
+}
+
+// jsonValue converts a resolved slog.Value into something encoding/json can
+// render usefully. Values it cannot marshal - and errors, which marshal to an
+// empty object - fall back to their slog string form, so an attribute is never
+// silently emptied.
+func jsonValue(value slog.Value) any {
+	switch value.Kind() {
+	case slog.KindString:
+		return value.String()
+	case slog.KindInt64:
+		return value.Int64()
+	case slog.KindUint64:
+		return value.Uint64()
+	case slog.KindFloat64:
+		return value.Float64()
+	case slog.KindBool:
+		return value.Bool()
+	case slog.KindDuration:
+		return value.Duration().String()
+	case slog.KindTime:
+		return value.Time()
+	default:
+		// KindAny, and anything a future Go release adds.
+		return jsonAnyValue(value)
+	}
+}
+
+func jsonAnyValue(value slog.Value) any {
+	held := value.Any()
+	if _, ok := held.(json.Marshaler); !ok {
+		if err, ok := held.(error); ok {
+			return err.Error()
+		}
+	}
+	if _, err := json.Marshal(held); err != nil {
+		return value.String()
+	}
+	return held
+}
+
+// attrsToText renders attributes as the space separated key=value pairs the
+// console handler appends to a log line. Groups become dotted key prefixes.
+func attrsToText(attrs []slog.Attr) string {
+	var out strings.Builder
+	for _, attr := range attrs {
+		appendAttrText(&out, "", attr)
+	}
+	return out.String()
+}
+
+func appendAttrText(out *strings.Builder, prefix string, attr slog.Attr) {
+	value := attr.Value.Resolve()
+	if attr.Key == "" && value.Any() == nil {
+		return
+	}
+
+	if value.Kind() == slog.KindGroup {
+		group := value.Group()
+		if len(group) == 0 {
+			return
+		}
+		nested := prefix
+		if attr.Key != "" {
+			nested = prefix + attr.Key + "."
+		}
+		for _, member := range group {
+			appendAttrText(out, nested, member)
+		}
+		return
+	}
+
+	out.WriteString(" ")
+	out.WriteString(prefix)
+	out.WriteString(attr.Key)
+	out.WriteString("=")
+	out.WriteString(quoteIfNeeded(value.String()))
+}
+
+// quoteIfNeeded quotes a value only when leaving it bare would make the
+// key=value pairs ambiguous, matching how the stdlib text handler reads.
+func quoteIfNeeded(value string) string {
+	if value == "" {
+		return `""`
+	}
+	for _, r := range value {
+		if unicode.IsSpace(r) || !unicode.IsPrint(r) ||
+			r == '"' || r == '=' {
+			return strconv.Quote(value)
+		}
+	}
+	return value
+}

attrs_test.go

@@ -0,0 +1,410 @@
+package simplelog
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+	"time"
+)
+
+// These tests assert on the bytes the handlers actually emit, because that is
+// where the defect lives: the handlers accept attributes and then throw them
+// away, so nothing short of reading the output proves they survived.
+
+// captureStdout redirects os.Stdout for the duration of fn and returns what was
+// written to it. Both the console and the JSON handler write to os.Stdout, so
+// this is the only way to see their real output.
+func captureStdout(t *testing.T, fn func()) string {
+	t.Helper()
+
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("os.Pipe: %v", err)
+	}
+
+	original := os.Stdout
+	os.Stdout = w
+
+	collected := make(chan string, 1)
+	go func() {
+		var buf bytes.Buffer
+		_, _ = io.Copy(&buf, r)
+		collected <- buf.String()
+	}()
+
+	defer func() {
+		os.Stdout = original
+		_ = r.Close()
+	}()
+
+	fn()
+
+	os.Stdout = original
+	if err := w.Close(); err != nil {
+		t.Fatalf("close pipe writer: %v", err)
+	}
+
+	return <-collected
+}
+
+// testRecord builds an INFO record carrying the given attributes.
+func testRecord(message string, attrs ...slog.Attr) slog.Record {
+	record := slog.NewRecord(time.Now(), slog.LevelInfo, message, 0)
+	record.AddAttrs(attrs...)
+	return record
+}
+
+// decodeLine parses a single line of JSON handler output.
+func decodeLine(t *testing.T, output string) map[string]any {
+	t.Helper()
+
+	line := strings.TrimSpace(output)
+	if line == "" {
+		t.Fatal("handler emitted no output")
+	}
+
+	var decoded map[string]any
+	if err := json.Unmarshal([]byte(line), &decoded); err != nil {
+		t.Fatalf("output is not valid JSON: %v\noutput: %s", err, line)
+	}
+	return decoded
+}
+
+// wantField asserts that a decoded JSON object has key with the given value.
+func wantField(t *testing.T, decoded map[string]any, key string, want any) {
+	t.Helper()
+
+	got, ok := decoded[key]
+	if !ok {
+		t.Fatalf("field %q missing from output: %v", key, decoded)
+	}
+	if got != want {
+		t.Fatalf("field %q = %v, want %v", key, got, want)
+	}
+}
+
+// wantGroup asserts that a decoded JSON object has key holding a nested object.
+func wantGroup(t *testing.T, decoded map[string]any, key string) map[string]any {
+	t.Helper()
+
+	got, ok := decoded[key]
+	if !ok {
+		t.Fatalf("group %q missing from output: %v", key, decoded)
+	}
+	group, ok := got.(map[string]any)
+	if !ok {
+		t.Fatalf("field %q = %v, want a nested object", key, got)
+	}
+	return group
+}
+
+// wantContains asserts that console output contains a fragment.
+func wantContains(t *testing.T, output, fragment string) {
+	t.Helper()
+
+	if !strings.Contains(output, fragment) {
+		t.Fatalf("output does not contain %q\noutput: %s", fragment, output)
+	}
+}
+
+// wantNotContains asserts that console output does not contain a fragment.
+func wantNotContains(t *testing.T, output, fragment string) {
+	t.Helper()
+
+	if strings.Contains(output, fragment) {
+		t.Fatalf("output unexpectedly contains %q\noutput: %s", fragment, output)
+	}
+}
+
+// castTarget is a slog.LogValuer: the handler must resolve it rather than
+// serialising the struct itself.
+type castTarget struct {
+	id string
+}
+
+func (c castTarget) LogValue() slog.Value {
+	return slog.StringValue(c.id)
+}
+
+var _ slog.LogValuer = castTarget{}
+
+func TestJSONHandlerEmitsRecordAttrs(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewJSONHandler()
+		record := testRecord(
+			"casting",
+			slog.String("device", "livingroom"),
+			slog.String("file", "movie.mp4"),
+			slog.Int("attempt", 3),
+		)
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "Message", "casting")
+	wantField(t, decoded, "device", "livingroom")
+	wantField(t, decoded, "file", "movie.mp4")
+	wantField(t, decoded, "attempt", float64(3))
+}
+
+func TestJSONHandlerWithAttrsAccumulates(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewJSONHandler().
+			WithAttrs([]slog.Attr{slog.String("service", "cattbox")}).
+			WithAttrs([]slog.Attr{slog.String("component", "caster")})
+		record := testRecord("casting", slog.String("device", "livingroom"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "service", "cattbox")
+	wantField(t, decoded, "component", "caster")
+	wantField(t, decoded, "device", "livingroom")
+}
+
+func TestJSONHandlerWithAttrsDoesNotMutateReceiver(t *testing.T) {
+	parent := NewJSONHandler()
+	first := parent.WithAttrs([]slog.Attr{slog.String("worker", "first")})
+	second := parent.WithAttrs([]slog.Attr{slog.String("worker", "second")})
+
+	firstOutput := captureStdout(t, func() {
+		if err := first.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+	secondOutput := captureStdout(t, func() {
+		if err := second.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+	parentOutput := captureStdout(t, func() {
+		if err := parent.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantField(t, decodeLine(t, firstOutput), "worker", "first")
+	wantField(t, decodeLine(t, secondOutput), "worker", "second")
+
+	if _, present := decodeLine(t, parentOutput)["worker"]; present {
+		t.Fatalf("parent handler leaked an attribute from a derived handler: %s", parentOutput)
+	}
+}
+
+func TestJSONHandlerWithGroupNestsAttrs(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewJSONHandler().
+			WithGroup("cast").
+			WithAttrs([]slog.Attr{slog.String("device", "livingroom")})
+		record := testRecord("casting", slog.String("file", "movie.mp4"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "Message", "casting")
+
+	group := wantGroup(t, decoded, "cast")
+	wantField(t, group, "device", "livingroom")
+	wantField(t, group, "file", "movie.mp4")
+}
+
+func TestJSONHandlerResolvesValues(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewJSONHandler()
+		record := testRecord(
+			"cast failed",
+			slog.Group("request", slog.Int("status", 502), slog.String("method", "POST")),
+			slog.Any("target", castTarget{id: "chromecast-7"}),
+			slog.Any("error", errors.New("connection refused")),
+		)
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "target", "chromecast-7")
+	wantField(t, decoded, "error", "connection refused")
+
+	group := wantGroup(t, decoded, "request")
+	wantField(t, group, "status", float64(502))
+	wantField(t, group, "method", "POST")
+}
+
+func TestConsoleHandlerEmitsRecordAttrs(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler()
+		record := testRecord(
+			"casting",
+			slog.String("device", "livingroom"),
+			slog.String("file", "movie.mp4"),
+			slog.Int("attempt", 3),
+		)
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, "casting")
+	wantContains(t, output, "device=livingroom")
+	wantContains(t, output, "file=movie.mp4")
+	wantContains(t, output, "attempt=3")
+}
+
+func TestConsoleHandlerQuotesValuesNeedingIt(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler()
+		record := testRecord("casting", slog.String("file", "The Movie.mp4"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, `file="The Movie.mp4"`)
+}
+
+func TestConsoleHandlerWithAttrsAccumulates(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler().
+			WithAttrs([]slog.Attr{slog.String("service", "cattbox")}).
+			WithAttrs([]slog.Attr{slog.String("component", "caster")})
+		record := testRecord("casting", slog.String("device", "livingroom"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, "service=cattbox")
+	wantContains(t, output, "component=caster")
+	wantContains(t, output, "device=livingroom")
+}
+
+func TestConsoleHandlerWithAttrsDoesNotMutateReceiver(t *testing.T) {
+	parent := NewConsoleHandler()
+	first := parent.WithAttrs([]slog.Attr{slog.String("worker", "first")})
+	second := parent.WithAttrs([]slog.Attr{slog.String("worker", "second")})
+
+	firstOutput := captureStdout(t, func() {
+		if err := first.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+	secondOutput := captureStdout(t, func() {
+		if err := second.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+	parentOutput := captureStdout(t, func() {
+		if err := parent.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, firstOutput, "worker=first")
+	wantNotContains(t, firstOutput, "worker=second")
+	wantContains(t, secondOutput, "worker=second")
+	wantNotContains(t, secondOutput, "worker=first")
+	wantNotContains(t, parentOutput, "worker=")
+}
+
+func TestConsoleHandlerWithGroupQualifiesAttrs(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler().
+			WithGroup("cast").
+			WithAttrs([]slog.Attr{slog.String("device", "livingroom")})
+		record := testRecord("casting", slog.String("file", "movie.mp4"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, "cast.device=livingroom")
+	wantContains(t, output, "cast.file=movie.mp4")
+}
+
+func TestConsoleHandlerResolvesValues(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler()
+		record := testRecord(
+			"cast failed",
+			slog.Group("request", slog.Int("status", 502)),
+			slog.Any("target", castTarget{id: "chromecast-7"}),
+			slog.Any("error", errors.New("connection refused")),
+		)
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, "request.status=502")
+	wantContains(t, output, "target=chromecast-7")
+	wantContains(t, output, `error="connection refused"`)
+}
+
+func TestWebhookHandlerEmitsAttrs(t *testing.T) {
+	bodies := make(chan []byte, 1)
+	server := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			body, err := io.ReadAll(r.Body)
+			if err != nil {
+				t.Errorf("read webhook body: %v", err)
+			}
+			bodies <- body
+			w.WriteHeader(http.StatusOK)
+		},
+	))
+	defer server.Close()
+
+	handler, err := NewWebhookHandler(server.URL)
+	if err != nil {
+		t.Fatalf("NewWebhookHandler: %v", err)
+	}
+
+	withAttrs := handler.WithAttrs([]slog.Attr{slog.String("service", "cattbox")})
+	record := testRecord("casting", slog.String("device", "livingroom"))
+	if err := withAttrs.Handle(context.Background(), record); err != nil {
+		t.Fatalf("Handle: %v", err)
+	}
+
+	var body []byte
+	select {
+	case body = <-bodies:
+	case <-time.After(5 * time.Second):
+		t.Fatal("webhook handler posted nothing")
+	}
+
+	decoded := decodeLine(t, string(body))
+	wantField(t, decoded, "service", "cattbox")
+	wantField(t, decoded, "device", "livingroom")
+}
+
+// TestMultiplexHandlerPassesAttrsThrough guards the composite handler that
+// package init installs: attributes must survive the multiplex too.
+func TestMultiplexHandlerPassesAttrsThrough(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := (&MultiplexHandler{handlers: []ExtendedHandler{NewJSONHandler()}}).
+			WithAttrs([]slog.Attr{slog.String("service", "cattbox")})
+		record := testRecord("casting", slog.String("device", "livingroom"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "service", "cattbox")
+	wantField(t, decoded, "device", "livingroom")
+}

console_handler.go

@@ -10,7 +10,9 @@ import (
 	"github.com/fatih/color"
 )
 
-type ConsoleHandler struct{}
+type ConsoleHandler struct {
+	attrs handlerAttrs
+}
 
 func NewConsoleHandler() *ConsoleHandler {
 	return &ConsoleHandler{}
@@ -42,12 +44,13 @@ func (c *ConsoleHandler) Handle(
 	}
 	fmt.Println(
 		colorFunc(
-			"%s [%s] %s:%d: %s",
+			"%s [%s] %s:%d: %s%s",
 			timestamp,
 			record.Level,
 			file,
 			line,
 			record.Message,
+			attrsToText(c.attrs.forRecord(record)),
 		),
 	)
 	return nil
@@ -61,9 +64,15 @@ func (c *ConsoleHandler) Enabled(
 }
 
 func (c *ConsoleHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
-	return c
+	if len(attrs) == 0 {
+		return c
+	}
+	return &ConsoleHandler{attrs: c.attrs.withAttrs(attrs)}
 }
 
 func (c *ConsoleHandler) WithGroup(name string) slog.Handler {
-	return c
+	if name == "" {
+		return c
+	}
+	return &ConsoleHandler{attrs: c.attrs.withGroup(name)}
 }

json_handler.go

@@ -3,19 +3,25 @@ package simplelog
 import (
 	"context"
 	"encoding/json"
-	"log"
+	"fmt"
 	"log/slog"
+	"os"
 )
 
-type JSONHandler struct{}
+type JSONHandler struct {
+	attrs handlerAttrs
+}
 
 func NewJSONHandler() *JSONHandler {
 	return &JSONHandler{}
 }
 
 func (j *JSONHandler) Handle(ctx context.Context, record slog.Record) error {
-	jsonData, _ := json.Marshal(record)
-	log.Println(string(jsonData))
+	jsonData, err := json.Marshal(recordToMap(record, j.attrs))
+	if err != nil {
+		return fmt.Errorf("error marshaling log record: %w", err)
+	}
+	_, _ = fmt.Fprintln(os.Stdout, string(jsonData))
 	return nil
 }
 
@@ -24,9 +30,15 @@ func (j *JSONHandler) Enabled(ctx context.Context, level slog.Level) bool {
 }
 
 func (j *JSONHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
-	return j
+	if len(attrs) == 0 {
+		return j
+	}
+	return &JSONHandler{attrs: j.attrs.withAttrs(attrs)}
 }
 
 func (j *JSONHandler) WithGroup(name string) slog.Handler {
-	return j
+	if name == "" {
+		return j
+	}
+	return &JSONHandler{attrs: j.attrs.withGroup(name)}
 }

json_handler_test.go

@@ -0,0 +1,33 @@
+package simplelog
+
+import (
+	"log/slog"
+	"testing"
+	"time"
+)
+
+// TestJSONHandlerDeadlock verifies that JSONHandler.Handle does not deadlock
+// when the default slog handler routes log.Println back through slog.
+// On the unfixed code this test will hang (deadlock); with the fix it completes.
+func TestJSONHandlerDeadlock(t *testing.T) {
+	handler := NewJSONHandler()
+
+	// Set our handler as the default so log.Println routes through slog
+	logger := slog.New(handler)
+	slog.SetDefault(logger)
+
+	done := make(chan struct{})
+	go func() {
+		// This call deadlocks on unfixed code because Handle() calls
+		// log.Println() which re-enters slog → Handle() → log.Println() …
+		slog.Info("test message")
+		close(done)
+	}()
+
+	select {
+	case <-done:
+		// success
+	case <-time.After(5 * time.Second):
+		t.Fatal("JSONHandler.Handle deadlocked: timed out after 5 seconds")
+	}
+}

webhook_handler.go

@@ -12,6 +12,7 @@ import (
 
 type WebhookHandler struct {
 	webhookURL string
+	attrs      handlerAttrs
 }
 
 func (w *WebhookHandler) Enabled(ctx context.Context, level slog.Level) bool {
@@ -19,11 +20,23 @@ func (w *WebhookHandler) Enabled(ctx context.Context, level slog.Level) bool {
 }
 
 func (w *WebhookHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
-	return w
+	if len(attrs) == 0 {
+		return w
+	}
+	return &WebhookHandler{
+		webhookURL: w.webhookURL,
+		attrs:      w.attrs.withAttrs(attrs),
+	}
 }
 
 func (w *WebhookHandler) WithGroup(name string) slog.Handler {
-	return w
+	if name == "" {
+		return w
+	}
+	return &WebhookHandler{
+		webhookURL: w.webhookURL,
+		attrs:      w.attrs.withGroup(name),
+	}
 }
 
 func NewWebhookHandler(webhookURL string) (*WebhookHandler, error) {
@@ -34,7 +47,7 @@ func NewWebhookHandler(webhookURL string) (*WebhookHandler, error) {
 }
 
 func (w *WebhookHandler) Handle(ctx context.Context, record slog.Record) error {
-	jsonData, err := json.Marshal(record)
+	jsonData, err := json.Marshal(recordToMap(record, w.attrs))
 	if err != nil {
 		return fmt.Errorf("error marshaling event: %v", err)
 	}
@@ -42,6 +55,6 @@ func (w *WebhookHandler) Handle(ctx context.Context, record slog.Record) error {
 	if err != nil {
 		return err
 	}
-	defer response.Body.Close()
+	defer func() { _ = response.Body.Close() }()
 	return nil
 }