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.

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

@@ -24,6 +24,10 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
 
 # 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
@@ -46,6 +50,8 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
     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)

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
+}

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 {
+	if len(attrs) == 0 {
 		return c
 	}
+	return &ConsoleHandler{attrs: c.attrs.withAttrs(attrs)}
+}
 
 func (c *ConsoleHandler) WithGroup(name string) slog.Handler {
+	if name == "" {
 		return c
 	}
+	return &ConsoleHandler{attrs: c.attrs.withGroup(name)}
+}

json_handler.go

@@ -8,15 +8,20 @@ import (
 	"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)
-	fmt.Fprintln(os.Stdout, 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
 }
 
@@ -25,9 +30,15 @@ func (j *JSONHandler) Enabled(ctx context.Context, level slog.Level) bool {
 }
 
 func (j *JSONHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	if len(attrs) == 0 {
 		return j
 	}
+	return &JSONHandler{attrs: j.attrs.withAttrs(attrs)}
+}
 
 func (j *JSONHandler) WithGroup(name string) slog.Handler {
+	if name == "" {
 		return j
 	}
+	return &JSONHandler{attrs: j.attrs.withGroup(name)}
+}

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 {
@@ -19,12 +20,24 @@ func (w *WebhookHandler) Enabled(ctx context.Context, level slog.Level) bool {
 }
 
 func (w *WebhookHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	if len(attrs) == 0 {
 		return w
 	}
+	return &WebhookHandler{
+		webhookURL: w.webhookURL,
+		attrs:      w.attrs.withAttrs(attrs),
+	}
+}
 
 func (w *WebhookHandler) WithGroup(name string) slog.Handler {
+	if name == "" {
 		return w
 	}
+	return &WebhookHandler{
+		webhookURL: w.webhookURL,
+		attrs:      w.attrs.withGroup(name),
+	}
+}
 
 func NewWebhookHandler(webhookURL string) (*WebhookHandler, error) {
 	if _, err := url.ParseRequestURI(webhookURL); err != nil {
@@ -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
 }