From 964ac8b0e88fb5789b87e33273e42363958d0afb Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Sat, 27 Apr 2024 01:02:22 +0200
Subject: [PATCH] libutil: de-callback-ify computeClosure

only two users of this function exist. only one used it in a way that
even bears resemblance to asynchronicity, and even that one didn't do
it right. fully async and parallel computation would have only worked
if any getEdgesAsync never calls the continuation it receives itself,
only from more derived callbacks running on other threads. calling it
directly would cause the decoupling promise to be awaited immediately
*on the original thread*, completely negating all nice async effects.

Change-Id: I0aa640950cf327533a32dee410105efdabb448df
---
 src/libstore/misc.cc          | 32 +++++------------
 src/libstore/realisation.cc   | 15 +-------
 src/libutil/closure.hh        | 68 ++++++++---------------------------
 tests/unit/libutil/closure.cc | 35 +++---------------
 4 files changed, 27 insertions(+), 123 deletions(-)

diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
index c8646083b..22f6b67ee 100644
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@@ -14,10 +14,9 @@ namespace nix {
 void Store::computeFSClosure(const StorePathSet & startPaths,
     StorePathSet & paths_, bool flipDirection, bool includeOutputs, bool includeDerivers)
 {
-    std::function<std::set<StorePath>(const StorePath & path, std::future<ref<const ValidPathInfo>> &)> queryDeps;
+    std::function<std::set<StorePath>(const StorePath & path, ref<const ValidPathInfo>)> queryDeps;
     if (flipDirection)
-        queryDeps = [&](const StorePath& path,
-                        std::future<ref<const ValidPathInfo>> & fut) {
+        queryDeps = [&](const StorePath& path, ref<const ValidPathInfo>) {
             StorePathSet res;
             StorePathSet referrers;
             queryReferrers(path, referrers);
@@ -36,10 +35,8 @@ void Store::computeFSClosure(const StorePathSet & startPaths,
             return res;
         };
     else
-        queryDeps = [&](const StorePath& path,
-                        std::future<ref<const ValidPathInfo>> & fut) {
+        queryDeps = [&](const StorePath& path, ref<const ValidPathInfo> info) {
             StorePathSet res;
-            auto info = fut.get();
             for (auto& ref : info->references)
                 if (ref != path)
                     res.insert(ref);
@@ -54,24 +51,11 @@ void Store::computeFSClosure(const StorePathSet & startPaths,
             return res;
         };
 
-    computeClosure<StorePath>(
-        startPaths, paths_,
-        [&](const StorePath& path,
-            std::function<void(std::promise<std::set<StorePath>>&)>
-                processEdges) {
-            std::promise<std::set<StorePath>> promise;
-            std::function<void(std::future<ref<const ValidPathInfo>>)>
-                getDependencies =
-                    [&](std::future<ref<const ValidPathInfo>> fut) {
-                        try {
-                            promise.set_value(queryDeps(path, fut));
-                        } catch (...) {
-                            promise.set_exception(std::current_exception());
-                        }
-                    };
-            queryPathInfo(path, getDependencies);
-            processEdges(promise);
-        });
+    paths_.merge(computeClosure<StorePath>(
+        startPaths,
+        [&](const StorePath& path) -> std::set<StorePath> {
+            return queryDeps(path, queryPathInfo(path));
+        }));
 }
 
 void Store::computeFSClosure(const StorePath & startPath,
diff --git a/src/libstore/realisation.cc b/src/libstore/realisation.cc
index 93ddb5b20..50c911da0 100644
--- a/src/libstore/realisation.cc
+++ b/src/libstore/realisation.cc
@@ -43,20 +43,7 @@ void Realisation::closure(Store & store, const std::set<Realisation> & startOutp
         return res;
     };
 
-    computeClosure<Realisation>(
-        startOutputs, res,
-        [&](const Realisation& current,
-            std::function<void(std::promise<std::set<Realisation>>&)>
-                processEdges) {
-            std::promise<std::set<Realisation>> promise;
-            try {
-                auto res = getDeps(current);
-                promise.set_value(res);
-            } catch (...) {
-                promise.set_exception(std::current_exception());
-            }
-            return processEdges(promise);
-        });
+    res.merge(computeClosure<Realisation>(startOutputs, getDeps));
 }
 
 nlohmann::json Realisation::toJSON() const {
diff --git a/src/libutil/closure.hh b/src/libutil/closure.hh
index 16e3b93e4..146931fc8 100644
--- a/src/libutil/closure.hh
+++ b/src/libutil/closure.hh
@@ -1,72 +1,32 @@
 #pragma once
 ///@file
 
+#include <functional>
 #include <set>
-#include <future>
-#include "sync.hh"
-
-using std::set;
 
 namespace nix {
 
 template<typename T>
-using GetEdgesAsync = std::function<void(const T &, std::function<void(std::promise<set<T>> &)>)>;
-
-template<typename T>
-void computeClosure(
-    const set<T> startElts,
-    set<T> & res,
-    GetEdgesAsync<T> getEdgesAsync
+std::set<T> computeClosure(
+    std::set<T> startElts,
+    std::function<std::set<T>(const T &)> getEdges
 )
 {
-    struct State
-    {
-        size_t pending;
-        set<T> & res;
-        std::exception_ptr exc;
-    };
+    std::set<T> res, queue = std::move(startElts);
 
-    Sync<State> state_(State{0, res, 0});
+    while (!queue.empty()) {
+        std::set<T> next;
 
-    std::function<void(const T &)> enqueue;
-
-    std::condition_variable done;
-
-    enqueue = [&](const T & current) -> void {
-        {
-            auto state(state_.lock());
-            if (state->exc) return;
-            if (!state->res.insert(current).second) return;
-            state->pending++;
+        for (auto & e : queue) {
+            if (res.insert(e).second) {
+                next.merge(getEdges(e));
+            }
         }
 
-        getEdgesAsync(current, [&](std::promise<set<T>> & prom) {
-            try {
-                auto children = prom.get_future().get();
-                for (auto & child : children)
-                    enqueue(child);
-                {
-                    auto state(state_.lock());
-                    assert(state->pending);
-                    if (!--state->pending) done.notify_one();
-                }
-            } catch (...) {
-                auto state(state_.lock());
-                if (!state->exc) state->exc = std::current_exception();
-                assert(state->pending);
-                if (!--state->pending) done.notify_one();
-            };
-        });
-    };
-
-    for (auto & startElt : startElts)
-        enqueue(startElt);
-
-    {
-        auto state(state_.lock());
-        while (state->pending) state.wait(done);
-        if (state->exc) std::rethrow_exception(state->exc);
+        queue = std::move(next);
     }
+
+    return res;
 }
 
 }
diff --git a/tests/unit/libutil/closure.cc b/tests/unit/libutil/closure.cc
index 7597e7807..b4eaad6f9 100644
--- a/tests/unit/libutil/closure.cc
+++ b/tests/unit/libutil/closure.cc
@@ -16,15 +16,11 @@ map<string, set<string>> testGraph = {
 };
 
 TEST(closure, correctClosure) {
-    set<string> aClosure;
     set<string> expectedClosure = {"A", "B", "C", "F", "G"};
-    computeClosure<string>(
+    set<string> aClosure = computeClosure<string>(
         {"A"},
-        aClosure,
-        [&](const string currentNode, function<void(promise<set<string>> &)> processEdges) {
-            promise<set<string>> promisedNodes;
-            promisedNodes.set_value(testGraph[currentNode]);
-            processEdges(promisedNodes);
+        [&](const string currentNode) {
+            return testGraph[currentNode];
         }
     );
 
@@ -33,12 +29,10 @@ TEST(closure, correctClosure) {
 
 TEST(closure, properlyHandlesDirectExceptions) {
     struct TestExn {};
-    set<string> aClosure;
     EXPECT_THROW(
         computeClosure<string>(
             {"A"},
-            aClosure,
-            [&](const string currentNode, function<void(promise<set<string>> &)> processEdges) {
+            [&](const string currentNode) -> set<string> {
                 throw TestExn();
             }
         ),
@@ -46,25 +40,4 @@ TEST(closure, properlyHandlesDirectExceptions) {
     );
 }
 
-TEST(closure, properlyHandlesExceptionsInPromise) {
-    struct TestExn {};
-    set<string> aClosure;
-    EXPECT_THROW(
-        computeClosure<string>(
-            {"A"},
-            aClosure,
-            [&](const string currentNode, function<void(promise<set<string>> &)> processEdges) {
-                promise<set<string>> promise;
-                try {
-                    throw TestExn();
-                } catch (...) {
-                    promise.set_exception(std::current_exception());
-                }
-                processEdges(promise);
-            }
-        ),
-        TestExn
-    );
-}
-
 }