libstore: turn HookReply into a variant type

we'll need this once we want to pass extra information out of accepting
replies, such as fd sets or possibly even async output reader promises.

Change-Id: I5e2f18cdb80b0d2faf3067703cc18bd263329b3f

src/libstore/build/derivation-goal.cc

@@ -732,15 +732,18 @@ Goal::WorkResult DerivationGoal::tryToBuild(bool inBuildSlot)
         && settings.maxBuildJobs.get() != 0;
 
     if (!buildLocally) {
-        switch (tryBuildHook(inBuildSlot)) {
-            case rpAccept:
+        auto hookReply = tryBuildHook(inBuildSlot);
+        auto result = std::visit(
+            overloaded{
+                [&](HookReply::Accept) -> std::optional<WorkResult> {
                     /* Yes, it has started doing so.  Wait until we get
                        EOF from the hook. */
                     actLock.reset();
                     buildResult.startTime = time(0); // inexact
                     state = &DerivationGoal::buildDone;
                     return started();
-            case rpPostpone:
+                },
+                [&](HookReply::Postpone) -> std::optional<WorkResult> {
                     /* Not now; wait until at least one child finishes or
                        the wake-up timeout expires. */
                     if (!actLock)
@@ -748,9 +751,15 @@ Goal::WorkResult DerivationGoal::tryToBuild(bool inBuildSlot)
                             fmt("waiting for a machine to build '%s'", Magenta(worker.store.printStorePath(drvPath))));
                     outputLocks.unlock();
                     return WaitForAWhile{};
-            case rpDecline:
+                },
+                [&](HookReply::Decline) -> std::optional<WorkResult> {
                     /* We should do it ourselves. */
-                break;
+                    return std::nullopt;
+                },
+            },
+            hookReply);
+        if (result) {
+            return std::move(*result);
         }
     }
 
@@ -1113,7 +1122,7 @@ Goal::WorkResult DerivationGoal::resolvedFinished(bool inBuildSlot)
 
 HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
 {
-    if (!worker.hook.available || !useDerivation) return rpDecline;
+    if (!worker.hook.available || !useDerivation) return HookReply::Decline{};
 
     if (!worker.hook.instance)
         worker.hook.instance = std::make_unique<HookInstance>();
@@ -1156,14 +1165,14 @@ HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
         debug("hook reply is '%1%'", reply);
 
         if (reply == "decline")
-            return rpDecline;
+            return HookReply::Decline{};
         else if (reply == "decline-permanently") {
             worker.hook.available = false;
             worker.hook.instance.reset();
-            return rpDecline;
+            return HookReply::Decline{};
         }
         else if (reply == "postpone")
-            return rpPostpone;
+            return HookReply::Postpone{};
         else if (reply != "accept")
             throw Error("bad hook reply '%s'", reply);
 
@@ -1173,7 +1182,7 @@ HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
                 "build hook died unexpectedly: %s",
                 chomp(drainFD(worker.hook.instance->fromHook.get())));
             worker.hook.instance.reset();
-            return rpDecline;
+            return HookReply::Decline{};
         } else
             throw;
     }
@@ -1215,7 +1224,7 @@ HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
     builderOutFD = &hook->builderOut;
     worker.childStarted(shared_from_this(), fds, false);
 
-    return rpAccept;
+    return HookReply::Accept{};
 }
 
 

src/libstore/build/derivation-goal.hh

@@ -14,7 +14,19 @@ using std::map;
 
 struct HookInstance;
 
-typedef enum {rpAccept, rpDecline, rpPostpone} HookReply;
+struct HookReplyBase {
+    struct [[nodiscard]] Accept {};
+    struct [[nodiscard]] Decline {};
+    struct [[nodiscard]] Postpone {};
+};
+
+struct [[nodiscard]] HookReply
+    : HookReplyBase,
+      std::variant<HookReplyBase::Accept, HookReplyBase::Decline, HookReplyBase::Postpone>
+{
+    HookReply() = delete;
+    using variant::variant;
+};
 
 /**
  * Unless we are repairing, we don't both to test validity and just assume it,