From 3e9a9979f016496a0804255aa3ce02e5297aa1b3 Mon Sep 17 00:00:00 2001
From: Nikita Massalitin <17572243+lohmag@users.noreply.github.com>
Date: Sun, 25 Oct 2026 23:16:06 +0200
Subject: [PATCH 1/2] mysqlnd: Do not compress packets smaller than 512 bytes

---
 .../mysqli_compression_packet_sizes.phpt      | 39 +++++++++++++++++++
 ext/mysqlnd/mysqlnd_protocol_frame_codec.c    |  3 +-
 ext/mysqlnd/mysqlnd_wireprotocol.h            |  2 +
 3 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 ext/mysqli/tests/mysqli_compression_packet_sizes.phpt

diff --git a/ext/mysqli/tests/mysqli_compression_packet_sizes.phpt b/ext/mysqli/tests/mysqli_compression_packet_sizes.phpt
new file mode 100644
index 000000000000..51a748cfe313
--- /dev/null
+++ b/ext/mysqli/tests/mysqli_compression_packet_sizes.phpt
@@ -0,0 +1,39 @@
+--TEST--
+Compressed protocol: packets on both sides of the compression threshold
+--EXTENSIONS--
+mysqli
+--SKIPIF--
+<?php
+require_once 'skipifconnectfailure.inc';
+?>
+--FILE--
+<?php
+require_once 'connect.inc';
+
+$link = mysqli_init();
+if (!my_mysqli_real_connect($link, $host, $user, $passwd, $db, $port, $socket, MYSQLI_CLIENT_COMPRESS)) {
+    printf("[001] Cannot connect: [%d] %s\n", mysqli_connect_errno(), mysqli_connect_error());
+}
+var_dump($link->query("SHOW SESSION STATUS LIKE 'Compression'")->fetch_row()[1]);
+
+foreach ([1, 230, 235, 236, 237, 238, 240, 600, 4000, 100000, 3000000] as $length) {
+    $value = substr(str_repeat('compressed protocol packet ', intdiv($length, 27) + 1), 0, $length);
+    $row = $link->query("SELECT '$value' AS v, LENGTH('$value') AS l")->fetch_assoc();
+    if ($row['v'] !== $value || (int) $row['l'] !== $length) {
+        printf("[002] Round trip failed for %d bytes\n", $length);
+    }
+}
+
+for ($i = 0; $i < 200; $i++) {
+    $value = bin2hex(random_bytes(1 + $i * 37));
+    if ($link->query("SELECT '$value' AS v")->fetch_assoc()['v'] !== $value) {
+        printf("[003] Round trip failed for %d bytes\n", strlen($value));
+    }
+}
+
+$link->close();
+print "done!";
+?>
+--EXPECT--
+string(2) "ON"
+done!
diff --git a/ext/mysqlnd/mysqlnd_protocol_frame_codec.c b/ext/mysqlnd/mysqlnd_protocol_frame_codec.c
index 70a81befdd71..2663a101ed0a 100644
--- a/ext/mysqlnd/mysqlnd_protocol_frame_codec.c
+++ b/ext/mysqlnd/mysqlnd_protocol_frame_codec.c
@@ -55,7 +55,8 @@ static ssize_t write_compressed_packet(
 	/* here we need to compress the data and then write it, first comes the compressed header */
 	size_t tmp_complen = to_be_sent;
 	size_t payload_size;
-	if (PASS == pfc->data->m.encode((compress_buf + COMPRESSED_HEADER_SIZE + MYSQLND_HEADER_SIZE), &tmp_complen,
+	if (to_be_sent >= MYSQLND_MIN_COMPRESS_LENGTH
+		&& PASS == pfc->data->m.encode((compress_buf + COMPRESSED_HEADER_SIZE + MYSQLND_HEADER_SIZE), &tmp_complen,
 							   uncompressed_payload, to_be_sent))
 	{
 		int3store(compress_buf + MYSQLND_HEADER_SIZE, to_be_sent);
diff --git a/ext/mysqlnd/mysqlnd_wireprotocol.h b/ext/mysqlnd/mysqlnd_wireprotocol.h
index ad452ec4f928..134c389ced04 100644
--- a/ext/mysqlnd/mysqlnd_wireprotocol.h
+++ b/ext/mysqlnd/mysqlnd_wireprotocol.h
@@ -18,6 +18,8 @@
 
 #define MYSQLND_HEADER_SIZE 4
 #define COMPRESSED_HEADER_SIZE 3
+/* Packets smaller than this are sent uncompressed: they barely shrink. */
+#define MYSQLND_MIN_COMPRESS_LENGTH 512
 
 #define MYSQLND_NULL_LENGTH	(zend_ulong) ~0
 

From 3d3b1c94765a9d3b59cad35fae18eb3a32624967 Mon Sep 17 00:00:00 2001
From: Nikita Massalitin <17572243+lohmag@users.noreply.github.com>
Date: Mon, 26 Oct 2026 15:53:05 +0200
Subject: [PATCH 2/2] Use incompressible data in the compression packet size
 test

---
 ext/mysqli/tests/mysqli_compression_packet_sizes.phpt | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/ext/mysqli/tests/mysqli_compression_packet_sizes.phpt b/ext/mysqli/tests/mysqli_compression_packet_sizes.phpt
index 51a748cfe313..c9b11216dd26 100644
--- a/ext/mysqli/tests/mysqli_compression_packet_sizes.phpt
+++ b/ext/mysqli/tests/mysqli_compression_packet_sizes.phpt
@@ -24,12 +24,19 @@ foreach ([1, 230, 235, 236, 237, 238, 240, 600, 4000, 100000, 3000000] as $lengt
     }
 }
 
+$stmt = $link->prepare("SELECT HEX(?)");
 for ($i = 0; $i < 200; $i++) {
-    $value = bin2hex(random_bytes(1 + $i * 37));
-    if ($link->query("SELECT '$value' AS v")->fetch_assoc()['v'] !== $value) {
+    $value = random_bytes(1 + $i * 37);
+    $stmt->bind_param('s', $value);
+    $stmt->execute();
+    $stmt->bind_result($hex);
+    $stmt->fetch();
+    $stmt->free_result();
+    if ($hex !== strtoupper(bin2hex($value))) {
         printf("[003] Round trip failed for %d bytes\n", strlen($value));
     }
 }
+$stmt->close();
 
 $link->close();
 print "done!";