libutil: add topoSortAsync

Change-Id: Icad14f3e168f9577e7b99ac5f3f552f8625aebcd

lix/libutil/topo-sort.hh

@@ -2,6 +2,8 @@
 ///@file
 
 #include "lix/libutil/error.hh"
+#include "lix/libutil/result.hh"
+#include <kj/async.h>
 
 namespace nix {
 
@@ -40,4 +42,47 @@ std::vector<T> topoSort(std::set<T> items,
     return sorted;
 }
 
+template<typename T>
+kj::Promise<Result<std::vector<T>>> topoSortAsync(std::set<T> items,
+        std::function<kj::Promise<Result<std::set<T>>>(const T &)> getChildren,
+        std::function<Error(const T &, const T &)> makeCycleError)
+try {
+    std::vector<T> sorted;
+    std::set<T> visited, parents;
+
+    std::function<kj::Promise<Result<void>>(const T & path, const T * parent)> dfsVisit;
+
+    // NOLINTNEXTLINE(cppcoreguidelines-avoid-capturing-lambda-coroutines)
+    dfsVisit = [&](const T & path, const T * parent) -> kj::Promise<Result<void>> {
+        try {
+            if (parents.count(path)) throw makeCycleError(path, *parent);
+
+            if (!visited.insert(path).second) co_return result::success();
+            parents.insert(path);
+
+            std::set<T> references = TRY_AWAIT(getChildren(path));
+
+            for (auto & i : references)
+                /* Don't traverse into items that don't exist in our starting set. */
+                if (i != path && items.count(i))
+                    TRY_AWAIT(dfsVisit(i, &path));
+
+            sorted.push_back(path);
+            parents.erase(path);
+            co_return result::success();
+        } catch (...) {
+            co_return result::current_exception();
+        }
+    };
+
+    for (auto & i : items)
+        TRY_AWAIT(dfsVisit(i, nullptr));
+
+    std::reverse(sorted.begin(), sorted.end());
+
+    co_return sorted;
+} catch (...) {
+    co_return result::current_exception();
+}
+
 }