Hi Martin,
Thank you for the report and the analysis.
Indeed, 4-bytes communication is not efficient at all.
It will be fixed in git master in similar way as suggested. In adding. a
log warning will be added to notify about such situations.
I'll let you know when fix landed in the master branch.
--
Evgeny
On 24.03.2023 14:36, Martin Hejnfelt wrote:
Hi,
So I'm experiencing an issue with upgraded connections running
libmicrohttpd with a thread per connection. What seemingly happens is
that after receiving the headers, the connections read buffers are
shrunk, and the write buffer is afterwards increased to the full size
of the memory pool.
This is never "undone" meaning that *if* a connection is upgraded, the
memory pool is "empty" and the upgraded connection then starts using
the emergency buffer (e_buf) which by default has a size of 8 bytes, 4
is allocated to write, 4 to read.
This means suddenly all communication between the application and the
library and again between the library and the upgraded connection
socket is segmented into 4 byte reads/writes.
Now as such, this doesn't necessarily post a "big" problem, besides it
for sure isn't intended functionality, however if the connections are
https based, each 4 byte "chunk" is now encrypted, causing a massive
overhead (in my case each 4 byte chunk is then encrypted a 26 byte chunk).
The seemingly "obvious" choice I found for "easily" fixing this, at
least in my case, was to reenable the disabled function
"connection_shrink_write_buffer" and call if after the headers have
been sent, just before upgrading the connection. In the bottom is a
patch for reference that fixes is in one case at least. I have not
determined other possible code paths that might trigger the same problem.
Best regards,
Martin Hejnfelt
avoid-upgraded-connection-memory-starvation.patch
Index: libmicrohttpd-0.9.76/src/microhttpd/connection.c
===================================================================
--- libmicrohttpd-0.9.76.orig/src/microhttpd/connection.c
<http://libmicrohttpd-0.9.76.orig/src/microhttpd/connection.c>
+++ libmicrohttpd-0.9.76/src/microhttpd/connection.c
@@ -1612,8 +1612,6 @@ connection_maximize_write_buffer (struct
return c->write_buffer_size - c->write_buffer_append_offset;
}
-
-#if 0 /* disable unused function */
/**
* Shrink connection write buffer to the size of unsent data.
*
@@ -1654,10 +1652,6 @@ connection_shrink_write_buffer (struct M
c->write_buffer = new_buf;
}
-
-#endif /* unused function */
-
-
/**
* Switch connection from recv mode to send mode.
*
@@ -4700,6 +4694,8 @@ MHD_connection_handle_idle (struct MHD_C
#ifdef UPGRADE_SUPPORT
if (NULL != connection->response->upgrade_handler)
{
+ connection_shrink_write_buffer(connection);
+
connection->state = MHD_CONNECTION_UPGRADE;
/* This connection is "upgraded". Pass socket to application. */
if (MHD_NO ==