Send diagnostics to stderr and stop dropping log attributes (closes #82)

Two defects in internal/log, fixed together because both live in the
handler construction path.

Logger on stdout (#82). Initialize built both handlers over os.Stdout.
Every --json subcommand writes its document to that same stream, and
WARN/ERROR are never suppressed by any flag, so a log record could land
inside a JSON document and break the parse. This was not theoretical: a
config file with group- or world-readable permissions triggers a WARN
during startup, which was enough to make `snapshot list --json | jq`
fail. Diagnostics now go to stderr, and the TTY/JSON format choice
follows stderr's TTY-ness rather than stdout's -- testing the wrong
stream would colorize records on a redirected stderr whenever stdout
happened to be a terminal, and emit JSON to a terminal in the reverse
case.

This is user-visible: --verbose and --debug output moves to stderr as
well, so `vaultik snapshot list -v > out.txt` no longer captures the
diagnostics. README.md documents the split under a new "stdout and
stderr" section. --quiet and --cron semantics are untouched: level
selection is unchanged, and warnings and errors are still emitted in
both modes.

It also retires the local workaround in internal/vaultik/snapshot_list.go.
warnWhileListing had been hand-rolling structured-log formatting to reach
a writer that was not stdout, and the jsonOutput parameter threaded
through the remote-listing helpers existed only to choose between the two
writers; both are gone, and those warnings go through log.Warn in every
mode. The collect-then-emit machinery around listingWarning stays, on
its remaining merit rather than its original one: slog handlers are safe
for concurrent use, but emitting from the manifest-fetch workers would
order warnings by network timing, where collecting and emitting in key
order after group.Wait makes two runs over the same damaged store
produce the same diagnostics in the same order.

TTYHandler dropped attributes (#97). WithAttrs and WithGroup discarded
their arguments and returned the receiver, while their doc comments
claimed the opposite. Because the handler is chosen by TTY-ness, this
failed only on a terminal and worked correctly in CI -- so it broke
exactly when someone was debugging interactively. Both now return a new
handler: the receiver is never written to, since slog permits a handler
to be shared and derived from concurrently, and the derived handler
copies its slices rather than reslicing so two concurrent derivations
cannot overwrite each other's attributes. The mutex became a pointer so
handlers sharing a stream keep sharing one lock. Attributes persist
across every subsequent record, and grouping is implemented as dotted
key prefixes, which is the only honest rendering for a format with
nowhere to nest.

Tests: an attribute attached through the exported log.With reaches
TTYHandler output; attributes persist across records; groups qualify
keys; deriving does not leak between siblings or back to the parent;
sixteen goroutines derive from and write through one handler under
-race; and the TTY and JSON handlers are fed identical derivation chains
and compared attribute set by attribute set, which is the test that
would have caught the original defect and the one that stops the two
paths drifting again. All of these fail against the unfixed handler.

The existing snapshot-list tests that asserted these warnings on an
injected writer now capture the process's real stderr, which is where
they go; the assertions are otherwise unchanged.

internal/log/tty_handler_test.go

@@ -0,0 +1,377 @@
+package log_test
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"math"
+	"regexp"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/vaultik/internal/log"
+)
+
+// ansiEscape matches the SGR sequences TTYHandler wraps every field in.
+// Stripping them is what lets a test compare TTYHandler's rendering with
+// slog.JSONHandler's.
+var ansiEscape = regexp.MustCompile(`\x1b\[[0-9;]*m`)
+
+// countKey is an attribute key reused across the comparison cases.
+const countKey = "count"
+
+// debugHandlerOptions enables every level, so a test never has to reason
+// about the default level while reasoning about attributes.
+func debugHandlerOptions() *slog.HandlerOptions {
+	return &slog.HandlerOptions{Level: slog.LevelDebug}
+}
+
+// ttyAttrs renders one record through a TTYHandler and returns its
+// attributes as key -> value, with color stripped.
+//
+// TTYHandler emits " key=value" per attribute after the message, and the
+// message itself is the last thing before the first attribute, so
+// splitting on spaces and keeping the tokens containing "=" recovers the
+// attribute set. Test values below therefore avoid spaces and "=".
+func ttyAttrs(t *testing.T, derive func(*slog.Logger) *slog.Logger,
+	msg string, args ...any,
+) map[string]string {
+	t.Helper()
+
+	var buf bytes.Buffer
+
+	logger := slog.New(log.NewTTYHandler(&buf, debugHandlerOptions()))
+	derive(logger).Info(msg, args...)
+
+	line := ansiEscape.ReplaceAllString(buf.String(), "")
+	attrs := make(map[string]string)
+
+	for token := range strings.FieldsSeq(line) {
+		key, value, found := strings.Cut(token, "=")
+		if !found {
+			continue
+		}
+
+		attrs[key] = value
+	}
+
+	return attrs
+}
+
+// jsonAttrs renders one record through slog.JSONHandler and returns its
+// attributes flattened to the same dotted-key form TTYHandler uses, so
+// the two are directly comparable. The built-in time/level/msg fields
+// are dropped: they are the record, not its attributes.
+func jsonAttrs(t *testing.T, derive func(*slog.Logger) *slog.Logger,
+	msg string, args ...any,
+) map[string]string {
+	t.Helper()
+
+	var buf bytes.Buffer
+
+	logger := slog.New(slog.NewJSONHandler(&buf, debugHandlerOptions()))
+	derive(logger).Info(msg, args...)
+
+	var decoded map[string]any
+
+	require.NoError(t, json.Unmarshal(buf.Bytes(), &decoded))
+
+	delete(decoded, slog.TimeKey)
+	delete(decoded, slog.LevelKey)
+	delete(decoded, slog.MessageKey)
+
+	attrs := make(map[string]string)
+	flattenJSON(attrs, "", decoded)
+
+	return attrs
+}
+
+// flattenJSON turns JSONHandler's nested group objects into the dotted
+// keys TTYHandler writes.
+func flattenJSON(dst map[string]string, prefix string, src map[string]any) {
+	for key, value := range src {
+		full := key
+		if prefix != "" {
+			full = prefix + "." + key
+		}
+
+		nested, ok := value.(map[string]any)
+		if ok {
+			flattenJSON(dst, full, nested)
+
+			continue
+		}
+
+		dst[full] = valueString(value)
+	}
+}
+
+// valueString renders a decoded JSON scalar the way slog.Value.String
+// renders the corresponding Go value, so the two handlers' outputs can
+// be compared as strings. encoding/json decodes every number as
+// float64, so an integral one is rendered back as an integer — which is
+// what the Go value that produced it was.
+func valueString(v any) string {
+	switch typed := v.(type) {
+	case string:
+		return typed
+	case bool:
+		return strconv.FormatBool(typed)
+	case float64:
+		if typed == math.Trunc(typed) {
+			return strconv.FormatInt(int64(typed), 10)
+		}
+
+		return strconv.FormatFloat(typed, 'g', -1, 64)
+	default:
+		return fmt.Sprint(v)
+	}
+}
+
+// TestTTYHandlerWithAttrsEmitsAttributes is the direct regression test
+// for the reported defect: WithAttrs discarded its argument, so an
+// attribute attached to a logger never reached the output.
+func TestTTYHandlerWithAttrsEmitsAttributes(t *testing.T) {
+	t.Parallel()
+
+	attrs := ttyAttrs(t, func(l *slog.Logger) *slog.Logger {
+		return l.With("key", "value")
+	}, "hello")
+
+	assert.Equal(t, "value", attrs["key"],
+		"an attribute attached with With must appear on every record")
+}
+
+// TestTTYHandlerWithAttrsPersistsAcrossRecords checks that the
+// attributes are retained rather than emitted once. A handler that
+// stored them but consumed them would pass the test above.
+func TestTTYHandlerWithAttrsPersistsAcrossRecords(t *testing.T) {
+	t.Parallel()
+
+	var buf bytes.Buffer
+
+	logger := slog.New(log.NewTTYHandler(&buf, debugHandlerOptions())).
+		With("request", "abc123")
+
+	logger.Info("first")
+	logger.Info("second")
+
+	plain := ansiEscape.ReplaceAllString(buf.String(), "")
+	lines := strings.Split(strings.TrimSuffix(plain, "\n"), "\n")
+
+	require.Len(t, lines, 2)
+
+	for _, line := range lines {
+		assert.Contains(t, line, "request=abc123")
+	}
+}
+
+// TestTTYHandlerWithGroupQualifiesKeys checks that WithGroup does
+// something real rather than being discarded. This format has no
+// nesting, so grouping shows up as a dotted key prefix.
+func TestTTYHandlerWithGroupQualifiesKeys(t *testing.T) {
+	t.Parallel()
+
+	attrs := ttyAttrs(t, func(l *slog.Logger) *slog.Logger {
+		return l.WithGroup("db").With("rows", 3)
+	}, "queried", "table", "chunks")
+
+	assert.Equal(t, "3", attrs["db.rows"],
+		"an attribute added under a group must be qualified by it")
+	assert.Equal(t, "chunks", attrs["db.table"],
+		"a record attribute must also be qualified by the open group")
+	assert.NotContains(t, attrs, "rows")
+}
+
+// TestTTYHandlerMatchesJSONHandlerAttributes is the drift guard. The
+// handler is chosen by TTY-ness, so a difference between these two is
+// invisible in whichever environment the developer is not in — which is
+// how the original defect survived: attributes vanished on a terminal
+// and were correct in CI.
+func TestTTYHandlerMatchesJSONHandlerAttributes(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name   string
+		derive func(*slog.Logger) *slog.Logger
+		args   []any
+	}{
+		{
+			name:   "record attributes only",
+			derive: func(l *slog.Logger) *slog.Logger { return l },
+			args:   []any{"path", "/etc/vaultik", countKey, 7},
+		},
+		{
+			name: "handler attributes",
+			derive: func(l *slog.Logger) *slog.Logger {
+				return l.With("host", "alpha")
+			},
+			args: []any{countKey, 7},
+		},
+		{
+			name: "handler attributes accumulate",
+			derive: func(l *slog.Logger) *slog.Logger {
+				return l.With("host", "alpha").With("snapshot", "s1")
+			},
+			args: []any{countKey, 7},
+		},
+		{
+			name: "group qualifies later attributes",
+			derive: func(l *slog.Logger) *slog.Logger {
+				return l.WithGroup("db").With("rows", 3)
+			},
+			args: []any{"table", "chunks"},
+		},
+		{
+			name: "nested groups",
+			derive: func(l *slog.Logger) *slog.Logger {
+				return l.WithGroup("outer").WithGroup("inner").
+					With("leaf", "v")
+			},
+			args: []any{"other", "w"},
+		},
+		{
+			name: "attributes before and after a group",
+			derive: func(l *slog.Logger) *slog.Logger {
+				return l.With("top", "t").WithGroup("g").With("in", "i")
+			},
+			args: []any{"rec", "r"},
+		},
+		{
+			name:   "inline group value on the record",
+			derive: func(l *slog.Logger) *slog.Logger { return l },
+			args: []any{slog.Group("net",
+				slog.String("proto", "s3"), slog.Int("retries", 2))},
+		},
+	}
+
+	for _, testCase := range cases {
+		t.Run(testCase.name, func(t *testing.T) {
+			t.Parallel()
+
+			tty := ttyAttrs(t, testCase.derive, "message", testCase.args...)
+			js := jsonAttrs(t, testCase.derive, "message", testCase.args...)
+
+			assert.Equal(t, sortedKeys(js), sortedKeys(tty),
+				"TTY and JSON handlers must emit the same attribute keys")
+			assert.Equal(t, js, tty,
+				"TTY and JSON handlers must emit the same attribute values")
+		})
+	}
+}
+
+// sortedKeys returns m's keys in order, for a stable comparison message.
+func sortedKeys(m map[string]string) []string {
+	keys := make([]string, 0, len(m))
+	for key := range m {
+		keys = append(keys, key)
+	}
+
+	sort.Strings(keys)
+
+	return keys
+}
+
+// TestTTYHandlerWithAttrsDoesNotMutateReceiver checks that deriving does
+// not write through to the parent or to a sibling. slog permits a
+// handler to be shared, so a WithAttrs that appended into the receiver's
+// state would leak attributes between unrelated loggers.
+func TestTTYHandlerWithAttrsDoesNotMutateReceiver(t *testing.T) {
+	t.Parallel()
+
+	var buf bytes.Buffer
+
+	base := slog.New(log.NewTTYHandler(&buf, debugHandlerOptions()))
+	first := base.With("branch", "one")
+	second := base.With("branch", "two")
+
+	base.Info("base")
+	first.Info("first")
+	second.Info("second")
+
+	plain := ansiEscape.ReplaceAllString(buf.String(), "")
+	lines := strings.Split(strings.TrimSuffix(plain, "\n"), "\n")
+
+	require.Len(t, lines, 3)
+
+	assert.NotContains(t, lines[0], "branch=",
+		"deriving must not add attributes to the handler derived from")
+	assert.Contains(t, lines[1], "branch=one")
+	assert.NotContains(t, lines[1], "branch=two")
+	assert.Contains(t, lines[2], "branch=two")
+	assert.NotContains(t, lines[2], "branch=one")
+}
+
+// TestTTYHandlerConcurrentDerivation exercises the same handler being
+// derived from and written through by several goroutines at once, which
+// is what slog permits and what a mutating WithAttrs would make a data
+// race. Run under -race by script/test.
+func TestTTYHandlerConcurrentDerivation(t *testing.T) {
+	t.Parallel()
+
+	const workers = 16
+
+	var buf bytes.Buffer
+
+	base := slog.New(log.NewTTYHandler(&buf, debugHandlerOptions())).
+		With("shared", "yes")
+
+	var group sync.WaitGroup
+
+	group.Add(workers)
+
+	for worker := range workers {
+		go func() {
+			defer group.Done()
+
+			base.With("worker", worker).
+				WithGroup("g").
+				With("nested", worker).
+				Info("concurrent")
+		}()
+	}
+
+	group.Wait()
+
+	plain := ansiEscape.ReplaceAllString(buf.String(), "")
+	lines := strings.Split(strings.TrimSuffix(plain, "\n"), "\n")
+
+	require.Len(t, lines, workers)
+
+	for _, line := range lines {
+		assert.Contains(t, line, "shared=yes")
+		assert.Contains(t, line, "worker=")
+		assert.Contains(t, line, "g.nested=")
+	}
+}
+
+// TestTTYHandlerEmptyGroupAndAttrsAreNoOps covers the slog.Handler
+// contract corners: WithGroup("") and WithAttrs(nil) change nothing, and
+// an empty Attr is dropped rather than rendered as "=".
+func TestTTYHandlerEmptyGroupAndAttrsAreNoOps(t *testing.T) {
+	t.Parallel()
+
+	var buf bytes.Buffer
+
+	handler := log.NewTTYHandler(&buf, debugHandlerOptions())
+
+	assert.Same(t, handler, handler.WithGroup(""),
+		"an empty group name must not open a group")
+	assert.Same(t, handler, handler.WithAttrs(nil),
+		"deriving with no attributes must not allocate a handler")
+
+	slog.New(handler).LogAttrs(context.Background(), slog.LevelInfo, "msg",
+		slog.Attr{}, slog.String("kept", "yes"))
+
+	plain := ansiEscape.ReplaceAllString(buf.String(), "")
+
+	assert.Contains(t, plain, "kept=yes")
+	assert.NotContains(t, plain, " =")
+}