[Uid] Fix UuidV7 collisions within the same ms

nicolas-grekas · nicolas-grekas · commit e3bf65b362d0 · 2023-10-30T14:24:38.000+01:00
diff --git a/src/Symfony/Component/Uid/UuidV7.php b/src/Symfony/Component/Uid/UuidV7.php
@@ -64,6 +64,17 @@ public static function generate(\DateTimeInterface $time = null): string
             self::$rand[1] &= 0x03FF;
             self::$time = $time;
         } else {
+            // Within the same ms, we increment the rand part by a random 24-bit number.
+            // Instead of getting this number from random_bytes(), which is slow, we get
+            // it by sha512-hashing self::$seed. This produces 64 bytes of entropy,
+            // which we need to split in a list of 24-bit numbers. unpack() first splits
+            // them into 16 x 32-bit numbers; we take the first byte of each of these
+            // numbers to get 5 extra 24-bit numbers. Then, we consume those numbers
+            // one-by-one and run this logic every 21 iterations.
+            // self::$rand holds the random part of the UUID, split into 5 x 16-bit
+            // numbers for x86 portability. We increment this random part by the next
+            // 24-bit number in the self::$seedParts list and decrement self::$seedIndex.
+
             if (!self::$seedIndex) {
                 $s = unpack('l*', self::$seed = hash('sha512', self::$seed, true));
                 $s[] = ($s[1] >> 8 & 0xFF0000) | ($s[2] >> 16 & 0xFF00) | ($s[3] >> 24 & 0xFF);
@@ -75,7 +86,7 @@ public static function generate(\DateTimeInterface $time = null): string
                 self::$seedIndex = 21;
             }
 
-            self::$rand[5] = 0xFFFF & $carry = self::$rand[5] + (self::$seedParts[self::$seedIndex--] & 0xFFFFFF);
+            self::$rand[5] = 0xFFFF & $carry = self::$rand[5] + 1 + (self::$seedParts[self::$seedIndex--] & 0xFFFFFF);
             self::$rand[4] = 0xFFFF & $carry = self::$rand[4] + ($carry >> 16);
             self::$rand[3] = 0xFFFF & $carry = self::$rand[3] + ($carry >> 16);
             self::$rand[2] = 0xFFFF & $carry = self::$rand[2] + ($carry >> 16);
