random: Fix off-by-one in fast path selection of Randomizer::getBytesFromString() (#10449)

With a single byte we can choose offsets between 0x00 and 0xff, thus 0x100
different offsets. We only need to use the slow path for sources of more than
0x100 bytes.

The previous version was correct with regard to the output expectations, it was
just slower than necessary. Better fix this now while we still can before being
bound by our BC guarantees with regard to emitted sequences.

This also adds a test to verify the behavior: For powers of two we never reject
any values during rejection sampling, we just need to mask off the unneeded
bits. Thus we can specifically verify that the number of calls to the engine
match the expected amount. We also verify that all the possible values are
emitted to make sure the masking does not remove any required bits. For inputs
longer than 0x100 bytes we need trust the `range()` implementation to be
unbiased, but still verify the number of engine calls and perform a basic
output check.

Author: TimWolla

ext/random/randomizer.c

@@ -401,7 +401,7 @@ PHP_METHOD(Random_Randomizer, getBytesFromString)
 
 	retval = zend_string_alloc(length, 0);
 
-	if (source_length > 0xFF) {
+	if (source_length > 0x100) {
 		while (total_size < length) {
 			uint64_t offset = randomizer->algo->range(randomizer->status, 0, source_length - 1);
 

ext/random/tests/03_randomizer/methods/getBytesFromString_fast_path.phpt

@@ -0,0 +1,112 @@
+--TEST--
+Random: Randomizer: getBytesFromString(): Fast Path Masking
+--FILE--
+<?php
+
+use Random\Engine\Test\TestWrapperEngine;
+use Random\Engine\Xoshiro256StarStar;
+use Random\Randomizer;
+
+require __DIR__ . "/../../engines.inc";
+
+$allBytes = implode('', array_map(
+    fn ($byte) => chr($byte),
+    range(0x00, 0xff)
+));
+
+// Xoshiro256** is the fastest engine available.
+$xoshiro = new Xoshiro256StarStar();
+
+var_dump(strlen($allBytes));
+echo PHP_EOL;
+
+// Fast path: Inputs less than or equal to 256.
+for ($i = 1; $i <= strlen($allBytes); $i *= 2) {
+    echo "{$i}:", PHP_EOL;
+
+    $wrapper = new TestWrapperEngine($xoshiro);
+    $r = new Randomizer($wrapper);
+    $result = $r->getBytesFromString(substr($allBytes, 0, $i), 20000);
+
+    // Xoshiro256** is a 64 Bit engine and thus generates 8 bytes at once.
+    // For powers of two we expect no rejections and thus exactly
+    // 20000/8 = 2500 calls to the engine.
+    var_dump($wrapper->getCount());
+
+    $count = [];
+    for ($j = 0; $j < strlen($result); $j++) {
+        $b = $result[$j];
+        $count[ord($b)] ??= 0;
+        $count[ord($b)]++;
+    }
+
+    // We also expect that each possible value appears at least once, if
+    // not is is very likely that some bits were erroneously masked away.
+    var_dump(count($count));
+
+    echo PHP_EOL;
+}
+
+echo "Slow Path:", PHP_EOL;
+
+$wrapper = new TestWrapperEngine($xoshiro);
+$r = new Randomizer($wrapper);
+$result = $r->getBytesFromString($allBytes . $allBytes, 20000);
+
+// In the slow path we expect one call per byte, i.e. 20000
+var_dump($wrapper->getCount());
+
+$count = [];
+for ($j = 0; $j < strlen($result); $j++) {
+    $b = $result[$j];
+    $count[ord($b)] ??= 0;
+    $count[ord($b)]++;
+}
+
+// We also expect that each possible value appears at least once, if
+// not is is very likely that some bits were erroneously masked away.
+var_dump(count($count));
+
+?>
+--EXPECT--
+int(256)
+
+1:
+int(2500)
+int(1)
+
+2:
+int(2500)
+int(2)
+
+4:
+int(2500)
+int(4)
+
+8:
+int(2500)
+int(8)
+
+16:
+int(2500)
+int(16)
+
+32:
+int(2500)
+int(32)
+
+64:
+int(2500)
+int(64)
+
+128:
+int(2500)
+int(128)
+
+256:
+int(2500)
+int(256)
+
+Slow Path:
+int(20000)
+int(256)

ext/random/tests/engines.inc

@@ -27,14 +27,23 @@ final class TestShaEngine implements Engine
 
 final class TestWrapperEngine implements Engine
 {
+    private int $count = 0;
+
     public function __construct(private readonly Engine $engine)
     {
     }
 
     public function generate(): string
     {
+        $this->count++;
+
         return $this->engine->generate();
     }
+
+    public function getCount(): int
+    {
+        return $this->count;
+    }
 }
 
 final class TestXoshiro128PlusPlusEngine implements Engine