From fdf5313e7ea9ba6476734dc7b2c20b74855e95a4 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 18 Jan 2024 17:16:34 +0100
Subject: [PATCH] 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.

(cherry picked from commit dca0a802405be9798e12ad8be2ec6d227d9a2fa2)
---
 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 fe1903ec6..43ce70cd2 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -982,6 +982,11 @@ 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);