Fix echo server to use advance-then-check pattern

The session loop checked ec before using the read bytes, silently
dropping partial data delivered alongside an error (e.g. EOF).
Apply the canonical advance-then-check pattern from the ReadStream
contract: always write back the received bytes before inspecting ec.

Author: sgerbino

doc/modules/ROOT/pages/3.tutorials/3a.echo-server.adoc

@@ -60,14 +60,13 @@ class echo_server : public corosio::tcp_server
     {
         corosio::io_context& ctx_;
         corosio::tcp_socket sock_;
-        std::string buf_;
+        char buf_[4096];
 
     public:
         explicit worker(corosio::io_context& ctx)
             : ctx_(ctx)
             , sock_(ctx)
         {
-            buf_.reserve(4096);
         }
 
         corosio::tcp_socket& socket() override
@@ -101,22 +100,13 @@ capy::task<> echo_server::worker::do_session()
 {
     for (;;)
     {
-        buf_.resize(4096);
-
-        // Read some data
         auto [ec, n] = co_await sock_.read_some(
-            capy::mutable_buffer(buf_.data(), buf_.size()));
-
-        if (ec || n == 0)
-            break;
-
-        buf_.resize(n);
+            capy::mutable_buffer(buf_, sizeof buf_));
 
-        // Echo it back
         auto [wec, wn] = co_await corosio::write(
-            sock_, capy::const_buffer(buf_.data(), buf_.size()));
+            sock_, capy::const_buffer(buf_, n));
 
-        if (wec)
+        if (wec || ec)
             break;
     }
 
@@ -127,7 +117,8 @@ capy::task<> echo_server::worker::do_session()
 Notice:
 
 * We reuse the worker's buffer across reads
-* `read_some()` returns when _any_ data arrives
+* `read_some()` returns when _any_ data arrives — it may deliver bytes alongside an error
+* We always write before checking the error (advance-then-check); writing zero bytes is a no-op
 * `corosio::write()` writes _all_ data (it's a composed operation)
 * When the coroutine ends, the launcher returns the worker to the pool
 
@@ -218,12 +209,14 @@ For echo servers, we want complete message delivery.
 
 === Why Not Use Exceptions?
 
-The session loop needs to handle EOF gracefully. Using structured bindings:
+The session loop needs to handle EOF gracefully. Using structured bindings
+with advance-then-check, we always act on `n` before inspecting `ec`:
 
 [source,cpp]
 ----
 auto [ec, n] = co_await sock.read_some(buf);
-if (ec || n == 0)
+auto [wec, wn] = co_await corosio::write(sock, const_buffer(buf.data(), n));
+if (wec || ec)
     break;  // Normal termination path
 ----
 

example/echo-server/echo_server.cpp

@@ -14,7 +14,6 @@
 
 #include <cstdlib>
 #include <iostream>
-#include <string>
 
 namespace corosio = boost::corosio;
 namespace capy = boost::capy;
@@ -23,14 +22,13 @@ class echo_worker : public corosio::tcp_server::worker_base
 {
     corosio::io_context& ctx_;
     corosio::tcp_socket sock_;
-    std::string buf_;
+    char buf_[4096];
 
 public:
     explicit echo_worker(corosio::io_context& ctx)
         : ctx_(ctx)
         , sock_(ctx)
     {
-        buf_.reserve(4096);
     }
 
     corosio::tcp_socket& socket() override
@@ -47,22 +45,13 @@ class echo_worker : public corosio::tcp_server::worker_base
     {
         for (;;)
         {
-            buf_.resize(4096);
-
-            // Read some data
             auto [ec, n] = co_await sock_.read_some(
-                capy::mutable_buffer(buf_.data(), buf_.size()));
-
-            if (ec)
-                break;
-
-            buf_.resize(n);
+                capy::mutable_buffer(buf_, sizeof buf_));
 
-            // Echo it back
             auto [wec, wn] = co_await capy::write(
-                sock_, capy::const_buffer(buf_.data(), buf_.size()));
+                sock_, capy::const_buffer(buf_, n));
 
-            if (wec)
+            if (wec || ec)
                 break;
         }
 

test/unit/tcp_server.cpp

@@ -45,8 +45,7 @@ class test_worker : public tcp_server::worker_base
                 char buf[64];
                 auto [ec, n] = co_await sock->read_some(
                     capy::mutable_buffer(buf, sizeof(buf)));
-                if (!ec)
-                    (void)co_await sock->write_some(capy::const_buffer(buf, n));
+                (void)co_await sock->write_some(capy::const_buffer(buf, n));
                 sock->close();
             }(&sock_));
     }