implemented qdb_memfd_fill_fd function to enhance file descriptor writing using FFI

lib/read_db_cache.php

@@ -113,6 +113,7 @@ function qdb_linux_libc_ffi() {
     try {
         $ffi = FFI::cdef(
             'int memfd_create(const char *name, unsigned int flags);
+             long write(int fd, const void *buf, unsigned long count);
              int close(int fd);',
             'libc.so.6'
         );
@@ -137,6 +138,38 @@ function qdb_close_fd(int $fd): void {
     }
 }
 
+/**
+ * Fill a raw fd (e.g. memfd). /proc/self/fd/N writes are blocked on some hosts;
+ * fall back to libc write() via FFI.
+ */
+function qdb_memfd_fill_fd(int $fd, string $bytes): bool {
+    $len = strlen($bytes);
+    if ($len === 0) {
+        return true;
+    }
+    $written = @file_put_contents('/proc/self/fd/' . $fd, $bytes);
+    if ($written === $len) {
+        return true;
+    }
+    $ffi = qdb_linux_libc_ffi();
+    if ($ffi === null) {
+        return false;
+    }
+    $offset = 0;
+    while ($offset < $len) {
+        $chunkLen = min(1024 * 1024, $len - $offset);
+        $chunk = substr($bytes, $offset, $chunkLen);
+        $buf = FFI::new('char[' . $chunkLen . ']');
+        FFI::memcpy($buf, $chunk, $chunkLen);
+        $n = (int)$ffi->write($fd, $buf, $chunkLen);
+        if ($n <= 0) {
+            return false;
+        }
+        $offset += $n;
+    }
+    return true;
+}
+
 /**
  * @return array{0: int, 1: string}|null [fd, /proc/self/fd/N]
  */
@@ -151,8 +184,7 @@ function qdb_memfd_write(string $bytes): ?array {
         if ($fd < 0) {
             return null;
         }
-        $written = @file_put_contents('/proc/self/fd/' . $fd, $bytes);
-        if ($written === false || $written !== strlen($bytes)) {
+        if (!qdb_memfd_fill_fd($fd, $bytes)) {
             qdb_close_fd($fd);
             return null;
         }