comfortable multibytes cutoff, ref guzzle#588

src/Message.php

@@ -67,6 +67,25 @@ public static function bodySummary(MessageInterface $message, int $truncateAt =
             return null;
         }
 
+        do {
+            $body->rewind();
+            $body->seek($truncateAt);
+            $char = $body->read(1);
+            $bin = str_pad(base_convert(strval(ord($char)), 10, 2), 8, '0', STR_PAD_LEFT);
+            // detect the character binary conversion
+            // stoped at 0x00000000 - 0x0000007F or the first byte of the utf8 code points
+            // @see https://man7.org/linux/man-pages/man7/utf-8.7.html
+            if (!$bin[0] || ($bin[0] && $bin[1])) {
+                $body->rewind();
+                break;
+            }
+            $truncateAt -= 1;
+        } while ($bin[0]);
+        // fast return while won't need reading the stream
+        if ($truncateAt === 0) {
+            return null;
+        }
+
         $body->rewind();
         $summary = $body->read($truncateAt);
         $body->rewind();

tests/MessageTest.php

@@ -271,6 +271,16 @@ public function testMessageBodySummaryWithSpecialUTF8Characters(): void
         self::assertSame('’é€௵ဪ‱', Psr7\Message::bodySummary($message));
     }
 
+    public function testMessageBodySummaryWithComfortableUTF8Characters(): void
+    {
+        $message = new Psr7\Response(200, [], '必填性规则校验失败，此字段为必填项');
+        $wellformed = '必填性规则校验失 (truncated...)';
+        // One chinese character is three bytes in the utf8 character range.
+        self::assertSame($wellformed, Psr7\Message::bodySummary($message, 24));
+        self::assertSame($wellformed, Psr7\Message::bodySummary($message, 25));
+        self::assertSame($wellformed, Psr7\Message::bodySummary($message, 26));
+    }
+
     public function testMessageBodySummaryWithSpecialUTF8CharactersAndLargeBody(): void
     {
         $message = new Psr7\Response(200, [], '🤦🏾‍♀️');