From 480426a364f09e7992230b32f2941a09fb52d729 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Fri, 5 Feb 2021 15:57:33 +0100
Subject: [PATCH] Add more instrumentation for #4270

---
 src/libstore/build/derivation-goal.cc | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 190adf31c..eeaec4f2c 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -1784,12 +1784,14 @@ void DerivationGoal::startBuilder()
     worker.childStarted(shared_from_this(), {builderOut.readSide.get()}, true, true);
 
     /* Check if setting up the build environment failed. */
+    std::vector<std::string> msgs;
     while (true) {
         string msg = [&]() {
             try {
                 return readLine(builderOut.readSide.get());
             } catch (Error & e) {
-                e.addTrace({}, "while reading the response of setting up the build environment");
+                e.addTrace({}, "while waiting for the build environment to initialize (previous messages: %s)",
+                    concatStringsSep("|", msgs));
                 throw e;
             }
         }();
@@ -1801,6 +1803,7 @@ void DerivationGoal::startBuilder()
             throw ex;
         }
         debug("sandbox setup: " + msg);
+        msgs.push_back(std::move(msg));
     }
 }