Make `DerivationGoal::drv` a full Derivation

This field used to be a `BasicDerivation`, but this `BasicDerivation`
was downcasted to a `Derivation` when needed (implicitely or not), so we
might as well make it a full `Derivation` and upcast it when needed.

This also allows getting rid of a weird duplication in the way we
compute the static output hashes for the derivation. We had to
do it differently and in a different place depending on whether the
derivation was a full derivation or just a basic drv, but we can now do
it unconditionally on the full derivation.

Fix #4559

src/libstore/build/derivation-goal.cc

@@ -123,17 +123,7 @@ DerivationGoal::DerivationGoal(const StorePath & drvPath, const BasicDerivation
     , wantedOutputs(wantedOutputs)
     , buildMode(buildMode)
 {
-    this->drv = std::make_unique<BasicDerivation>(BasicDerivation(drv));
-
-    auto outputHashes = staticOutputHashes(worker.store, drv);
-    for (auto &[outputName, outputHash] : outputHashes)
-      initialOutputs.insert({
-            outputName,
-            InitialOutput{
-                .wanted = true, // Will be refined later
-                .outputHash = outputHash
-            }
-          });
+    this->drv = std::make_unique<Derivation>(drv);
 
     state = &DerivationGoal::haveDerivation;
     name = fmt(
@@ -271,18 +261,8 @@ void DerivationGoal::loadDerivation()
 
     auto fullDrv = new Derivation(worker.store.derivationFromPath(drvPath));
 
-    auto outputHashes = staticOutputHashes(worker.store, *fullDrv);
-    for (auto &[outputName, outputHash] : outputHashes)
-      initialOutputs.insert({
-            outputName,
-            InitialOutput{
-                .wanted = true, // Will be refined later
-                .outputHash = outputHash
-            }
-          });
-
     /* Get the derivation. */
-    drv = std::unique_ptr<BasicDerivation>(fullDrv);
+    drv = std::unique_ptr<Derivation>(fullDrv);
 
     haveDerivation();
 }
@@ -301,6 +281,16 @@ void DerivationGoal::haveDerivation()
         if (i.second.second)
             worker.store.addTempRoot(*i.second.second);
 
+    auto outputHashes = staticOutputHashes(worker.store, *drv);
+    for (auto &[outputName, outputHash] : outputHashes)
+      initialOutputs.insert({
+            outputName,
+            InitialOutput{
+                .wanted = true, // Will be refined later
+                .outputHash = outputHash
+            }
+          });
+
     /* Check what outputs paths are not already valid. */
     checkPathValidity();
     bool allValid = true;
@@ -3517,10 +3507,9 @@ void DerivationGoal::registerOutputs()
        but it's fine to do in all cases. */
 
     if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
-        auto outputHashes = staticOutputHashes(worker.store, *drv);
         for (auto& [outputName, newInfo] : infos)
             worker.store.registerDrvOutput(Realisation{
-                .id = DrvOutput{outputHashes.at(outputName), outputName},
+                .id = DrvOutput{initialOutputs.at(outputName).outputHash, outputName},
                 .outPath = newInfo.path});
     }
 }

src/libstore/build/derivation-goal.hh

@@ -64,7 +64,7 @@ struct DerivationGoal : public Goal
     bool retrySubstitution;
 
     /* The derivation stored at drvPath. */
-    std::unique_ptr<BasicDerivation> drv;
+    std::unique_ptr<Derivation> drv;
 
     std::unique_ptr<ParsedDerivation> parsedDrv;