bugfix: read full message checksum on partial event stream reads

.changes/nextrelease/fix-eventstream-partial-checksum-read.json

@@ -0,0 +1,7 @@
+[
+    {
+        "type": "bugfix",
+        "category": "",
+        "description": "Fixed an issue in NonSeekableStreamDecodingEventStreamIterator where a message checksum returned across multiple partial reads caused a checksum mismatch."
+    }
+]

src/Api/Parser/NonSeekableStreamDecodingEventStreamIterator.php

@@ -50,7 +50,21 @@ protected function parseEvent(): array
             )
         );
         $calculatedCrc = hash_final($this->hashContext, true);
-        $messageCrc = $this->stream->read(4);
+        $messageCrc = '';
+        while (
+            strlen($messageCrc) < self::BYTES_TRAILING
+            && !$this->stream->eof()
+        ) {
+            $chunk = $this->stream->read(
+                self::BYTES_TRAILING - strlen($messageCrc)
+            );
+            if ($chunk === '') {
+                break;
+            }
+
+            $messageCrc .= $chunk;
+        }
+
         if ($calculatedCrc !== $messageCrc) {
             throw new ParserException('Message checksum mismatch.');
         }

tests/Api/Parser/NonSeekableStreamDecodingEventStreamIteratorTest.php

@@ -119,6 +119,40 @@ public function testReadAndHashBytesHandlesPartialReads(
         );
     }
 
+    #[DataProvider('readAndHashBytesHandlesPartialReadsProvider')]
+    public function testParseEventHandlesPartialChecksumReads(
+        string $eventName,
+        array $expected
+    ): void
+    {
+        $eventPath = self::EVENT_STREAMS_DIR . "events/$eventName";
+        $eventStream = Utils::streamFor(
+            base64_decode(
+                file_get_contents($eventPath)
+            )
+        );
+        $partialReadStream = $this->createMock(StreamInterface::class);
+        $partialReadStream->method('isSeekable')->willReturn(false);
+        $partialReadStream->method('isReadable')->willReturn(true);
+        $partialReadStream->method('read')
+            ->willReturnCallback(function ($length) use ($eventStream) {
+                return $eventStream->read(min($length, 1));
+            });
+        $iterator = new NonSeekableStreamDecodingEventStreamIterator(
+            $partialReadStream
+        );
+        $iterator->rewind();
+        $event = $iterator->current();
+        $this->assertEquals(
+            $expected['headers'],
+            $event['headers']
+        );
+        $this->assertEquals(
+            $expected['payload'],
+            json_decode($event['payload']->getContents(), true)
+        );
+    }
+
     public static function readAndHashBytesHandlesPartialReadsProvider(): \Generator
     {
         $cases = json_decode(