From e52fe2cab4061058980f2b0ef9b3ac66f14bfbff Mon Sep 17 00:00:00 2001
From: Wolfgang Walther <walther@technowledgy.de>
Date: Tue, 17 Jun 2025 08:45:29 +0200
Subject: [PATCH] libstore: fix race condition when creating state directories

Running parallel nix in nix can lead to multiple instances trying to
create the state directories and failing on the `createSymlink` step,
because the link already exists.

`replaceSymlink` is already idempotent, so let's use that.

Resolves #2706

(cherry picked from commit d64c92216409b08ca40f43e601fbdae59114e812)
---
 src/libstore/local-store.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 339d3c926..63108fab4 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -126,7 +126,7 @@ LocalStore::LocalStore(std::string_view scheme, PathView path, const Params & pa
     Path gcRootsDir = stateDir + "/gcroots";
     if (!pathExists(gcRootsDir)) {
         createDirs(gcRootsDir);
-        createSymlink(profilesDir, gcRootsDir + "/profiles");
+        replaceSymlink(profilesDir, gcRootsDir + "/profiles");
     }
 
     for (auto & perUserDir : {profilesDir + "/per-user", gcRootsDir + "/per-user"}) {