From f1a47a96b6ab55d0b0017df5b94b3da69c65bf21 Mon Sep 17 00:00:00 2001
From: Ben Burdette <bburdette@gmail.com>
Date: Wed, 5 Aug 2020 10:58:00 -0600
Subject: [PATCH 1/3] error messages for issue 2238

---
 src/build-remote/build-remote.cc | 33 +++++++++++++++++++++++++++++++-
 src/libstore/build.cc            | 13 +++++++++++--
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/src/build-remote/build-remote.cc b/src/build-remote/build-remote.cc
index 3579d8fff..2414a47c1 100644
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@@ -103,7 +103,7 @@ static int _main(int argc, char * * argv)
             drvPath = store->parseStorePath(readString(source));
             auto requiredFeatures = readStrings<std::set<std::string>>(source);
 
-             auto canBuildLocally = amWilling
+            auto canBuildLocally = amWilling
                  &&  (  neededSystem == settings.thisSystem
                      || settings.extraPlatforms.get().count(neededSystem) > 0)
                  &&  allSupportedLocally(requiredFeatures);
@@ -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;
                 }
 
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 94c398b2f..76baa1a6e 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -4876,8 +4876,17 @@ void Worker::run(const Goals & _topGoals)
             waitForInput();
         else {
             if (awake.empty() && 0 == settings.maxBuildJobs)
-                throw Error("unable to start any build; either increase '--max-jobs' "
-                            "or enable remote builds");
+            {
+                if (getMachines().empty())
+                   throw Error("unable to start any build; either increase '--max-jobs' "
+                            "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());
         }
     }

From e4eae078a52fa4209bb5a46d21dbed09dd9c54ec Mon Sep 17 00:00:00 2001
From: Ben Burdette <bburdette@gmail.com>
Date: Wed, 5 Aug 2020 11:21:36 -0600
Subject: [PATCH 2/3] add derivation path to hint

---
 src/build-remote/build-remote.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/build-remote/build-remote.cc b/src/build-remote/build-remote.cc
index 2414a47c1..723e1463e 100644
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@@ -172,7 +172,7 @@ static int _main(int argc, char * * argv)
                     else
                     {
                         // build the hint template.
-                        string hintstring =  "required (system, features): (%s, %s)";
+                        string hintstring =  "derivation: %s\nrequired (system, features): (%s, %s)";
                         hintstring += "\n%s available machines:";
                         hintstring += "\n(systems, maxjobs, supportedFeatures, mandatoryFeatures)";
 
@@ -183,6 +183,7 @@ static int _main(int argc, char * * argv)
                         // add the template values.
                         auto hint = hintformat(hintstring);
                         hint
+                          % drvPath->to_string()
                           % neededSystem
                           % concatStringsSep<StringSet>(", ", requiredFeatures)
                           % machines.size();

From 31f1af0cabb65e3c5f8f4539500c6b236c387780 Mon Sep 17 00:00:00 2001
From: Ben Burdette <bburdette@gmail.com>
Date: Wed, 5 Aug 2020 11:26:06 -0600
Subject: [PATCH 3/3] don't crash if there's no drvPath

---
 src/build-remote/build-remote.cc | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/build-remote/build-remote.cc b/src/build-remote/build-remote.cc
index 723e1463e..5247cefa6 100644
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@@ -181,9 +181,15 @@ static int _main(int argc, char * * argv)
                         }
 
                         // add the template values.
+                        string drvstr;
+                        if (drvPath.has_value())
+                            drvstr = drvPath->to_string();
+                        else
+                            drvstr = "<unknown>";
+
                         auto hint = hintformat(hintstring);
                         hint
-                          % drvPath->to_string()
+                          % drvstr
                           % neededSystem
                           % concatStringsSep<StringSet>(", ", requiredFeatures)
                           % machines.size();