From 7889aef7927f61c1aaa54d1afcfe564b7bc69005 Mon Sep 17 00:00:00 2001
From: Samuel Dionne-Riel <samuel@dionne-riel.com>
Date: Mon, 30 Mar 2020 14:36:16 -0400
Subject: [PATCH] hydra-eval-jobs: Mirror eval errors in STDERR

Otherwise, errors will not be shown to end-users, which makes debugging
long evals pretty much impossible.

(cherry picked from commit 76299b9174d7cba06427220166dc4707b43d407d)
---
 src/hydra-eval-jobs/hydra-eval-jobs.cc | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/hydra-eval-jobs/hydra-eval-jobs.cc b/src/hydra-eval-jobs/hydra-eval-jobs.cc
index 4a4084e0..f3367e11 100644
--- a/src/hydra-eval-jobs/hydra-eval-jobs.cc
+++ b/src/hydra-eval-jobs/hydra-eval-jobs.cc
@@ -238,7 +238,12 @@ static void worker(
             else throw TypeError("attribute '%s' is %s, which is not supported", attrPath, showType(*v));
 
         } catch (EvalError & e) {
+            // Transmits the error we got from the previous evaluation
+            // in the JSON output.
             reply["error"] = filterANSIEscapes(e.msg(), true);
+            // Don't forget to print it into the STDERR log, this is
+            // what's shown in the Hydra UI.
+            printError("error: %s", reply["error"]);
         }
 
         writeLine(to.get(), reply.dump());
@@ -328,6 +333,9 @@ int main(int argc, char * * argv)
                                     nlohmann::json err;
                                     err["error"] = e.what();
                                     writeLine(to->get(), err.dump());
+                                    // Don't forget to print it into the STDERR log, this is
+                                    // what's shown in the Hydra UI.
+                                    printError("error: %s", err["error"]);
                                 }
                             },
                             ProcessOptions { .allowVfork = false });