From 3a64d3c0da2b169383256fd3198cf7d18f8ab163 Mon Sep 17 00:00:00 2001
From: Sergei Zimmerman <sergei@zimmerman.foo>
Date: Sun, 28 Sep 2025 17:42:19 +0300
Subject: [PATCH 1/2] libstore: Call canonPath for constructing
 LocalFSStoreConfig::rootDir

This mirrors what OptionalPathSetting does. Otherwise we run into
an assertion failure for relative paths specified as the authority + path:

nix build nixpkgs#hello --store "local://a/b"
nix: ../posix-source-accessor.cc:13: nix::PosixSourceAccessor::PosixSourceAccessor(std::filesystem::__cxx11::path&&): Assertion `root.empty() || root.is_absolute()' failed.

This is now diagnosed properly:

error: not an absolute path: 'a/b'

Just as you'd specify the root via a query parameter:

nix build nixpkgs#hello --store "local?root=a/b"
---
 src/libstore/local-fs-store.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libstore/local-fs-store.cc b/src/libstore/local-fs-store.cc
index 66ae85d89..b16fc86e9 100644
--- a/src/libstore/local-fs-store.cc
+++ b/src/libstore/local-fs-store.cc
@@ -24,7 +24,7 @@ LocalFSStoreConfig::LocalFSStoreConfig(PathView rootDir, const Params & params)
     // FIXME don't duplicate description once we don't have root setting
     , rootDir{
           this,
-          !rootDir.empty() && params.count("root") == 0 ? (std::optional<Path>{rootDir}) : std::nullopt,
+          !rootDir.empty() && params.count("root") == 0 ? (std::optional{canonPath(rootDir)}) : std::nullopt,
           "root",
           "Directory prefixed to all other paths."}
 {

From 0866ba0b4ad14ddc1aa7ad7d3211ab4a981b9c5d Mon Sep 17 00:00:00 2001
From: Sergei Zimmerman <sergei@zimmerman.foo>
Date: Sun, 28 Sep 2025 18:38:57 +0300
Subject: [PATCH 2/2] libstore: Deduplicate LocalFSStoreConfig::rootDir
 initializers

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>
---
 .../include/nix/store/local-fs-store.hh        | 14 +++++++++++++-
 src/libstore/local-fs-store.cc                 | 18 +++++++++++-------
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/src/libstore/include/nix/store/local-fs-store.hh b/src/libstore/include/nix/store/local-fs-store.hh
index f7d6d65b1..08f8e1656 100644
--- a/src/libstore/include/nix/store/local-fs-store.hh
+++ b/src/libstore/include/nix/store/local-fs-store.hh
@@ -9,6 +9,18 @@ namespace nix {
 
 struct LocalFSStoreConfig : virtual StoreConfig
 {
+private:
+    static OptionalPathSetting makeRootDirSetting(LocalFSStoreConfig & self, std::optional<Path> defaultValue)
+    {
+        return {
+            &self,
+            std::move(defaultValue),
+            "root",
+            "Directory prefixed to all other paths.",
+        };
+    }
+
+public:
     using StoreConfig::StoreConfig;
 
     /**
@@ -20,7 +32,7 @@ struct LocalFSStoreConfig : virtual StoreConfig
      */
     LocalFSStoreConfig(PathView path, const Params & params);
 
-    OptionalPathSetting rootDir{this, std::nullopt, "root", "Directory prefixed to all other paths."};
+    OptionalPathSetting rootDir = makeRootDirSetting(*this, std::nullopt);
 
 private:
 
diff --git a/src/libstore/local-fs-store.cc b/src/libstore/local-fs-store.cc
index b16fc86e9..28069dcaf 100644
--- a/src/libstore/local-fs-store.cc
+++ b/src/libstore/local-fs-store.cc
@@ -20,13 +20,17 @@ Path LocalFSStoreConfig::getDefaultLogDir()
 
 LocalFSStoreConfig::LocalFSStoreConfig(PathView rootDir, const Params & params)
     : StoreConfig(params)
-    // Default `?root` from `rootDir` if non set
-    // FIXME don't duplicate description once we don't have root setting
-    , rootDir{
-          this,
-          !rootDir.empty() && params.count("root") == 0 ? (std::optional{canonPath(rootDir)}) : std::nullopt,
-          "root",
-          "Directory prefixed to all other paths."}
+    /* Default `?root` from `rootDir` if non set
+     * NOTE: We would like to just do rootDir.set(...), which would take care of
+     * all normalization and error checking for us. Unfortunately we cannot do
+     * that because of the complicated initialization order of other fields with
+     * the virtual class hierarchy of nix store configs, and the design of the
+     * settings system. As such, we have no choice but to redefine the field and
+     * manually repeat the same normalization logic.
+     */
+    , rootDir{makeRootDirSetting(
+          *this,
+          !rootDir.empty() && params.count("root") == 0 ? std::optional<Path>{canonPath(rootDir)} : std::nullopt)}
 {
 }