refactor: Impure derivation type isPure -> isImpure

To quote the method doc:

Non-impure derivations can still behave impurely, to the degree permitted
by the sandbox. Hence why this method isn't `isPure`: impure derivations
are not the negation of pure derivations. Purity can not be ascertained
except by rather heavy tools.

src/libstore/build/derivation-goal.cc

@@ -223,7 +223,7 @@ void DerivationGoal::haveDerivation()
     if (!drv->type().hasKnownOutputPaths())
         experimentalFeatureSettings.require(Xp::CaDerivations);
 
-    if (!drv->type().isPure()) {
+    if (drv->type().isImpure()) {
         experimentalFeatureSettings.require(Xp::ImpureDerivations);
 
         for (auto & [outputName, output] : drv->outputs) {
@@ -304,7 +304,7 @@ void DerivationGoal::outputsSubstitutionTried()
 {
     trace("all outputs substituted (maybe)");
 
-    assert(drv->type().isPure());
+    assert(!drv->type().isImpure());
 
     if (nrFailed > 0 && nrFailed > nrNoSubstituters + nrIncompleteClosure && !settings.tryFallback) {
         done(BuildResult::TransientFailure, {},
@@ -397,9 +397,9 @@ void DerivationGoal::gaveUpOnSubstitution()
         for (const auto & [inputDrvPath, inputNode] : dynamic_cast<Derivation *>(drv.get())->inputDrvs.map) {
             /* Ensure that pure, non-fixed-output derivations don't
                depend on impure derivations. */
-            if (experimentalFeatureSettings.isEnabled(Xp::ImpureDerivations) && drv->type().isPure() && !drv->type().isFixed()) {
+            if (experimentalFeatureSettings.isEnabled(Xp::ImpureDerivations) && !drv->type().isImpure() && !drv->type().isFixed()) {
                 auto inputDrv = worker.evalStore.readDerivation(inputDrvPath);
-                if (!inputDrv.type().isPure())
+                if (inputDrv.type().isImpure())
                     throw Error("pure derivation '%s' depends on impure derivation '%s'",
                         worker.store.printStorePath(drvPath),
                         worker.store.printStorePath(inputDrvPath));
@@ -439,7 +439,7 @@ void DerivationGoal::gaveUpOnSubstitution()
 
 void DerivationGoal::repairClosure()
 {
-    assert(drv->type().isPure());
+    assert(!drv->type().isImpure());
 
     /* If we're repairing, we now know that our own outputs are valid.
        Now check whether the other paths in the outputs closure are
@@ -1100,7 +1100,7 @@ void DerivationGoal::resolvedFinished()
                   worker.store.printStorePath(resolvedDrvGoal->drvPath), outputName);
             }();
 
-            if (drv->type().isPure()) {
+            if (!drv->type().isImpure()) {
                 auto newRealisation = realisation;
                 newRealisation.id = DrvOutput { initialOutput->outputHash, outputName };
                 newRealisation.signatures.clear();
@@ -1395,7 +1395,7 @@ void DerivationGoal::flushLine()
 
 std::map<std::string, std::optional<StorePath>> DerivationGoal::queryPartialDerivationOutputMap()
 {
-    assert(drv->type().isPure());
+    assert(!drv->type().isImpure());
     if (!useDerivation || drv->type().hasKnownOutputPaths()) {
         std::map<std::string, std::optional<StorePath>> res;
         for (auto & [name, output] : drv->outputs)
@@ -1411,7 +1411,7 @@ std::map<std::string, std::optional<StorePath>> DerivationGoal::queryPartialDeri
 
 OutputPathMap DerivationGoal::queryDerivationOutputMap()
 {
-    assert(drv->type().isPure());
+    assert(!drv->type().isImpure());
     if (!useDerivation || drv->type().hasKnownOutputPaths()) {
         OutputPathMap res;
         for (auto & [name, output] : drv->outputsAndOptPaths(worker.store))
@@ -1428,7 +1428,7 @@ OutputPathMap DerivationGoal::queryDerivationOutputMap()
 
 std::pair<bool, SingleDrvOutputs> DerivationGoal::checkPathValidity()
 {
-    if (!drv->type().isPure()) return { false, {} };
+    if (drv->type().isImpure()) return { false, {} };
 
     bool checkHash = buildMode == bmRepair;
     auto wantedOutputsLeft = std::visit(overloaded {

src/libstore/build/local-derivation-goal.cc

@@ -2724,7 +2724,7 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
             .outPath = newInfo.path
         };
         if (experimentalFeatureSettings.isEnabled(Xp::CaDerivations)
-            && drv->type().isPure())
+            && !drv->type().isImpure())
         {
             signRealisation(thisRealisation);
             worker.store.registerDrvOutput(thisRealisation);

src/libstore/derivations.cc

@@ -110,17 +110,17 @@ bool DerivationType::isSandboxed() const
 }
 
 
-bool DerivationType::isPure() const
+bool DerivationType::isImpure() const
 {
     return std::visit(overloaded {
         [](const InputAddressed & ia) {
-            return true;
+            return false;
         },
         [](const ContentAddressed & ca) {
-            return true;
+            return false;
         },
         [](const Impure &) {
-            return false;
+            return true;
         },
     }, raw);
 }
@@ -840,7 +840,7 @@ DrvHash hashDerivationModulo(Store & store, const Derivation & drv, bool maskOut
         };
     }
 
-    if (!type.isPure()) {
+    if (type.isImpure()) {
         std::map<std::string, Hash> outputHashes;
         for (const auto & [outputName, _] : drv.outputs)
             outputHashes.insert_or_assign(outputName, impureOutputHash);

src/libstore/derivations.hh

@@ -253,12 +253,17 @@ struct DerivationType {
     bool isSandboxed() const;
 
     /**
-     * Whether the derivation is expected to produce the same result
+     * Whether the derivation is expected to produce a different result
-     * every time, and therefore it only needs to be built once. This is
+     * every time, and therefore it needs to be rebuilt every time. This is
-     * only false for derivations that have the attribute '__impure =
+     * only true for derivations that have the attribute '__impure =
      * true'.
+     *
+     * Non-impure derivations can still behave impurely, to the degree permitted
+     * by the sandbox. Hence why this method isn't `isPure`: impure derivations
+     * are not the negation of pure derivations. Purity can not be ascertained
+     * except by rather heavy tools.
      */
-    bool isPure() const;
+    bool isImpure() const;
 
     /**
      * Does the derivation knows its own output paths?