Remove modulo bias from generated IDs

main.go

@@ -510,17 +510,34 @@ func generateUniqueID() (string, error) {
 	return "", fmt.Errorf("failed to generate unique ID after %d retries", maxRetries)
 }
 
+// generateID returns a random ID drawn uniformly from idCharset. Bytes landing
+// in the last, partial repetition of the charset within a byte's range are
+// discarded rather than folded in with a bare modulo: 256 = 7*36 + 4, so
+// folding would make "a" through "d" about 14% more likely than the other 32
+// characters. Only 4 of 256 values are rejected, so a second read is rarely
+// needed.
 func generateID() string {
-	bytes := make([]byte, idLength)
-	if _, err := rand.Read(bytes); err != nil {
+	const unbiasedLimit = byte(256 - 256%len(idCharset))
+
+	id := make([]byte, 0, idLength)
+	buf := make([]byte, idLength)
+	for len(id) < idLength {
+		if _, err := rand.Read(buf); err != nil {
 			panic(err)
 		}
 
-	for i, b := range bytes {
-		bytes[i] = idCharset[int(b)%len(idCharset)]
+		for _, b := range buf {
+			if b >= unbiasedLimit {
+				continue
+			}
+			id = append(id, idCharset[int(b)%len(idCharset)])
+			if len(id) == idLength {
+				break
+			}
+		}
 	}
 
-	return string(bytes)
+	return string(id)
 }
 
 func getHost(r *http.Request) string {