Start building the scheduler for Windows

Building derivations is a lot harder, but the downloading goals is
portable enough.

The "common channel" code is due to Volth. I wonder if there is a way we
can factor it out into separate functions / files to avoid some
within-function CPP.

Co-authored-by: volth <volth@volth.com>

src/libstore/build/goal.cc

@@ -0,0 +1,109 @@
+#include "goal.hh"
+#include "worker.hh"
+
+namespace nix {
+
+
+bool CompareGoalPtrs::operator() (const GoalPtr & a, const GoalPtr & b) const {
+    std::string s1 = a->key();
+    std::string s2 = b->key();
+    return s1 < s2;
+}
+
+
+BuildResult Goal::getBuildResult(const DerivedPath & req) const {
+    BuildResult res { buildResult };
+
+    if (auto pbp = std::get_if<DerivedPath::Built>(&req)) {
+        auto & bp = *pbp;
+
+        /* Because goals are in general shared between derived paths
+           that share the same derivation, we need to filter their
+           results to get back just the results we care about.
+         */
+
+        for (auto it = res.builtOutputs.begin(); it != res.builtOutputs.end();) {
+            if (bp.outputs.contains(it->first))
+                ++it;
+            else
+                it = res.builtOutputs.erase(it);
+        }
+    }
+
+    return res;
+}
+
+
+void addToWeakGoals(WeakGoals & goals, GoalPtr p)
+{
+    if (goals.find(p) != goals.end())
+        return;
+    goals.insert(p);
+}
+
+
+void Goal::addWaitee(GoalPtr waitee)
+{
+    waitees.insert(waitee);
+    addToWeakGoals(waitee->waiters, shared_from_this());
+}
+
+
+void Goal::waiteeDone(GoalPtr waitee, ExitCode result)
+{
+    assert(waitees.count(waitee));
+    waitees.erase(waitee);
+
+    trace(fmt("waitee '%s' done; %d left", waitee->name, waitees.size()));
+
+    if (result == ecFailed || result == ecNoSubstituters || result == ecIncompleteClosure) ++nrFailed;
+
+    if (result == ecNoSubstituters) ++nrNoSubstituters;
+
+    if (result == ecIncompleteClosure) ++nrIncompleteClosure;
+
+    if (waitees.empty() || (result == ecFailed && !settings.keepGoing)) {
+
+        /* If we failed and keepGoing is not set, we remove all
+           remaining waitees. */
+        for (auto & goal : waitees) {
+            goal->waiters.extract(shared_from_this());
+        }
+        waitees.clear();
+
+        worker.wakeUp(shared_from_this());
+    }
+}
+
+
+void Goal::amDone(ExitCode result, std::optional<Error> ex)
+{
+    trace("done");
+    assert(exitCode == ecBusy);
+    assert(result == ecSuccess || result == ecFailed || result == ecNoSubstituters || result == ecIncompleteClosure);
+    exitCode = result;
+
+    if (ex) {
+        if (!waiters.empty())
+            logError(ex->info());
+        else
+            this->ex = std::move(*ex);
+    }
+
+    for (auto & i : waiters) {
+        GoalPtr goal = i.lock();
+        if (goal) goal->waiteeDone(shared_from_this(), result);
+    }
+    waiters.clear();
+    worker.removeGoal(shared_from_this());
+
+    cleanup();
+}
+
+
+void Goal::trace(std::string_view s)
+{
+    debug("%1%: %2%", name, s);
+}
+
+}