Server helper: Suppress error of UDPServer over max_bytes on Windows

This fix just suppresses the error message and output a info level log.

On Windows, `UDPSocket::recv` and `UDPSocket::recvfrom` can
raise `Errno::EMSGSIZE` error when the receiving data size is
larger than the `maxlen`, the value specified as the method's
argument.
(On non-Windows, this error doesn't happen, and we can receive data
that is cut off to the specified size.)

We need to consider this case since this causes an unexpected
error message as follows.

     unexpected error in processing UDP data
     error_class=Errno::EMSGSIZE
     error="A message sent on a datagram socket was larger than
     the internal message buffer or some other network limit,
     or the buffer used to receive a datagram into was smaller
     than the datagram itself. - recvfrom(2)"

On Windows, we can't cut data off the same way as Non-Windows.
So, the data is simply ignored on Windows.
It is not a big problem since cutting data off usually causes a
failure of parsing and the data is eventually ignored.

We want information about what data is ignored, but it is
difficult to know it. That is an issue for the future.

Ref: `winsock` specification

* https://learn.microsoft.com/en-us/windows/win32/api/winsock/nf-winsock-recvfrom
* https://learn.microsoft.com/en-us/windows/win32/winsock/windows-sockets-error-codes-2

Ref: Ruby implementation

* https://github.com/ruby/ruby/blob/e51014f9c05aa65cbf203442d37fef7c12390015/win32/win32.c#L3540-L3589
* https://github.com/ruby/ruby/blob/e51014f9c05aa65cbf203442d37fef7c12390015/win32/win32.c#L3730-L3782

Signed-off-by: Daijiro Fukuda <[email protected]>

lib/fluent/plugin_helper/server.rb

@@ -545,6 +545,10 @@ def on_readable_without_sock
               data = @sock.recv(@max_bytes, @flags)
             rescue Errno::EAGAIN, Errno::EWOULDBLOCK, Errno::EINTR, Errno::ECONNRESET, IOError, Errno::EBADF
               return
+            rescue Errno::EMSGSIZE
+              # Windows ONLY: This happens when the data size is larger than `@max_bytes`.
+              @log.info "A received data was ignored since it was too large."
+              return
             end
             @callback.call(data)
           rescue => e
@@ -558,6 +562,10 @@ def on_readable_with_sock
               data, addr = @sock.recvfrom(@max_bytes)
             rescue Errno::EAGAIN, Errno::EWOULDBLOCK, Errno::EINTR, Errno::ECONNRESET, IOError, Errno::EBADF
               return
+            rescue Errno::EMSGSIZE
+              # Windows ONLY: This happens when the data size is larger than `@max_bytes`.
+              @log.info "A received data was ignored since it was too large."
+              return
             end
             @callback.call(data, UDPCallbackSocket.new(@sock, addr, close_socket: @close_socket))
           rescue => e

test/plugin_helper/test_server.rb

@@ -29,6 +29,7 @@ class Dummy < Fluent::Plugin::TestBase
     ENV['SERVERENGINE_SOCKETMANAGER_PATH'] = @socket_manager_path.to_s
 
     @d = Dummy.new
+    @d.under_plugin_development = true
     @d.start
     @d.after_start
   end
@@ -794,6 +795,50 @@ class Dummy < Fluent::Plugin::TestBase
         end
       end
     end
+
+    sub_test_case 'over max_bytes' do
+      data("cut off on Non-Windows", { max_bytes: 32, records: ["a" * 40], expected: ["a" * 32] }, keep: true) unless Fluent.windows?
+      data("drop on Windows", { max_bytes: 32, records: ["a" * 40], expected: [] }, keep: true) if Fluent.windows?
+      test 'with sock' do |data|
+        max_bytes, records, expected = data.values
+
+        actual_records = []
+        @d.server_create_udp(:myserver, @port, max_bytes: max_bytes) do |data, sock|
+          actual_records << data
+        end
+
+        open_client(:udp, "127.0.0.1", @port) do |sock|
+          records.each do |record|
+            sock.send(record, 0)
+          end
+        end
+
+        waiting(10) { sleep 0.1 until actual_records.size >= expected.size }
+        sleep 1 if expected.size == 0 # To confirm no record recieved.
+
+        assert_equal expected, actual_records
+      end
+
+      test 'without sock' do |data|
+        max_bytes, records, expected = data.values
+
+        actual_records = []
+        @d.server_create_udp(:myserver, @port, max_bytes: max_bytes) do |data|
+          actual_records << data
+        end
+
+        open_client(:udp, "127.0.0.1", @port) do |sock|
+          records.each do |record|
+            sock.send(record, 0)
+          end
+        end
+
+        waiting(10) { sleep 0.1 until actual_records.size >= expected.size }
+        sleep 1 if expected.size == 0 # To confirm no record recieved.
+
+        assert_equal expected, actual_records
+      end
+    end
   end
 
   module CertUtil
@@ -1575,6 +1620,10 @@ def assert_certificate(cert, expected_extensions)
 
   def open_client(proto, addr, port)
     client = case proto
+             when :udp
+               c = UDPSocket.open
+               c.connect(addr, port)
+               c
              when :tcp
                TCPSocket.open(addr, port)
              when :tls