From a987d92bd0162cadb36b76f8dfd5ac2cbddc97fb 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: asyncify LocalStore::autoGC

sadly we need both a synchronous and an asynchronous promise for this
since destructors cannot be async. we also cannot use forked promises
since multiple threads may be waiting for an auto-gc to complete, but
forked promises are bound to an event loop (and thus a single thread)

Change-Id: I7b4fdbd229c4a1e01bbf80858e93347024f2e333
---
 lix/libstore/build/worker.cc |  2 +-
 lix/libstore/gc.cc           | 29 ++++++++++++++++++++---------
 lix/libstore/local-store.cc  | 10 +++++-----
 lix/libstore/local-store.hh  |  6 ++++--
 4 files changed, 30 insertions(+), 17 deletions(-)

diff --git a/lix/libstore/build/worker.cc b/lix/libstore/build/worker.cc
index ff8ba2ab4..f38e6cb6a 100644
--- a/lix/libstore/build/worker.cc
+++ b/lix/libstore/build/worker.cc
@@ -313,7 +313,7 @@ kj::Promise<Result<Worker::Results>> Worker::boopGC(LocalStore & localStore)
 try {
     while (true) {
         co_await AIO().provider.getTimer().afterDelay(10 * kj::SECONDS);
-        localStore.autoGC(false);
+        TRY_AWAIT(localStore.autoGC(false));
     }
 } catch (...) {
     co_return result::current_exception();
diff --git a/lix/libstore/gc.cc b/lix/libstore/gc.cc
index c8c78b5f4..3e62a2e85 100644
--- a/lix/libstore/gc.cc
+++ b/lix/libstore/gc.cc
@@ -8,6 +8,7 @@
 #include "lix/libutil/strings.hh"
 #include "lix/libutil/thread-name.hh"
 
+#include <kj/async.h>
 #include <queue>
 #include <regex>
 
@@ -868,8 +869,8 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
 }
 
 
-void LocalStore::autoGC(bool sync)
-{
+kj::Promise<Result<void>> LocalStore::autoGC(bool sync)
+try {
     static auto fakeFreeSpaceFile = getEnv("_NIX_TEST_FREE_SPACE_FILE");
 
     auto getAvail = [this]() -> uint64_t {
@@ -883,33 +884,35 @@ void LocalStore::autoGC(bool sync)
         return (uint64_t) st.f_bavail * st.f_frsize;
     };
 
-    std::shared_future<void> future;
+    auto pfp = kj::newPromiseAndCrossThreadFulfiller<void>();
 
     {
         auto state(_gcState.lock());
 
         if (state->gcRunning) {
-            future = state->gcFuture;
+            state->gcWaiters.push_back(std::move(pfp.fulfiller));
             debug("waiting for auto-GC to finish");
             goto sync;
         }
 
         auto now = std::chrono::steady_clock::now();
 
-        if (now < state->lastGCCheck + std::chrono::seconds(settings.minFreeCheckInterval)) return;
+        if (now < state->lastGCCheck + std::chrono::seconds(settings.minFreeCheckInterval)) {
+            co_return result::success();
+        }
 
         auto avail = getAvail();
 
         state->lastGCCheck = now;
 
-        if (avail >= settings.minFree || avail >= settings.maxFree) return;
+        if (avail >= settings.minFree || avail >= settings.maxFree) co_return result::success();
 
-        if (avail > state->availAfterGC * 0.97) return;
+        if (avail > state->availAfterGC * 0.97) co_return result::success();
 
         state->gcRunning = true;
 
         std::promise<void> promise;
-        future = state->gcFuture = promise.get_future().share();
+        state->gcFuture = promise.get_future();
 
         std::thread([promise{std::move(promise)}, this, avail, getAvail]() mutable {
             setCurrentThreadName("auto gc");
@@ -922,6 +925,10 @@ void LocalStore::autoGC(bool sync)
                     state->gcRunning = false;
                     state->lastGCCheck = std::chrono::steady_clock::now();
                     promise.set_value();
+                    for (auto & waiter : state->gcWaiters) {
+                        waiter->fulfill();
+                    }
+                    state->gcWaiters.clear();
                 });
 
                 GCOptions options;
@@ -946,7 +953,11 @@ void LocalStore::autoGC(bool sync)
 
  sync:
     // Wait for the future outside of the state lock.
-    if (sync) future.get();
+    if (sync) co_await pfp.promise;
+
+    co_return result::success();
+} catch (...) {
+    co_return result::current_exception();
 }
 
 
diff --git a/lix/libstore/local-store.cc b/lix/libstore/local-store.cc
index 8d802fea5..9e06b7117 100644
--- a/lix/libstore/local-store.cc
+++ b/lix/libstore/local-store.cc
@@ -458,12 +458,12 @@ AutoCloseFD LocalStore::openGCLock()
 
 LocalStore::~LocalStore()
 {
-    std::shared_future<void> future;
+    std::future<void> future;
 
     {
         auto state(_gcState.lock());
         if (state->gcRunning)
-            future = state->gcFuture;
+            future = std::move(state->gcFuture);
     }
 
     if (future.valid()) {
@@ -1266,7 +1266,7 @@ try {
                 }
             }
 
-            autoGC();
+            TRY_AWAIT(autoGC());
 
             canonicalisePathMetaData(realPath, {});
 
@@ -1381,7 +1381,7 @@ try {
 
             deletePath(realPath);
 
-            autoGC();
+            TRY_AWAIT(autoGC());
 
             if (inMemory) {
                 StringSource dumpSource { dump };
@@ -1448,7 +1448,7 @@ try {
 
             deletePath(realPath);
 
-            autoGC();
+            TRY_AWAIT(autoGC());
 
             writeFile(realPath, s);
 
diff --git a/lix/libstore/local-store.hh b/lix/libstore/local-store.hh
index 3ce451fbb..66263fe15 100644
--- a/lix/libstore/local-store.hh
+++ b/lix/libstore/local-store.hh
@@ -9,6 +9,7 @@
 
 #include <chrono>
 #include <future>
+#include <kj/async.h>
 #include <string>
 #include <mutex>
 #include <memory>
@@ -118,7 +119,8 @@ private:
          * the GC to finish.
          */
         bool gcRunning = false;
-        std::shared_future<void> gcFuture;
+        std::future<void> gcFuture;
+        std::list<kj::Own<kj::CrossThreadPromiseFulfiller<void>>> gcWaiters;
 
         /**
          * How much disk space was available after the previous
@@ -297,7 +299,7 @@ public:
      * If free disk space in /nix/store if below minFree, delete
      * garbage until it exceeds maxFree.
      */
-    void autoGC(bool sync = true);
+    kj::Promise<Result<void>> autoGC(bool sync = true);
 
     /**
      * Register the store path 'output' as the output named 'outputName' of