various: cleanup

Fixes a status code conflict in `LimitedResponseWriter`, and a clock
skew bug that I probably introduced last time I dealt with time. I hate
computers.

We now use `tie.Since()`, which employs a monotonic clock that is immune
to system wall clock changes. 

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Iec3147c21c5a295170f48cbf1a4620596a6a6964

cmd/watchdog/root.go

@@ -198,9 +198,19 @@ func rateLimitMiddleware(next http.Handler, limiter *ratelimit.TokenBucket) http
 // Wraps http.ResponseWriter to enforce max response size
 type limitedResponseWriter struct {
 	http.ResponseWriter
-	maxSize       int
-	written       int
-	limitExceeded bool
+	maxSize        int
+	written        int
+	limitExceeded  bool
+	headersWritten bool
+	statusCode     int
+}
+
+func (w *limitedResponseWriter) WriteHeader(statusCode int) {
+	if !w.headersWritten {
+		w.statusCode = statusCode
+		w.headersWritten = true
+		w.ResponseWriter.WriteHeader(statusCode)
+	}
 }
 
 func (w *limitedResponseWriter) Write(p []byte) (int, error) {
@@ -208,11 +218,33 @@ func (w *limitedResponseWriter) Write(p []byte) (int, error) {
 		return 0, fmt.Errorf("response size limit exceeded")
 	}
 
+	// Check if adding this data would exceed the limit
 	if w.written+len(p) > w.maxSize {
 		w.limitExceeded = true
-		w.Header().Set("X-Response-Truncated", "true")
-		http.Error(w.ResponseWriter, "Response size limit exceeded", http.StatusInternalServerError)
-		return 0, fmt.Errorf("response size limit exceeded: %d bytes", w.maxSize)
+
+		// If headers haven't been written yet, we can send an error response
+		if !w.headersWritten {
+			w.Header().Set("X-Response-Truncated", "true")
+			http.Error(w.ResponseWriter, "Response size limit exceeded", http.StatusInternalServerError)
+			return 0, fmt.Errorf("response size limit exceeded: %d bytes", w.maxSize)
+		}
+
+		// Headers already written - we can't change status code
+		// Write only up to the limit and then stop
+		remaining := w.maxSize - w.written
+		if remaining > 0 {
+			_, _ = w.ResponseWriter.Write(p[:remaining])
+			w.written = w.maxSize
+		}
+		return len(p), fmt.Errorf("response size limit exceeded after headers sent")
+	}
+
+	// Normal write
+	if !w.headersWritten {
+		w.headersWritten = true
+		if w.statusCode == 0 {
+			w.statusCode = http.StatusOK
+		}
 	}
 	n, err := w.ResponseWriter.Write(p)
 	w.written += n

internal/aggregate/uniques.go

@@ -107,6 +107,7 @@ func DailySalt(t time.Time) string {
 }
 
 // Save persists the HLL state to disk
+// Format: saltKey\nsalt\nHLLdata
 func (u *UniquesEstimator) Save(path string) error {
 	u.mu.Lock()
 	defer u.mu.Unlock()
@@ -116,11 +117,19 @@ func (u *UniquesEstimator) Save(path string) error {
 		return err
 	}
 
-	// Save both HLL data and current salt
-	return os.WriteFile(path, append([]byte(u.salt+"\n"), data...), 0600)
+	// Save saltKey, salt, and HLL data
+	var buf bytes.Buffer
+	buf.WriteString(u.saltKey)
+	buf.WriteByte('\n')
+	buf.WriteString(u.salt)
+	buf.WriteByte('\n')
+	buf.Write(data)
+
+	return os.WriteFile(path, buf.Bytes(), 0600)
 }
 
 // Load restores the HLL state from disk
+// Supports both new format (saltKey\nsalt\nHLLdata) and old format (salt\nHLLdata)
 func (u *UniquesEstimator) Load(path string) error {
 	data, err := os.ReadFile(path)
 	if err != nil {
@@ -133,8 +142,32 @@ func (u *UniquesEstimator) Load(path string) error {
 	u.mu.Lock()
 	defer u.mu.Unlock()
 
-	// Parse saved salt and HLL data
-	parts := bytes.SplitN(data, []byte("\n"), 2)
+	// Try new format first: saltKey\nsalt\nHLLdata
+	parts := bytes.SplitN(data, []byte("\n"), 3)
+	if len(parts) == 3 {
+		savedSaltKey := string(parts[0])
+		savedSalt := string(parts[1])
+		hllData := parts[2]
+
+		now := time.Now()
+		currentKey := getSaltKey(now, u.rotation)
+
+		// Only restore if it's the same period
+		if savedSaltKey == currentKey {
+			u.salt = savedSalt
+			u.saltKey = savedSaltKey
+			return u.hll.UnmarshalBinary(hllData)
+		}
+
+		// Different period, start fresh
+		u.hll = hyperloglog.New()
+		u.salt = generateSaltFromKey(currentKey)
+		u.saltKey = currentKey
+		return nil
+	}
+
+	// Try old format for backward compatibility: salt\nHLLdata
+	parts = bytes.SplitN(data, []byte("\n"), 2)
 	if len(parts) != 2 {
 		return fmt.Errorf("invalid state file format")
 	}

internal/ratelimit/limiter.go

@@ -27,13 +27,14 @@ func NewTokenBucket(capacity, refillPerInterval int, interval time.Duration) *To
 }
 
 // Allow checks if a request should be allowed
+// Uses monotonic time via time.Since() to prevent clock skew issues
 func (tb *TokenBucket) Allow() bool {
 	tb.mu.Lock()
 	defer tb.mu.Unlock()
 
-	// Refill tokens based on elapsed time
-	now := time.Now()
-	elapsed := now.Sub(tb.lastFill)
+	// Refill tokens based on elapsed time using monotonic clock
+	// time.Since() uses monotonic readings when available, unaffected by wall clock changes
+	elapsed := time.Since(tb.lastFill)
 	if elapsed >= tb.interval {
 		periods := int(elapsed / tb.interval)
 		tb.tokens += periods * tb.refill