From ab786e22f16eed0d95123d5698eb71079c312584 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 18 Jan 2024 15:29:54 +0100
Subject: [PATCH 1/3] Show what goal is waiting for a build slot

---
 src/libstore/build/worker.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libstore/build/worker.cc b/src/libstore/build/worker.cc
index 974a9f510..d57e22393 100644
--- a/src/libstore/build/worker.cc
+++ b/src/libstore/build/worker.cc
@@ -251,7 +251,7 @@ void Worker::childTerminated(Goal * goal, bool wakeSleepers)
 
 void Worker::waitForBuildSlot(GoalPtr goal)
 {
-    debug("wait for build slot");
+    goal->trace("wait for build slot");
     bool isSubstitutionGoal = goal->jobCategory() == JobCategory::Substitution;
     if ((!isSubstitutionGoal && getNrLocalBuilds() < settings.maxBuildJobs) ||
         (isSubstitutionGoal && getNrSubstitutions() < settings.maxSubstitutionJobs))

From a18d8d688a826ff535b3eeff289ef51db33a413b Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 18 Jan 2024 17:01:45 +0100
Subject: [PATCH 2/3] LocalStore::addToStore(): Ignore exceptions from
 parseDump()

In the "discard" case (i.e. when the store path already exists
locally), when we call parseDump() from a Finally and it throws an
exception (e.g. if the download of the NAR fails), Nix crashes:

   terminate called after throwing an instance of 'nix::SubstituteGone'
     what():  error: file 'nar/06br3254rx4gz4cvjzxlv028jrx80zg5i4jr62vjmn416dqihgr7.nar.xz' does not exist in binary cache 'http://localhost'
   Aborted (core dumped)
---
 src/libstore/local-store.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 5a399c8be..07068f8f8 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1049,7 +1049,11 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
     Finally cleanup = [&]() {
         if (!narRead) {
             NullParseSink sink;
-            parseDump(sink, source);
+            try {
+                parseDump(sink, source);
+            } catch (...) {
+                ignoreException();
+            }
         }
     };
 

From dca0a802405be9798e12ad8be2ec6d227d9a2fa2 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 18 Jan 2024 17:16:34 +0100
Subject: [PATCH 3/3] copyStorePath(): Bail out early if the store path already
 exists

In rare cases (e.g. when using allowSubstitutes = false), it's
possible that we simultaneously have a DerivationGoal *and* a
SubstitutionGoal building the same path. So if a DerivationGoal
already built the path while the SubstitutionGoal was waiting for a
download slot, it saves us a superfluous download to exit early.
---
 src/libstore/store-api.cc | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 0c37ecd30..9cb187e66 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -981,6 +981,11 @@ void copyStorePath(
     RepairFlag repair,
     CheckSigsFlag checkSigs)
 {
+    /* Bail out early (before starting a download from srcStore) if
+       dstStore already has this path. */
+    if (!repair && dstStore.isValidPath(storePath))
+        return;
+
     auto srcUri = srcStore.getUri();
     auto dstUri = dstStore.getUri();
     auto storePathS = srcStore.printStorePath(storePath);