From 3b9020d37c48446841e7fc2aab186e345e2c4da9 Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Tue, 11 Feb 2025 01:06:15 +0100
Subject: [PATCH] libstore/libfetchers: do more async lazy init

while these two are currently safe using synchronous mutexes we can't
easily guarantee this forever. since we have async mutexes too now we
may as well use them; the overhead they incur is small enough that we
can just ignore it here. this can be revisited later, when necessary.

Change-Id: Ia4cb2361d5638aa48e3b982a05e75ae9c4f254cd
---
 lix/libfetchers/registry.cc | 21 ++++++++-------------
 lix/libstore/store-api.cc   | 21 ++++++++-------------
 2 files changed, 16 insertions(+), 26 deletions(-)

diff --git a/lix/libfetchers/registry.cc b/lix/libfetchers/registry.cc
index 42c57f69d..f0f321ddf 100644
--- a/lix/libfetchers/registry.cc
+++ b/lix/libfetchers/registry.cc
@@ -1,6 +1,7 @@
 #include "lix/libfetchers/registry.hh"
 #include "lix/libfetchers/fetchers.hh"
 #include "lix/libutil/async.hh"
+#include "lix/libutil/sync.hh"
 #include "lix/libutil/types.hh"
 #include "lix/libutil/users.hh"
 #include "lix/libstore/globals.hh"
@@ -12,7 +13,6 @@
 #include <memory>
 #include <mutex>
 #include <nlohmann/json.hpp>
-#include <shared_mutex>
 
 namespace nix::fetchers {
 
@@ -160,21 +160,16 @@ void overrideRegistry(
 
 static kj::Promise<Result<std::shared_ptr<Registry>>> getGlobalRegistry(ref<Store> store)
 try {
-    static std::shared_mutex mtx;
-    static std::shared_ptr<Registry> reg;
+    static Sync<std::shared_ptr<Registry>, AsyncMutex> reg;
 
-    if (std::shared_lock l(mtx); reg) {
-        co_return reg;
-    }
+    auto lk = co_await reg.lock();
 
-    std::scoped_lock l(mtx);
-
-    if (!reg) {
+    if (!*lk) {
         auto path = fetchSettings.flakeRegistry.get();
         if (path == "") {
-            reg = std::make_shared<Registry>(Registry::Global); // empty registry
+            *lk = std::make_shared<Registry>(Registry::Global); // empty registry
         } else if (path == "vendored") {
-            reg = Registry::read(settings.nixDataDir + "/flake-registry.json", Registry::Global);
+            *lk = Registry::read(settings.nixDataDir + "/flake-registry.json", Registry::Global);
         } else {
             if (!path.starts_with("/")) {
                 warn(
@@ -193,11 +188,11 @@ try {
                 path = store->toRealPath(storePath);
             }
 
-            reg = Registry::read(path, Registry::Global);
+            *lk = Registry::read(path, Registry::Global);
         }
     };
 
-    co_return reg;
+    co_return *lk;
 } catch (...) {
     co_return result::current_exception();
 }
diff --git a/lix/libstore/store-api.cc b/lix/libstore/store-api.cc
index 99028c640..06d165636 100644
--- a/lix/libstore/store-api.cc
+++ b/lix/libstore/store-api.cc
@@ -5,6 +5,7 @@
 #include "lix/libstore/nar-info-disk-cache.hh"
 #include "lix/libutil/async.hh"
 #include "lix/libutil/result.hh"
+#include "lix/libutil/sync.hh"
 #include "lix/libutil/thread-pool.hh"
 #include "lix/libutil/url.hh"
 #include "lix/libutil/archive.hh"
@@ -19,7 +20,6 @@
 #include <mutex>
 #include <nlohmann/json.hpp>
 #include <regex>
-#include <shared_mutex>
 
 using json = nlohmann::json;
 
@@ -1598,34 +1598,29 @@ try {
 
 kj::Promise<Result<std::list<ref<Store>>>> getDefaultSubstituters()
 try {
-    static std::shared_mutex mtx;
-    static std::optional<std::list<ref<Store>>> stores;
+    static Sync<std::optional<std::list<ref<Store>>>, AsyncMutex> stores;
 
-    if (std::shared_lock l(mtx); stores.has_value()) {
-        co_return *stores;
-    }
+    auto lk = co_await stores.lock();
 
-    std::lock_guard l(mtx);
-
-    if (!stores.has_value()) {
+    if (!lk->has_value()) {
         StringSet done;
 
-        stores.emplace();
+        lk->emplace();
         for (auto uri : settings.substituters.get()) {
             if (!done.insert(uri).second) continue;
             try {
-                stores->push_back(TRY_AWAIT(openStore(uri)));
+                (*lk)->push_back(TRY_AWAIT(openStore(uri)));
             } catch (Error & e) {
                 logWarning(e.info());
             }
         }
 
-        stores->sort([](ref<Store> & a, ref<Store> & b) {
+        (*lk)->sort([](ref<Store> & a, ref<Store> & b) {
             return a->config().priority < b->config().priority;
         });
     }
 
-    co_return *stores;
+    co_return **lk;
 } catch (...) {
     co_return result::current_exception();
 }