error messages for issue 2238

src/build-remote/build-remote.cc

@@ -170,7 +170,38 @@ static int _main(int argc, char * * argv)
                     if (rightType && !canBuildLocally)
                         std::cerr << "# postpone\n";
                     else
+                    {
+                        // build the hint template.
+                        string hintstring =  "required (system, features): (%s, %s)";
+                        hintstring += "\n%s available machines:";
+                        hintstring += "\n(systems, maxjobs, supportedFeatures, mandatoryFeatures)";
+
+                        for (unsigned int i = 0; i < machines.size(); ++i) {
+                          hintstring += "\n(%s, %s, %s, %s)";
+                        }
+
+                        // add the template values.
+                        auto hint = hintformat(hintstring);
+                        hint
+                          % neededSystem
+                          % concatStringsSep<StringSet>(", ", requiredFeatures)
+                          % machines.size();
+
+                        for (auto & m : machines) {
+                          hint % concatStringsSep<vector<string>>(", ", m.systemTypes)
+                            % m.maxJobs
+                            % concatStringsSep<StringSet>(", ", m.supportedFeatures)
+                            % concatStringsSep<StringSet>(", ", m.mandatoryFeatures);
+                        }
+
+                        logError({
+                              .name = "Remote build",
+                              .description = "Failed to find a machine for remote build!",
+                              .hint = hint
+                        });
+
                         std::cerr << "# decline\n";
+                    }
                     break;
                 }
 

src/libstore/build.cc

@@ -4876,8 +4876,17 @@ void Worker::run(const Goals & _topGoals)
             waitForInput();
         else {
             if (awake.empty() && 0 == settings.maxBuildJobs)
+            {
+                if (getMachines().empty())
                    throw Error("unable to start any build; either increase '--max-jobs' "
-                            "or enable remote builds");
+                            "or enable remote builds."
+                            "\nhttps://nixos.org/nix/manual/#chap-distributed-builds");
+                else
+                   throw Error("unable to start any build; remote machines may not have "
+                            "all required system features."
+                            "\nhttps://nixos.org/nix/manual/#chap-distributed-builds");
+
+            }
             assert(!awake.empty());
         }
     }