Make the Derived Path family of types inductive for dynamic derivations

We want to be able to write down `foo.drv^bar.drv^baz`:
`foo.drv^bar.drv` is the dynamic derivation (since it is itself a
derivation output, `bar.drv` from `foo.drv`).

To that end, we create `Single{Derivation,BuiltPath}` types, that are
very similar except instead of having multiple outputs (in a set or
map), they have a single one. This is for everything to the left of the
rightmost `^`.

`NixStringContextElem` has an analogous change, and now can reuse
`SingleDerivedPath` at the top level. In fact, if we ever get rid of
`DrvDeep`, `NixStringContextElem` could be replaced with
`SingleDerivedPath` entirely!

Important note: some JSON formats have changed.

We already can *produce* dynamic derivations, but we can't refer to them
directly. Today, we can merely express building or example at the top
imperatively over time by building `foo.drv^bar.drv`, and then with a
second nix invocation doing `<result-from-first>^baz`, but this is not
declarative. The ethos of Nix of being able to write down the full plan
everything you want to do, and then execute than plan with a single
command, and for that we need the new inductive form of these types.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libstore/derived-path.cc

@@ -7,52 +7,133 @@
 
 namespace nix {
 
-nlohmann::json DerivedPath::Opaque::toJSON(ref<Store> store) const {
+#define CMP_ONE(CHILD_TYPE, MY_TYPE, FIELD, COMPARATOR) \
+    bool MY_TYPE ::operator COMPARATOR (const MY_TYPE & other) const \
+    { \
+        const MY_TYPE* me = this; \
+        auto fields1 = std::make_tuple<const CHILD_TYPE &, const FIELD_TYPE &>(*me->drvPath, me->FIELD); \
+        me = &other; \
+        auto fields2 = std::make_tuple<const CHILD_TYPE &, const FIELD_TYPE &>(*me->drvPath, me->FIELD); \
+        return fields1 COMPARATOR fields2; \
+    }
+#define CMP(CHILD_TYPE, MY_TYPE, FIELD) \
+    CMP_ONE(CHILD_TYPE, MY_TYPE, FIELD, ==) \
+    CMP_ONE(CHILD_TYPE, MY_TYPE, FIELD, !=) \
+    CMP_ONE(CHILD_TYPE, MY_TYPE, FIELD, <)
+
+#define FIELD_TYPE std::string
+CMP(SingleDerivedPath, SingleDerivedPathBuilt, output)
+#undef FIELD_TYPE
+
+#define FIELD_TYPE OutputsSpec
+CMP(SingleDerivedPath, DerivedPathBuilt, outputs)
+#undef FIELD_TYPE
+
+#undef CMP
+#undef CMP_ONE
+
+nlohmann::json DerivedPath::Opaque::toJSON(const Store & store) const
+{
+    return store.printStorePath(path);
+}
+
+nlohmann::json SingleDerivedPath::Built::toJSON(Store & store) const {
     nlohmann::json res;
-    res["path"] = store->printStorePath(path);
+    res["drvPath"] = drvPath->toJSON(store);
+    // Fallback for the input-addressed derivation case: We expect to always be
+    // able to print the output paths, so let’s do it
+    // FIXME try-resolve on drvPath
+    const auto outputMap = store.queryPartialDerivationOutputMap(resolveDerivedPath(store, *drvPath));
+    res["output"] = output;
+    auto outputPathIter = outputMap.find(output);
+    if (outputPathIter == outputMap.end())
+        res["outputPath"] = nullptr;
+    else if (std::optional p = outputPathIter->second)
+        res["outputPath"] = store.printStorePath(*p);
+    else
+        res["outputPath"] = nullptr;
     return res;
 }
 
-nlohmann::json DerivedPath::Built::toJSON(ref<Store> store) const {
+nlohmann::json DerivedPath::Built::toJSON(Store & store) const {
     nlohmann::json res;
-    res["drvPath"] = store->printStorePath(drvPath);
+    res["drvPath"] = drvPath->toJSON(store);
     // Fallback for the input-addressed derivation case: We expect to always be
     // able to print the output paths, so let’s do it
-    const auto outputMap = store->queryPartialDerivationOutputMap(drvPath);
+    // FIXME try-resolve on drvPath
+    const auto outputMap = store.queryPartialDerivationOutputMap(resolveDerivedPath(store, *drvPath));
     for (const auto & [output, outputPathOpt] : outputMap) {
         if (!outputs.contains(output)) continue;
         if (outputPathOpt)
-            res["outputs"][output] = store->printStorePath(*outputPathOpt);
+            res["outputs"][output] = store.printStorePath(*outputPathOpt);
         else
             res["outputs"][output] = nullptr;
     }
     return res;
 }
 
+nlohmann::json SingleDerivedPath::toJSON(Store & store) const
+{
+    return std::visit([&](const auto & buildable) {
+        return buildable.toJSON(store);
+    }, raw());
+}
+
+nlohmann::json DerivedPath::toJSON(Store & store) const
+{
+    return std::visit([&](const auto & buildable) {
+        return buildable.toJSON(store);
+    }, raw());
+}
+
 std::string DerivedPath::Opaque::to_string(const Store & store) const
 {
     return store.printStorePath(path);
 }
 
+std::string SingleDerivedPath::Built::to_string(const Store & store) const
+{
+    return drvPath->to_string(store) + "^" + output;
+}
+
+std::string SingleDerivedPath::Built::to_string_legacy(const Store & store) const
+{
+    return drvPath->to_string(store) + "!" + output;
+}
+
 std::string DerivedPath::Built::to_string(const Store & store) const
 {
-    return store.printStorePath(drvPath)
+    return drvPath->to_string(store)
         + '^'
         + outputs.to_string();
 }
 
 std::string DerivedPath::Built::to_string_legacy(const Store & store) const
 {
-    return store.printStorePath(drvPath)
-        + '!'
+    return drvPath->to_string_legacy(store)
+        + "!"
         + outputs.to_string();
 }
 
+std::string SingleDerivedPath::to_string(const Store & store) const
+{
+    return std::visit(
+        [&](const auto & req) { return req.to_string(store); },
+        raw());
+}
+
 std::string DerivedPath::to_string(const Store & store) const
+{
+    return std::visit(
+        [&](const auto & req) { return req.to_string(store); },
+        raw());
+}
+
+std::string SingleDerivedPath::to_string_legacy(const Store & store) const
 {
     return std::visit(overloaded {
-        [&](const DerivedPath::Built & req) { return req.to_string(store); },
-        [&](const DerivedPath::Opaque & req) { return req.to_string(store); },
+        [&](const SingleDerivedPath::Built & req) { return req.to_string_legacy(store); },
+        [&](const SingleDerivedPath::Opaque & req) { return req.to_string(store); },
     }, this->raw());
 }
 
@@ -70,30 +151,156 @@ DerivedPath::Opaque DerivedPath::Opaque::parse(const Store & store, std::string_
     return {store.parseStorePath(s)};
 }
 
-DerivedPath::Built DerivedPath::Built::parse(const Store & store, std::string_view drvS, std::string_view outputsS)
+void drvRequireExperiment(
+    const SingleDerivedPath & drv,
+    const ExperimentalFeatureSettings & xpSettings)
 {
+    std::visit(overloaded {
+        [&](const SingleDerivedPath::Opaque &) {
+            // plain drv path; no experimental features required.
+        },
+        [&](const SingleDerivedPath::Built &) {
+            xpSettings.require(Xp::DynamicDerivations);
+        },
+    }, drv.raw());
+}
+
+SingleDerivedPath::Built SingleDerivedPath::Built::parse(
+    const Store & store, ref<SingleDerivedPath> drv,
+    std::string_view output,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    drvRequireExperiment(*drv, xpSettings);
     return {
-        .drvPath = store.parseStorePath(drvS),
+        .drvPath = drv,
+        .output = std::string { output },
+    };
+}
+
+DerivedPath::Built DerivedPath::Built::parse(
+    const Store & store, ref<SingleDerivedPath> drv,
+    std::string_view outputsS,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    drvRequireExperiment(*drv, xpSettings);
+    return {
+        .drvPath = drv,
         .outputs = OutputsSpec::parse(outputsS),
     };
 }
 
-static inline DerivedPath parseWith(const Store & store, std::string_view s, std::string_view separator)
+static SingleDerivedPath parseWithSingle(
+    const Store & store, std::string_view s, std::string_view separator,
+    const ExperimentalFeatureSettings & xpSettings)
 {
-    size_t n = s.find(separator);
+    size_t n = s.rfind(separator);
+    return n == s.npos
+        ? (SingleDerivedPath) SingleDerivedPath::Opaque::parse(store, s)
+        : (SingleDerivedPath) SingleDerivedPath::Built::parse(store,
+            make_ref<SingleDerivedPath>(parseWithSingle(
+                store,
+                s.substr(0, n),
+                separator,
+                xpSettings)),
+            s.substr(n + 1),
+            xpSettings);
+}
+
+SingleDerivedPath SingleDerivedPath::parse(
+    const Store & store,
+    std::string_view s,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    return parseWithSingle(store, s, "^", xpSettings);
+}
+
+SingleDerivedPath SingleDerivedPath::parseLegacy(
+    const Store & store,
+    std::string_view s,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    return parseWithSingle(store, s, "!", xpSettings);
+}
+
+static DerivedPath parseWith(
+    const Store & store, std::string_view s, std::string_view separator,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    size_t n = s.rfind(separator);
     return n == s.npos
         ? (DerivedPath) DerivedPath::Opaque::parse(store, s)
-        : (DerivedPath) DerivedPath::Built::parse(store, s.substr(0, n), s.substr(n + 1));
+        : (DerivedPath) DerivedPath::Built::parse(store,
+            make_ref<SingleDerivedPath>(parseWithSingle(
+                store,
+                s.substr(0, n),
+                separator,
+                xpSettings)),
+            s.substr(n + 1),
+            xpSettings);
 }
 
-DerivedPath DerivedPath::parse(const Store & store, std::string_view s)
+DerivedPath DerivedPath::parse(
+    const Store & store,
+    std::string_view s,
+    const ExperimentalFeatureSettings & xpSettings)
 {
-	return parseWith(store, s, "^");
+    return parseWith(store, s, "^", xpSettings);
 }
 
-DerivedPath DerivedPath::parseLegacy(const Store & store, std::string_view s)
+DerivedPath DerivedPath::parseLegacy(
+    const Store & store,
+    std::string_view s,
+    const ExperimentalFeatureSettings & xpSettings)
 {
-	return parseWith(store, s, "!");
+    return parseWith(store, s, "!", xpSettings);
+}
+
+DerivedPath DerivedPath::fromSingle(const SingleDerivedPath & req)
+{
+    return std::visit(overloaded {
+        [&](const SingleDerivedPath::Opaque & o) -> DerivedPath {
+            return o;
+        },
+        [&](const SingleDerivedPath::Built & b) -> DerivedPath {
+            return DerivedPath::Built {
+                .drvPath = b.drvPath,
+                .outputs = OutputsSpec::Names { b.output },
+            };
+        },
+    }, req.raw());
+}
+
+const StorePath & SingleDerivedPath::Built::getBaseStorePath() const
+{
+	return drvPath->getBaseStorePath();
+}
+
+const StorePath & DerivedPath::Built::getBaseStorePath() const
+{
+	return drvPath->getBaseStorePath();
+}
+
+template<typename DP>
+static inline const StorePath & getBaseStorePath_(const DP & derivedPath)
+{
+    return std::visit(overloaded {
+        [&](const typename DP::Built & bfd) -> auto & {
+            return bfd.drvPath->getBaseStorePath();
+        },
+        [&](const typename DP::Opaque & bo) -> auto & {
+            return bo.path;
+        },
+    }, derivedPath.raw());
+}
+
+const StorePath & SingleDerivedPath::getBaseStorePath() const
+{
+	return getBaseStorePath_(*this);
+}
+
+const StorePath & DerivedPath::getBaseStorePath() const
+{
+	return getBaseStorePath_(*this);
 }
 
 }