nbd.Server: Enable to override device not found behavior.

Author: mefyl

nbd/src/elle/nbd/Server.cc

@@ -389,19 +389,19 @@ namespace elle
               auto& device =
                 [&] () -> Device&
                 {
-                  if (this->_devices.empty())
-                    elle::err("no devices available");
-                  if (data == elle::ConstWeakBuffer(""))
+                  if (data == elle::ConstWeakBuffer("") && !this->_devices.empty())
                     return this->_devices.front().get();
+                  auto res = boost::range::find_if(
+                    this->_devices,
+                    [&] (Device const& d)
+                    { return data == d.name(); });
+                  if (res != this->_devices.end())
+                    return (*res).get();
                   else
                   {
-                    auto res = boost::range::find_if(
-                      this->_devices,
-                      [&] (Device const& d)
-                      { return data == d.name(); });
-                    if (res != this->_devices.end())
-                      return (*res).get();
-                    elle::err("no such device: {}", data);
+                    auto& res = this->_device_not_found(data.string());
+                    this->add(res);
+                    return res;
                   }
                 }();
               ELLE_TRACE_SCOPE("export volume {}", device.name());
@@ -484,6 +484,12 @@ namespace elle
         }
     }
 
+    Server::Device&
+    Server::_device_not_found(std::string name)
+    {
+      elle::err("no such device: {}", name);
+    }
+
     void
     Server::add(Device& d)
     {

nbd/src/elle/nbd/Server.hh

@@ -90,15 +90,26 @@ namespace elle
         /// it to memory for instance could speed up things. Takin no action,
         /// which is the default implementation, is correct.
         ///
-        /// @param offset Index of the first byte to cache.  @param length
-        /// Number of bytes to cache.
+        /// @param offset Index of the first byte to cache.
+        /// @param length Number of bytes to cache.
         virtual
         void
         cache(uint64_t offset, uint32_t length);
       };
       /// Register a device to be exposed by the server.
       void
       add(Device& d);
+      /// Called when a requested device is not found.
+      ///
+      /// Subclasses may return a dynamically allocated device or throw an
+      /// exception to refuse doing so, which is the default behavior.
+      ///
+      /// @param name The name of the device that was requested and not found.
+      /// @return The device for that name.
+      /// @throw elle::Error Signifies the device does not exist.
+      virtual
+      Device&
+      _device_not_found(std::string name);
       ELLE_ATTRIBUTE_R(std::vector<std::reference_wrapper<Device>>, devices);
 
     /*-------.