Fix data race in http server examples

When using `beast::tcp_stream`, the user must make sure that async operations
are initiated from within the strand associated with the stream.

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

example/http/server/async-ssl/http_server_async_ssl.cpp

@@ -19,6 +19,7 @@
 #include <boost/beast/http.hpp>
 #include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
+#include <boost/asio/dispatch.hpp>
 #include <boost/asio/strand.hpp>
 #include <boost/config.hpp>
 #include <algorithm>
@@ -298,9 +299,24 @@ public:
     // Start the asynchronous operation
     void
     run()
+    {
+        // We need to be executing within a strand to perform async operations
+        // on the I/O objects in this session. Although not strictly necessary
+        // for single-threaded contexts, this example code is written to be
+        // thread-safe by default.
+        net::dispatch(
+            stream_.get_executor(),
+            beast::bind_front_handler(
+                &session::on_run,
+                shared_from_this()));
+    }
+
+    void
+    on_run()
     {
         // Set the timeout.
-        beast::get_lowest_layer(stream_).expires_after(std::chrono::seconds(30));
+        beast::get_lowest_layer(stream_).expires_after(
+            std::chrono::seconds(30));
 
         // Perform the SSL handshake
         stream_.async_handshake(

example/http/server/async/http_server_async.cpp

@@ -16,6 +16,7 @@
 #include <boost/beast/core.hpp>
 #include <boost/beast/http.hpp>
 #include <boost/beast/version.hpp>
+#include <boost/asio/dispatch.hpp>
 #include <boost/asio/strand.hpp>
 #include <boost/config.hpp>
 #include <algorithm>
@@ -273,7 +274,14 @@ public:
     void
     run()
     {
-        do_read();
+        // We need to be executing within a strand to perform async operations
+        // on the I/O objects in this session. Although not strictly necessary
+        // for single-threaded contexts, this example code is written to be
+        // thread-safe by default.
+        net::dispatch(stream_.get_executor(),
+                      beast::bind_front_handler(
+                          &session::do_read,
+                          shared_from_this()));
     }
 
     void

example/http/server/flex/http_server_flex.cpp

@@ -19,6 +19,7 @@
 #include <boost/beast/http.hpp>
 #include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
+#include <boost/asio/dispatch.hpp>
 #include <boost/asio/strand.hpp>
 #include <boost/config.hpp>
 #include <algorithm>
@@ -396,7 +397,14 @@ public:
     void
     run()
     {
-        do_read();
+        // We need to be executing within a strand to perform async operations
+        // on the I/O objects in this session. Although not strictly necessary
+        // for single-threaded contexts, this example code is written to be
+        // thread-safe by default.
+        net::dispatch(stream_.get_executor(),
+                      beast::bind_front_handler(
+                          &session::do_read,
+                          shared_from_this()));
     }
 
     void
@@ -442,17 +450,23 @@ public:
     void
     run()
     {
+        auto self = shared_from_this();
+        // We need to be executing within a strand to perform async operations
+        // on the I/O objects in this session.
+        net::dispatch(stream_.get_executor(), [self]() {
             // Set the timeout.
-        beast::get_lowest_layer(stream_).expires_after(std::chrono::seconds(30));
+            beast::get_lowest_layer(self->stream_).expires_after(
+                std::chrono::seconds(30));
 
             // Perform the SSL handshake
             // Note, this is the buffered version of the handshake.
-        stream_.async_handshake(
+            self->stream_.async_handshake(
                 ssl::stream_base::server,
-            buffer_.data(),
+                self->buffer_.data(),
                 beast::bind_front_handler(
                     &ssl_session::on_handshake,
-                shared_from_this()));
+                    self));
+        });
     }
 
     void

example/http/server/stackless-ssl/http_server_stackless_ssl.cpp

@@ -20,6 +20,7 @@
 #include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
 #include <boost/asio/coroutine.hpp>
+#include <boost/asio/dispatch.hpp>
 #include <boost/asio/strand.hpp>
 #include <boost/config.hpp>
 #include <algorithm>
@@ -304,7 +305,16 @@ public:
     void
     run()
     {
-        loop({}, 0, false);
+        // We need to be executing within a strand to perform async operations
+        // on the I/O objects in this session.Although not strictly necessary
+        // for single-threaded contexts, this example code is written to be
+        // thread-safe by default.
+        net::dispatch(stream_.get_executor(),
+                      beast::bind_front_handler(&session::loop,
+                                                shared_from_this(),
+                                                beast::error_code{},
+                                                0,
+                                                false));
     }
 
     #include <boost/asio/yield.hpp>

example/http/server/stackless/http_server_stackless.cpp

@@ -17,6 +17,7 @@
 #include <boost/beast/http.hpp>
 #include <boost/beast/version.hpp>
 #include <boost/asio/coroutine.hpp>
+#include <boost/asio/dispatch.hpp>
 #include <boost/asio/strand.hpp>
 #include <boost/config.hpp>
 #include <algorithm>
@@ -278,7 +279,16 @@ public:
     void
     run()
     {
-        loop(false, {}, 0);
+        // We need to be executing within a strand to perform async operations
+        // on the I/O objects in this session. Although not strictly necessary
+        // for single-threaded contexts, this example code is written to be
+        // thread-safe by default.
+        net::dispatch(stream_.get_executor(),
+                      beast::bind_front_handler(&session::loop,
+                                                shared_from_this(),
+                                                false,
+                                                beast::error_code{},
+                                                0));
     }
 
     #include <boost/asio/yield.hpp>