From 0b6b57f9acaa2ec648bf582ff67851331f8e6eef Mon Sep 17 00:00:00 2001
From: Karen Arutyunov <karen@codesynthesis.com>
Date: Mon, 14 Mar 2016 14:38:45 +0300
Subject: Use serializable transaction isolation level

---
 web/apache/log         |   6 +-
 web/apache/request     | 121 +++++++++++++++++++++++++---------------
 web/apache/request.cxx | 147 ++++++++++++++++++++++++++++++++++++++-----------
 web/apache/request.ixx |  43 ++++++++-------
 web/apache/service     |  13 +++--
 web/apache/service.cxx |  42 ++++++++------
 web/apache/service.txx |  44 +++++++++------
 web/apache/stream      |  92 +++++++++++++------------------
 web/module             |  46 ++++++++++------
 9 files changed, 345 insertions(+), 209 deletions(-)

(limited to 'web')

diff --git a/web/apache/log b/web/apache/log
index 7318e32..291f8de 100644
--- a/web/apache/log
+++ b/web/apache/log
@@ -5,7 +5,7 @@
 #ifndef WEB_APACHE_LOG
 #define WEB_APACHE_LOG
 
-#include <httpd.h>       // request_rec
+#include <httpd.h>       // request_rec, server_rec
 #include <http_log.h>
 #include <http_config.h> // module
 
@@ -44,7 +44,7 @@ namespace web
              const char* msg) const noexcept
       {
         if (file && *file)
-          file = nullptr; // skip file/line placeholder from log line.
+          file = nullptr; // Skip file/line placeholder from log line.
 
         level = std::min (level, APLOG_TRACE8);
 
@@ -59,7 +59,7 @@ namespace web
                         func,
                         msg);
         else
-          // skip function name placeholder from log line
+          // Skip function name placeholder from log line.
           //
           ap_log_error (file,
                         line,
diff --git a/web/apache/request b/web/apache/request
index ab69dff..9540561 100644
--- a/web/apache/request
+++ b/web/apache/request
@@ -5,23 +5,14 @@
 #ifndef WEB_APACHE_REQUEST
 #define WEB_APACHE_REQUEST
 
-#include <apr_strings.h>
+#include <httpd.h> // request_rec, HTTP_*, OK, M_POST
 
-#include <httpd.h>
-#include <http_core.h>
-#include <util_script.h>
-
-#include <ios>
 #include <chrono>
 #include <memory>    // unique_ptr
 #include <string>
-#include <cassert>
 #include <istream>
 #include <ostream>
-#include <utility>   // move()
 #include <streambuf>
-#include <stdexcept>
-#include <exception>
 
 #include <web/module>
 #include <web/apache/stream>
@@ -30,23 +21,63 @@ namespace web
 {
   namespace apache
   {
+    // The state of the request processing, reflecting an interaction with
+    // Apache API (like reading/writing content function calls), with no
+    // buffering taken into account. Any state different from the initial
+    // suppose that some irrevocable interaction with Apache API have
+    // happened, so request processing should be either completed, or
+    // reported as failed. State values are ordered in a sense that the
+    // higher value reflects the more advanced stage of processing, so the
+    // request current state value may not decrease.
+    //
+    enum class request_state
+    {
+      // Denotes the initial stage of the request handling. At this stage
+      // the request line and headers are already parsed by Apache.
+      //
+      initial,
+
+      // Reading the request content.
+      //
+      reading,
+
+      // Adding the response headers (cookies in particular).
+      //
+      headers,
+
+      // Writing the response content.
+      //
+      writing
+    };
+
     class request: public web::request,
                    public web::response,
-                   public write_state
+                   public stream_state
     {
       friend class service;
 
-      request (request_rec* rec) noexcept: rec_ (rec) {rec_->status = HTTP_OK;}
+      request (request_rec* rec) noexcept
+        : rec_ (rec)
+      {
+        rec_->status = HTTP_OK;
+      }
+
+      request_state
+      state () const noexcept {return state_;}
 
-      // Flush of buffered content.
+      // Flush the buffered response content if present. The returned value
+      // should be passed to Apache API on request handler exit.
       //
       int
       flush ();
 
-      // Return true if content have been sent to the client, false otherwise.
+      // Prepare for the request re-processing if possible (no unbuffered
+      // read/write operations have been done). Throw sequence_error
+      // otherwise. In particular, the preparation can include the response
+      // content buffer cleanup, the request content buffer rewind.
       //
-      bool
-      get_write_state () const noexcept {return write_state_;}
+      void
+      rewind ();
 
       // Get request path.
       //
@@ -56,7 +87,7 @@ namespace web
       // Get request body data stream.
       //
       virtual std::istream&
-      content ();
+      content (bool buffer = false);
 
       // Get request parameters.
       //
@@ -70,7 +101,8 @@ namespace web
 
       // Get response status code.
       //
-      status_code status () const noexcept {return rec_->status;}
+      status_code
+      status () const noexcept {return rec_->status;}
 
       // Set response status code.
       //
@@ -89,10 +121,11 @@ namespace web
       virtual void
       cookie (const char* name,
               const char* value,
-              const std::chrono::seconds* max_age = 0,
-              const char* path = 0,
-              const char* domain = 0,
-              bool secure = false);
+              const std::chrono::seconds* max_age = nullptr,
+              const char* path = nullptr,
+              const char* domain = nullptr,
+              bool secure = false,
+              bool buffer = true);
 
     private:
       // Get application/x-www-form-urlencoded form data.
@@ -103,37 +136,35 @@ namespace web
       void
       parse_parameters (const char* args);
 
+      // Advance the request processing state. Noop if new state is equal to
+      // the current one. Throw sequence_error if the new state is less then
+      // the current one. Can throw invalid_request if HTTP request is
+      // malformed.
+      //
+      void
+      state (request_state);
+
+      // stream_state members implementation.
+      //
       virtual void
-      set_write_state ()
-      {
-        if (!write_state_)
-        {
-          // Preparing to write a response read and discard request
-          // body if any.
-          //
-          int r (ap_discard_request_body (rec_));
-
-          if (r != OK)
-          {
-            throw invalid_request (r);
-          }
-
-          write_state_ = true;
-        }
-      }
+      set_read_state () {state (request_state::reading);}
+
+      virtual void
+      set_write_state () {state (request_state::writing);}
 
     private:
       request_rec* rec_;
-      bool buffer_ {true};
-      bool write_state_ {false};
-      std::unique_ptr<std::streambuf> out_buf_;
-      std::unique_ptr<std::ostream> out_;
-      std::unique_ptr<std::streambuf> in_buf_;
-      std::unique_ptr<std::istream> in_;
+      request_state state_ = request_state::initial;
+
       path_type path_;
       std::unique_ptr<name_values> parameters_;
       std::unique_ptr<name_values> cookies_;
       std::unique_ptr<std::string> form_data_;
+      std::unique_ptr<std::streambuf> in_buf_;
+      std::unique_ptr<std::istream> in_;
+
+      std::unique_ptr<std::streambuf> out_buf_;
+      std::unique_ptr<std::ostream> out_;
     };
   }
 }
diff --git a/web/apache/request.cxx b/web/apache/request.cxx
index 2e03190..b0b4d61 100644
--- a/web/apache/request.cxx
+++ b/web/apache/request.cxx
@@ -4,22 +4,30 @@
 
 #include <web/apache/request>
 
-#include <apr_tables.h>
+#include <apr_tables.h>  // apr_table_*, apr_array_header_t
+#include <apr_strings.h> // apr_pstrdup()
+
+#include <httpd.h>         // request_rec, HTTP_*, OK
+#include <http_protocol.h> // ap_*()
 
 #include <strings.h> // strcasecmp()
 
-#include <ios>
-#include <ctime>
+#include <ctime>     // strftime(), time_t
 #include <chrono>
 #include <memory>    // unique_ptr
+#include <string>
+#include <cassert>
 #include <sstream>
 #include <ostream>
 #include <istream>
-#include <cstring>
+#include <cstring>   // str*(), size_t
 #include <utility>   // move()
-#include <stdexcept>
+#include <stdexcept> // invalid_argument
+#include <exception> // current_exception()
 #include <streambuf>
 
+#include <butl/optional>
+
 #include <web/mime-url-encoding>
 
 using namespace std;
@@ -28,16 +36,92 @@ namespace web
 {
   namespace apache
   {
+    void request::
+    state (request_state s)
+    {
+      assert (s != request_state::initial);
+
+      if (s == state_)
+        return; // Noop.
+
+      if (s < state_)
+      {
+        // Can't "unwind" irrevocable interaction with Apache API.
+        //
+        static const char* names[] = {
+          "initial", "reading", "headers", "writing"};
+
+        string str ("web::apache::request::set_state: ");
+        str += names[static_cast<size_t> (state_)];
+        str += " to ";
+        str += names[static_cast<size_t> (s)];
+
+        throw sequence_error (move (str));
+      }
+
+      if (s == request_state::reading)
+      {
+        // Prepare request content for reading.
+        //
+        int r (ap_setup_client_block (rec_, REQUEST_CHUNKED_DECHUNK));
+
+        if (r != OK)
+          throw invalid_request (r);
+      }
+      else if (s > request_state::reading && state_ <= request_state::reading)
+      {
+        // Read request content if any, discard whatever is received.
+        //
+        int r (ap_discard_request_body (rec_));
+
+        if (r != OK)
+          throw invalid_request (r);
+      }
+
+      state_ = s;
+    }
+
+    void request::
+    rewind ()
+    {
+      // @@ Request content buffering, and response cookies buffering are not
+      //    supported yet. When done will be possible to rewind in broader
+      //    range of cases.
+      //
+
+      if (state_ == request_state::initial ||
+
+          // Form data have been read. Lucky case, can rewind.
+          //
+          (state_ == request_state::reading &&
+           dynamic_cast<stringbuf*> (in_buf_.get ()) != nullptr))
+      {
+        out_.reset ();
+        out_buf_.reset ();
+
+        rec_->status = HTTP_OK;
+
+        ap_set_content_type (rec_, nullptr);
+
+        if (in_)
+          in_->seekg (0);
+      }
+      else
+        throw sequence_error ("web::apache::request::rewind");
+    }
+
     istream& request::
-    content ()
+    content (bool buffer)
     {
+      assert (!buffer); // Request content buffering is not implemented yet.
+
       if (!in_)
       {
         unique_ptr<streambuf> in_buf (new istreambuf (rec_, *this));
 
         in_.reset (new istream (in_buf.get ()));
         in_buf_ = move (in_buf);
-        in_->exceptions (ios::failbit | ios::badbit);
+        in_->exceptions (istream::failbit | istream::badbit);
 
         // Save form data now otherwise will not be available to do later
         // when data already read from stream.
@@ -135,17 +219,29 @@ namespace web
     ostream& request::
     content (status_code status, const string& type, bool buffer)
     {
-      if (out_ && status == rec_->status && buffer == buffer_ &&
+      if (out_ &&
+
+          // Same status code.
+          //
+          status == rec_->status &&
+
+          // Same buffering flag.
+          //
+          buffer ==
+          (dynamic_cast<stringbuf*> (out_buf_.get ()) != nullptr) &&
+
+          // Same content type.
+          //
           strcasecmp (rec_->content_type ? rec_->content_type : "",
                       type.c_str ()) == 0)
       {
+        // No change, return the existing stream.
+        //
         return *out_;
       }
 
-      if (get_write_state ())
-      {
-        throw sequence_error ("::web::apache::request::content");
-      }
+      if (state_ >= request_state::writing)
+        throw sequence_error ("web::apache::request::content");
 
       if (!buffer)
         // Request body will be discarded prior first byte of content is
@@ -161,9 +257,8 @@ namespace web
 
       out_.reset (new ostream (out_buf.get ()));
       out_buf_ = move (out_buf);
-      out_->exceptions (ios::eofbit | ios::failbit | ios::badbit);
+      out_->exceptions (ostream::eofbit | ostream::failbit | ostream::badbit);
 
-      buffer_ = buffer;
       rec_->status = status;
 
       ap_set_content_type (
@@ -182,13 +277,10 @@ namespace web
         // where no sense to throw but still need to signal apache a
         // proper status code.
         //
-        if (get_write_state () && !current_exception ())
-        {
-          throw sequence_error ("::web::apache::request::status");
-        }
+        if (state_ >= request_state::writing && !current_exception ())
+          throw sequence_error ("web::apache::request::status");
 
         rec_->status = status;
-        buffer_ = true;
         out_.reset ();
         out_buf_.reset ();
         ap_set_content_type (rec_, nullptr);
@@ -201,14 +293,10 @@ namespace web
             const chrono::seconds* max_age,
             const char* path,
             const char* domain,
-            bool secure)
+            bool secure,
+            bool buffer)
     {
-      if (get_write_state ())
-      {
-        // Too late to send cookie if content is already written.
-        //
-        throw sequence_error ("::web::apache::request::cookie");
-      }
+      assert (!buffer); // Cookie buffering is not implemented yet.
 
       ostringstream s;
       mime_url_encode (name, s);
@@ -230,20 +318,15 @@ namespace web
       }
 
       if (path)
-      {
         s << ";Path=" << path;
-      }
 
       if (domain)
-      {
         s << ";Domain=" << domain;
-      }
 
       if (secure)
-      {
         s << ";Secure";
-      }
 
+      state (request_state::headers);
       apr_table_add (rec_->err_headers_out, "Set-Cookie", s.str ().c_str ());
     }
 
diff --git a/web/apache/request.ixx b/web/apache/request.ixx
index 8a3b32b..821aaba 100644
--- a/web/apache/request.ixx
+++ b/web/apache/request.ixx
@@ -4,10 +4,12 @@
 
 #include <strings.h> // strncasecmp()
 
-#include <iomanip>
+#include <apr_tables.h>  // apr_table_*
+
+#include <http_protocol.h> // ap_*()
+
 #include <sstream>
-#include <cstring>
-#include <cstdlib>
+#include <utility> // move()
 
 namespace web
 {
@@ -16,35 +18,34 @@ namespace web
     inline int request::
     flush ()
     {
-      if (buffer_ && out_buf_)
+      if (std::stringbuf* b = dynamic_cast<std::stringbuf*> (out_buf_.get ()))
       {
-        auto b (dynamic_cast<std::stringbuf*> (out_buf_.get ()));
-        assert (b);
-
+        // Response content is buffered.
+        //
         std::string s (b->str ());
 
         if (!s.empty ())
         {
-          // Before writing response read and discard request body if any.
-          //
-          int r (ap_discard_request_body (rec_));
-
-          if (r == OK)
+          try
           {
-            set_write_state ();
+            state (request_state::writing);
 
             if (ap_rwrite (s.c_str (), s.length (), rec_) < 0)
               rec_->status = HTTP_REQUEST_TIME_OUT;
           }
-          else
-            rec_->status = r;
+          catch (const invalid_request& e)
+          {
+            rec_->status = e.status;
+          }
         }
 
         out_.reset ();
         out_buf_.reset ();
       }
 
-      return rec_->status == HTTP_OK || get_write_state () ? OK : rec_->status;
+      return rec_->status == HTTP_OK || state_ >= request_state::writing
+        ? OK
+        : rec_->status;
     }
 
     inline const std::string& request::
@@ -64,19 +65,21 @@ namespace web
             std::istream& istr (content ());
 
             // Do not throw when eofbit is set (end of stream reached), and
-            // when failbit is set (getline() failed to extract any character).
+            // when failbit is set (getline() failed to extract any
+            // character).
             //
-            istr.exceptions (std::ios::badbit);
+            istr.exceptions (std::istream::badbit);
             std::getline (istr, *form_data_);
 
-            // Make this data the content of the input stream.
+            // Make this data the content of the input stream, so it's
+            // available for the application as well.
             //
             std::unique_ptr<std::streambuf> in_buf (
               new std::stringbuf (*form_data_));
 
             in_.reset (new std::istream (in_buf.get ()));
             in_buf_ = std::move (in_buf);
-            in_->exceptions (std::ios::failbit | std::ios::badbit);
+            in_->exceptions (std::istream::failbit | std::istream::badbit);
           }
         }
       }
diff --git a/web/apache/service b/web/apache/service
index 4c0d395..165ff90 100644
--- a/web/apache/service
+++ b/web/apache/service
@@ -5,8 +5,11 @@
 #ifndef WEB_APACHE_SERVICE
 #define WEB_APACHE_SERVICE
 
-#include <httpd.h>
-#include <http_config.h> // module, ap_hook_*()
+#include <apr_pools.h>   // apr_pool_t
+#include <apr_hooks.h>   // APR_HOOK_*
+
+#include <httpd.h>       // request_rec, server_rec, HTTP_*, DECLINED
+#include <http_config.h> // module, cmd_parms, ap_hook_*()
 
 #include <map>
 #include <memory>  // unique_ptr
@@ -130,7 +133,8 @@ namespace web
         // The worker_initializer() function is called right after Apache
         // worker process is started. Called for every new process spawned.
         //
-        ap_hook_child_init (&worker_initializer<M>, NULL, NULL, APR_HOOK_LAST);
+        ap_hook_child_init (
+          &worker_initializer<M>, NULL, NULL, APR_HOOK_LAST);
 
         // The request_handler () function is called for each client request.
         //
@@ -254,7 +258,8 @@ namespace web
       parse_option (cmd_parms* parms, void* conf, const char* args) noexcept;
 
       const char*
-      add_option (context_id id, const char* name, optional<std::string> value);
+      add_option (
+        context_id id, const char* name, optional<std::string> value);
 
       void
       finalize_config (server_rec*);
diff --git a/web/apache/service.cxx b/web/apache/service.cxx
index 2ca8cf0..da44530 100644
--- a/web/apache/service.cxx
+++ b/web/apache/service.cxx
@@ -4,10 +4,10 @@
 
 #include <web/apache/service>
 
-#include <apr_pools.h>
+#include <apr_pools.h> // apr_palloc()
 
-#include <httpd.h>
-#include <http_config.h>
+#include <httpd.h>       // server_rec
+#include <http_config.h> // command_rec, cmd_*, ap_get_module_config()
 
 #include <memory>    // unique_ptr
 #include <string>
@@ -16,7 +16,10 @@
 #include <cstring>   // strlen()
 #include <exception>
 
+#include <butl/optional>
+
 #include <web/module>
+#include <web/apache/log>
 
 using namespace std;
 
@@ -29,12 +32,12 @@ namespace web
     {
       assert (cmds == nullptr);
 
-      // Fill apache module directive definitions. Directives share
-      // common name space in apache configuration file, so to prevent name
-      // clash have to form directive name as a combination of module and
-      // option names: <module name>-<option name>. This why for option
-      // bar of module foo the corresponding directive will appear in apache
-      // configuration file as foo-bar.
+      // Fill apache module directive definitions. Directives share common
+      // name space in apache configuration file, so to prevent name clash
+      // have to form directive name as a combination of module and option
+      // names: <module name>-<option name>. This why for option bar of module
+      // foo the corresponding directive will appear in apache configuration
+      // file as foo-bar.
       //
       const option_descriptions& od (exemplar_.options ());
       unique_ptr<command_rec[]> directives (new command_rec[od.size () + 1]);
@@ -42,7 +45,8 @@ namespace web
 
       for (const auto& o: od)
       {
-        auto i (option_descriptions_.emplace (name_ + "-" + o.first, o.second));
+        auto i (
+          option_descriptions_.emplace (name_ + "-" + o.first, o.second));
         assert (i.second);
 
         *d++ =
@@ -73,8 +77,8 @@ namespace web
     create_server_context (apr_pool_t* pool, server_rec*) noexcept
     {
       // Create the object using the configuration memory pool provided by the
-      // Apache API. The lifetime of the object is equal to the lifetime of the
-      // pool.
+      // Apache API. The lifetime of the object is equal to the lifetime of
+      // the pool.
       //
       void* p (apr_palloc (pool, sizeof (context)));
       assert (p != nullptr);
@@ -104,10 +108,10 @@ namespace web
       service& srv (*reinterpret_cast<service*> (parms->cmd->cmd_data));
 
       if (srv.options_parsed_)
-        // Apache have started the second pass of its messy initialization cycle
-        // (more details at http://wiki.apache.org/httpd/ModuleLife). This time
-        // we are parsing for real. Cleanup the existing config, and start
-        // building the new one.
+        // Apache have started the second pass of its messy initialization
+        // cycle (more details at http://wiki.apache.org/httpd/ModuleLife).
+        // This time we are parsing for real. Cleanup the existing config, and
+        // start building the new one.
         //
         srv.clear_config ();
 
@@ -214,8 +218,12 @@ namespace web
       // context options as a result of the merge_server_context() calls.
       //
       for (const auto& o: options_)
-        if (o.first->server != nullptr) // Is a directory configuration context.
+      {
+        // Is a directory configuration context.
+        //
+        if (o.first->server != nullptr)
           complement (o.first, o.first->server);
+      }
 
       options_parsed_ = true;
     }
diff --git a/web/apache/service.txx b/web/apache/service.txx
index b57befc..961c19b 100644
--- a/web/apache/service.txx
+++ b/web/apache/service.txx
@@ -3,9 +3,9 @@
 // license   : MIT; see accompanying LICENSE file
 
 #include <unistd.h> // getppid()
-#include <signal.h> // kill()
+#include <signal.h> // kill(), SIGTERM
 
-#include <http_log.h>
+#include <http_log.h> // APLOG_*
 
 #include <utility>   // move()
 #include <exception>
@@ -132,21 +132,31 @@ namespace web
         const M* e (dynamic_cast<const M*> (exemplar));
         assert (e != nullptr);
 
-        M m (*e);
-
-        if (static_cast<module&> (m).handle (rq, rq, lg))
-          return rq.flush ();
+        for (M m (*e);;)
+        {
+          try
+          {
+            if (static_cast<module&> (m).handle (rq, rq, lg))
+              return rq.flush ();
 
-        if (!rq.get_write_state ())
-          return DECLINED;
+            if (rq.state () == request_state::initial)
+              return DECLINED;
 
-        lg.write (nullptr, 0, func_name.c_str (), APLOG_ERR,
-                  "handling declined while unbuffered content "
-                  "has been written");
+            lg.write (nullptr, 0, func_name.c_str (), APLOG_ERR,
+                      "handling declined being partially executed");
+            break;
+          }
+          catch (const module::retry&)
+          {
+            // Retry to handle the request.
+            //
+            rq.rewind ();
+          }
+        }
       }
       catch (const invalid_request& e)
       {
-        if (!e.content.empty () && !rq.get_write_state ())
+        if (!e.content.empty () && rq.state () < request_state::writing)
         {
           try
           {
@@ -165,11 +175,12 @@ namespace web
       {
         lg.write (nullptr, 0, func_name.c_str (), APLOG_ERR, e.what ());
 
-        if (*e.what () && !rq.get_write_state ())
+        if (*e.what () && rq.state () < request_state::writing)
         {
           try
           {
-            rq.content (HTTP_INTERNAL_SERVER_ERROR, "text/plain;charset=utf-8")
+            rq.content (
+              HTTP_INTERNAL_SERVER_ERROR, "text/plain;charset=utf-8")
               << e.what ();
 
             return rq.flush ();
@@ -184,11 +195,12 @@ namespace web
       {
         lg.write (nullptr, 0, func_name.c_str (), APLOG_ERR, "unknown error");
 
-        if (!rq.get_write_state ())
+        if (rq.state () < request_state::writing)
         {
           try
           {
-            rq.content (HTTP_INTERNAL_SERVER_ERROR, "text/plain;charset=utf-8")
+            rq.content (
+              HTTP_INTERNAL_SERVER_ERROR, "text/plain;charset=utf-8")
               << "unknown error";
 
             return rq.flush ();
diff --git a/web/apache/stream b/web/apache/stream
index 2ea9b7e..f05ea08 100644
--- a/web/apache/stream
+++ b/web/apache/stream
@@ -5,44 +5,56 @@
 #ifndef WEB_APACHE_STREAM
 #define WEB_APACHE_STREAM
 
-#include <httpd.h>
-#include <http_protocol.h>
+#include <httpd.h>         // request_rec, HTTP_*
+#include <http_protocol.h> // ap_*()
 
 #include <ios>       // streamsize
 #include <vector>
-#include <cstring>   // memmove()
+#include <cstring>   // memmove(), size_t
 #include <streambuf>
 #include <algorithm> // min(), max()
 
-#include <web/module>
+#include <web/module> // invalid_request
 
 namespace web
 {
   namespace apache
   {
-    // Object of a class implementing this interface is intended for
-    // keeping the state of writing response to the client.
+    // Object of a class implementing this interface is intended for keeping
+    // the state of communication with the client.
     //
-    struct write_state
+    struct stream_state
     {
-      // Called by ostreambuf methods when some content to be written to the
-      // client.
+      // Called by istreambuf functions when content is about to be read from
+      // the client. Can throw invalid_request or sequence_error.
       //
       virtual void
-      set_write_state () = 0;
+      set_read_state () = 0;
 
-      // Called to check if any data have already been written to the client.
-      // Such checks required for some operations which are impossible to
-      // execute after response is partially written.
+      // Called by ostreambuf functions when some content is about to be
+      // written to the client. Can throw invalid_request or sequence_error.
       //
-      virtual bool
-      get_write_state () const noexcept = 0;
+      virtual void
+      set_write_state () = 0;
+    };
+
+    // Base class for ostreambuf and istreambuf. References request and
+    // communication state structures.
+    //
+    class rbuf: public std::streambuf
+    {
+    protected:
+      rbuf (request_rec* r, stream_state& s): rec_ (r), state_ (s) {}
+
+    protected:
+      request_rec* rec_;
+      stream_state& state_;
     };
 
-    class ostreambuf: public std::streambuf
+    class ostreambuf: public rbuf
     {
     public:
-      ostreambuf (request_rec* rec, write_state& ws): rec_ (rec), ws_ (ws) {}
+      ostreambuf (request_rec* r, stream_state& s): rbuf (r, s) {}
 
     private:
       virtual int_type
@@ -50,7 +62,7 @@ namespace web
       {
         if (c != traits_type::eof ())
         {
-          ws_.set_write_state ();
+          state_.set_write_state ();
 
           char chr (c);
 
@@ -67,12 +79,10 @@ namespace web
       virtual std::streamsize
       xsputn (const char* s, std::streamsize num)
       {
-        ws_.set_write_state ();
+        state_.set_write_state ();
 
         if (ap_rwrite (s, num, rec_) < 0)
-        {
           throw invalid_request (HTTP_REQUEST_TIME_OUT);
-        }
 
         return num;
       }
@@ -81,59 +91,37 @@ namespace web
       sync ()
       {
         if (ap_rflush (rec_) < 0)
-        {
           throw invalid_request (HTTP_REQUEST_TIME_OUT);
-        }
 
         return 0;
       }
-
-    private:
-      request_rec* rec_;
-      write_state& ws_;
     };
 
-    class istreambuf: public std::streambuf
+    class istreambuf: public rbuf
     {
     public:
-      istreambuf (request_rec* rec,
-                  write_state& ws,
+      istreambuf (request_rec* r,
+                  stream_state& s,
                   size_t bufsize = 1024,
                   size_t putback = 1)
-          : rec_ (rec),
-            ws_ (ws),
+          : rbuf (r, s),
             bufsize_ (std::max (bufsize, (size_t)1)),
             putback_ (std::min (putback, bufsize_ - 1)),
             buf_ (bufsize_)
       {
-        if (ws_.get_write_state ())
-        {
-          throw sequence_error ("::web::apache::istreambuf::istreambuf");
-        }
-
         char* p (buf_.data () + putback_);
         setg (p, p, p);
-
-        int status (ap_setup_client_block (rec_, REQUEST_CHUNKED_DECHUNK));
-
-        if (status != OK)
-        {
-          throw invalid_request (status);
-        }
       }
 
     private:
       virtual int_type
       underflow ()
       {
-        if (ws_.get_write_state ())
-        {
-          throw sequence_error ("::web::apache::istreambuf::underflow");
-        }
-
         if (gptr () < egptr ())
           return traits_type::to_int_type (*gptr ());
 
+        state_.set_read_state ();
+
         size_t pb (std::min ((size_t)(gptr () - eback ()), putback_));
         std::memmove (buf_.data () + putback_ - pb, gptr () - pb, pb);
 
@@ -141,22 +129,16 @@ namespace web
         int rb (ap_get_client_block (rec_, p, bufsize_ - putback_));
 
         if (rb == 0)
-        {
           return traits_type::eof ();
-        }
 
         if (rb < 0)
-        {
           throw invalid_request (HTTP_REQUEST_TIME_OUT);
-        }
 
         setg (p - pb, p, p + rb);
         return traits_type::to_int_type (*gptr ());
       }
 
     private:
-      request_rec* rec_;
-      write_state& ws_;
       size_t bufsize_;
       size_t putback_;
       std::vector<char> buf_;
diff --git a/web/module b/web/module
index 85896c3..fcd7012 100644
--- a/web/module
+++ b/web/module
@@ -111,13 +111,13 @@ namespace web
     virtual const name_values&
     cookies () = 0;
 
-    // Get the stream to read the request content from. Note that
-    // reading content after any unbuffered content has been written
-    // is undefined behavior. The implementation may detect it and
-    // throw sequence_error but is not required to do so.
+    // Get the stream to read the request content from. If the buffer argument
+    // is false, then reading content after any unbuffered content has been
+    // written or after a retry is undefined behavior. The implementation may
+    // detect this and throw sequence_error but is not required to do so.
     //
     virtual std::istream&
-    content () = 0;
+    content (bool buffer = false) = 0;
   };
 
   class response
@@ -160,10 +160,11 @@ namespace web
     virtual void
     cookie (const char* name,
             const char* value,
-            const std::chrono::seconds* max_age = 0,
-            const char* path = 0,
-            const char* domain = 0,
-            bool secure = false) = 0;
+            const std::chrono::seconds* max_age = nullptr,
+            const char* path = nullptr,
+            const char* domain = nullptr,
+            bool secure = false,
+            bool buffer = true) = 0;
   };
 
   // A web server logging backend. The module can use it to log
@@ -229,15 +230,26 @@ namespace web
     // Return false if decline to handle the request. If handling have been
     // declined after any unbuffered content has been written, then the
     // implementation shall terminate the response in a suitable but
-    // unspecified manner. Any exception other than invalid_request described
-    // above that leaves this function is treated by the web server
-    // implementation as an internal server error (500). Similar to
-    // invalid_request, it will try to return the status and description
-    // (obtained by calling what() on std::exception) to the client, if
-    // possible. The description is assume to be encoded in UTF-8. The
-    // implementation may provide a configuration option to omit the
-    // description from the response, for security/privacy reasons.
+    // unspecified manner.
     //
+    // Throw retry if need to retry handling the request. The retry will
+    // happen on the same instance of the module and the implementation is
+    // expected to "rewind" the request and response objects to their initial
+    // state. This is only guaranteed to be possible if the relevant functions
+    // in the request and response objects were called in buffered mode (the
+    // buffer argument was true).
+    //
+    // Any exception other than retry and invalid_request described above that
+    // leaves this function is treated by the web server implementation as an
+    // internal server error (500). Similar to invalid_request, it will try to
+    // return the status and description (obtained by calling what() on
+    // std::exception) to the client, if possible. The description is assume
+    // to be encoded in UTF-8. The implementation may provide a configuration
+    // option to omit the description from the response, for security/privacy
+    // reasons.
+    //
+    struct retry {};
+
     virtual bool
     handle (request&, response&, log&) = 0;
   };
-- 
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