Bug#37385923 routertest_integration_routing_sharing select_overlong times out

$ routertest_integration_routing_sharing
--gtest_filter=Spec/ShareConnectionTest.select_overlong/ssl_modes_PREFERRED__DISABLED_socket

takes over 150s to complete if Router is built with
-DCMAKE_BUILD_TYPE=Debug.

With RelWithDebInfo and Release the test takes 0.2s.

Background
==========

Building with CMAKE_BUILD_TYPE=Debug disables optimizations which leads
to std::vector.resize() trying to initialize every byte before each read
from an encrypted socket.

For "select_overlong", 16Mbyte of memory are allocated, passed to
SSL_read() which only uses 16kByte, but all 16M were tried to be
initialized.

Change
======

- before SSL_read() only resize() the std::vector by 16k, the max size
  SSL_read() will return.

That reduces the test runtime from 150sec to 2sec.

Change-Id: I5b60712ca01ec84fcc33b23a7ab82864af7a6e65

Author: weigon

router/src/routing/src/channel.cc

@@ -235,10 +235,18 @@ stdx::expected<size_t, std::error_code> Channel::read_to_plain(size_t sz) {
   size_t bytes_read{};
   // decrypt from src-ssl into the ssl-plain-buf
   while (sz > 0) {
+    // read at most a SSL frame (16k) to avoid excessive resizes.
+    //
+    //      | Debug    | RelWithDebInfo
+    //   1k |  2100ms  | 256ms
+    //  16k |  1966ms  | 219ms
+    // 256k | *6509ms* | 189ms
+    const size_t to_read = std::min(sz, 16UL * 1024);
+
     auto dyn_buf = net::dynamic_buffer(plain_buf);
 
     // append to the plain buffer
-    const auto read_res = read(dyn_buf, sz);
+    const auto read_res = read(dyn_buf, to_read);
 
     // sync the plain-view as the read() may have resized it.
     view_sync_plain();
@@ -248,7 +256,6 @@ stdx::expected<size_t, std::error_code> Channel::read_to_plain(size_t sz) {
 
       sz -= transferred;
       bytes_read += transferred;
-
     } else {
       // read from client failed.