Fix nix build --dry-run with CA derivations

Don’t try and assume that we know the output paths when we’ve just built
with `--dry-run`. Instead make `--dry-run` follow a different code path
that won’t assume the knowledge of the output paths at all.

Fix #6275

src/libstore/derived-path.cc

@@ -11,6 +11,21 @@ nlohmann::json DerivedPath::Opaque::toJSON(ref<Store> store) const {
     return res;
 }
 
+nlohmann::json DerivedPath::Built::toJSON(ref<Store> store) const {
+    nlohmann::json res;
+    res["drvPath"] = store->printStorePath(drvPath);
+    // Fallback for the input-addressed derivation case: We expect to always be
+    // able to print the output paths, so let’s do it
+    auto knownOutputs = store->queryPartialDerivationOutputMap(drvPath);
+    for (const auto& output : outputs) {
+        if (knownOutputs.at(output))
+            res["outputs"][output] = store->printStorePath(knownOutputs.at(output).value());
+        else
+            res["outputs"][output] = nullptr;
+    }
+    return res;
+}
+
 nlohmann::json BuiltPath::Built::toJSON(ref<Store> store) const {
     nlohmann::json res;
     res["drvPath"] = store->printStorePath(drvPath);
@@ -35,16 +50,22 @@ StorePathSet BuiltPath::outPaths() const
     );
 }
 
-nlohmann::json derivedPathsWithHintsToJSON(const BuiltPaths & buildables, ref<Store> store) {
+template<typename T>
+nlohmann::json stuffToJSON(const std::vector<T> & ts, ref<Store> store) {
     auto res = nlohmann::json::array();
-    for (const BuiltPath & buildable : buildables) {
-        std::visit([&res, store](const auto & buildable) {
-            res.push_back(buildable.toJSON(store));
-        }, buildable.raw());
+    for (const T & t : ts) {
+        std::visit([&res, store](const auto & t) {
+            res.push_back(t.toJSON(store));
+        }, t.raw());
     }
     return res;
 }
 
+nlohmann::json derivedPathsWithHintsToJSON(const BuiltPaths & buildables, ref<Store> store)
+{ return stuffToJSON<BuiltPath>(buildables, store); }
+nlohmann::json derivedPathsToJSON(const DerivedPaths & paths, ref<Store> store)
+{ return stuffToJSON<DerivedPath>(paths, store); }
+
 
 std::string DerivedPath::Opaque::to_string(const Store & store) const {
     return store.printStorePath(path);

src/libstore/derived-path.hh

@@ -45,6 +45,7 @@ struct DerivedPathBuilt {
 
     std::string to_string(const Store & store) const;
     static DerivedPathBuilt parse(const Store & store, std::string_view);
+    nlohmann::json toJSON(ref<Store> store) const;
 };
 
 using _DerivedPathRaw = std::variant<
@@ -119,5 +120,6 @@ typedef std::vector<DerivedPath> DerivedPaths;
 typedef std::vector<BuiltPath> BuiltPaths;
 
 nlohmann::json derivedPathsWithHintsToJSON(const BuiltPaths & buildables, ref<Store> store);
+nlohmann::json derivedPathsToJSON(const DerivedPaths & , ref<Store> store);
 
 }

src/nix/build.cc

@@ -52,15 +52,26 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
 
     void run(ref<Store> store) override
     {
+        if (dryRun) {
+            std::vector<DerivedPath> pathsToBuild;
+
+            for (auto & i : installables) {
+                auto b = i->toDerivedPaths();
+                pathsToBuild.insert(pathsToBuild.end(), b.begin(), b.end());
+            }
+            printMissing(store, pathsToBuild, lvlError);
+            if (json)
+                logger->cout("%s", derivedPathsToJSON(pathsToBuild, store).dump());
+            return;
+        }
+
         auto buildables = Installable::build(
             getEvalStore(), store,
-            dryRun ? Realise::Derivation : Realise::Outputs,
+            Realise::Outputs,
             installables, buildMode);
 
         if (json) logger->cout("%s", derivedPathsWithHintsToJSON(buildables, store).dump());
 
-        if (dryRun) return;
-
         if (outLink != "")
             if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())
                 for (const auto & [_i, buildable] : enumerate(buildables)) {

tests/build-dry.sh

@@ -50,3 +50,22 @@ nix build -f dependencies.nix -o $RESULT --dry-run
 nix build -f dependencies.nix -o $RESULT
 
 [[ -h $RESULT ]]
+
+###################################################
+# Check the JSON output
+clearStore
+clearCache
+
+RES=$(nix build -f dependencies.nix --dry-run --json)
+
+if [[ -z "$NIX_TESTS_CA_BY_DEFAULT" ]]; then
+    echo "$RES" | jq '.[0] | [
+        (.drvPath | test("'$NIX_STORE_DIR'.*\\.drv")),
+        (.outputs.out | test("'$NIX_STORE_DIR'"))
+    ] | all'
+else
+    echo "$RES" | jq '.[0] | [
+        (.drvPath | test("'$NIX_STORE_DIR'.*\\.drv")),
+        .outputs.out == null
+    ] | all'
+fi

tests/ca/build-dry.sh

@@ -0,0 +1,6 @@
+source common.sh
+
+export NIX_TESTS_CA_BY_DEFAULT=1
+
+cd .. && source build-dry.sh
+