fix: good errors for failures caused by allowSubstitutes

This caused an absolute saga which I would not like anyone else to have
to experience. Let's put in a laser targeted error message that
diagnoses this exact problem.

Fixes: https://git.lix.systems/lix-project/lix/issues/484
Change-Id: I2a79f04aeb4a1b67c10115e5e39501d958836298

2 files changed, 38 insertions, 23 deletions

diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index f31c3acd5..0f082b193 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -261,24 +261,29 @@ Goal::WorkResult DerivationGoal::haveDerivation(bool inBuildSlot)
        through substitutes.  If that doesn't work, we'll build
        them. */
     WaitForGoals result;
-    if (settings.useSubstitutes && parsedDrv->substitutesAllowed())
-        for (auto & [outputName, status] : initialOutputs) {
-            if (!status.wanted) continue;
-            if (!status.known)
-                result.goals.insert(
-                    worker.makeDrvOutputSubstitutionGoal(
-                        DrvOutput{status.outputHash, outputName},
-                        buildMode == bmRepair ? Repair : NoRepair
-                    )
-                );
-            else {
-                auto * cap = getDerivationCA(*drv);
-                result.goals.insert(worker.makePathSubstitutionGoal(
-                    status.known->path,
-                    buildMode == bmRepair ? Repair : NoRepair,
-                    cap ? std::optional { *cap } : std::nullopt));
+    if (settings.useSubstitutes) {
+        if (parsedDrv->substitutesAllowed()) {
+            for (auto & [outputName, status] : initialOutputs) {
+                if (!status.wanted) continue;
+                if (!status.known)
+                    result.goals.insert(
+                        worker.makeDrvOutputSubstitutionGoal(
+                            DrvOutput{status.outputHash, outputName},
+                            buildMode == bmRepair ? Repair : NoRepair
+                        )
+                    );
+                else {
+                    auto * cap = getDerivationCA(*drv);
+                    result.goals.insert(worker.makePathSubstitutionGoal(
+                        status.known->path,
+                        buildMode == bmRepair ? Repair : NoRepair,
+                        cap ? std::optional { *cap } : std::nullopt));
+                }
             }
+        } else {
+            trace("skipping substitute because allowSubstitutes is false");
         }
+    }
 
     if (result.goals.empty()) { /* to prevent hang (no wake-up event) */
         return outputsSubstitutionTried(inBuildSlot);
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 1e3c4109d..dadda4de2 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -436,13 +436,23 @@ std::set<int> LocalDerivationGoal::startBuilder()
     killSandbox(false);
 
     /* Right platform? */
-    if (!parsedDrv->canBuildLocally(worker.store))
-        throw Error("a '%s' with features {%s} is required to build '%s', but I am a '%s' with features {%s}",
-            drv->platform,
-            concatStringsSep(", ", parsedDrv->getRequiredSystemFeatures()),
-            worker.store.printStorePath(drvPath),
-            settings.thisSystem,
-            concatStringsSep<StringSet>(", ", worker.store.systemFeatures));
+    if (!parsedDrv->canBuildLocally(worker.store)) {
+        HintFmt addendum{""};
+        if (settings.useSubstitutes && !parsedDrv->substitutesAllowed()) {
+            addendum = HintFmt("\n\nHint: the failing derivation has %s set to %s, forcing it to be built rather than substituted.\n"
+                "Passing %s to force substitution may resolve this failure if the path is available in a substituter.",
+                "allowSubstitutes", "false", "--always-allow-substitutes");
+        }
+        throw Error({
+            .msg = HintFmt("a '%s' with features {%s} is required to build '%s', but I am a '%s' with features {%s}%s",
+                drv->platform,
+                concatStringsSep(", ", parsedDrv->getRequiredSystemFeatures()),
+                worker.store.printStorePath(drvPath),
+                settings.thisSystem,
+                concatStringsSep<StringSet>(", ", worker.store.systemFeatures),
+                Uncolored(addendum))
+        });
+    }
 
     /* Create a temporary directory where the build will take
        place. */