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>

1 files changed, 33 insertions, 3 deletions

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 9d593a01f..4c7d134ec 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -515,6 +515,30 @@ ref<Installable> SourceExprCommand::parseInstallable(
     return installables.front();
 }
 
+static SingleBuiltPath getBuiltPath(ref<Store> evalStore, ref<Store> store, const SingleDerivedPath & b)
+{
+    return std::visit(
+        overloaded{
+            [&](const SingleDerivedPath::Opaque & bo) -> SingleBuiltPath {
+                return SingleBuiltPath::Opaque { bo.path };
+            },
+            [&](const SingleDerivedPath::Built & bfd) -> SingleBuiltPath {
+                auto drvPath = getBuiltPath(evalStore, store, *bfd.drvPath);
+                // Resolving this instead of `bfd` will yield the same result, but avoid duplicative work.
+                SingleDerivedPath::Built truncatedBfd {
+                    .drvPath = makeConstantStorePathRef(drvPath.outPath()),
+                    .output = bfd.output,
+                };
+                auto outputPath = resolveDerivedPath(*store, truncatedBfd, &*evalStore);
+                return SingleBuiltPath::Built {
+                    .drvPath = make_ref<SingleBuiltPath>(std::move(drvPath)),
+                    .output = { bfd.output, outputPath },
+                };
+            },
+        },
+        b.raw());
+}
+
 std::vector<BuiltPathWithResult> Installable::build(
     ref<Store> evalStore,
     ref<Store> store,
@@ -568,7 +592,10 @@ std::vector<std::pair<ref<Installable>, BuiltPathWithResult>> Installable::build
                     [&](const DerivedPath::Built & bfd) {
                         auto outputs = resolveDerivedPath(*store, bfd, &*evalStore);
                         res.push_back({aux.installable, {
-                            .path = BuiltPath::Built { bfd.drvPath, outputs },
+                            .path = BuiltPath::Built {
+                                .drvPath = make_ref<SingleBuiltPath>(getBuiltPath(evalStore, store, *bfd.drvPath)),
+                                .outputs = outputs,
+                             },
                             .info = aux.info}});
                     },
                     [&](const DerivedPath::Opaque & bo) {
@@ -597,7 +624,10 @@ std::vector<std::pair<ref<Installable>, BuiltPathWithResult>> Installable::build
                         for (auto & [outputName, realisation] : buildResult.builtOutputs)
                             outputs.emplace(outputName, realisation.outPath);
                         res.push_back({aux.installable, {
-                            .path = BuiltPath::Built { bfd.drvPath, outputs },
+                            .path = BuiltPath::Built {
+                                .drvPath = make_ref<SingleBuiltPath>(getBuiltPath(evalStore, store, *bfd.drvPath)),
+                                .outputs = outputs,
+                            },
                             .info = aux.info,
                             .result = buildResult}});
                     },
@@ -691,7 +721,7 @@ StorePathSet Installable::toDerivations(
                         : throw Error("argument '%s' did not evaluate to a derivation", i->what()));
                 },
                 [&](const DerivedPath::Built & bfd) {
-                    drvPaths.insert(bfd.drvPath);
+                    drvPaths.insert(resolveDerivedPath(*store, *bfd.drvPath));
                 },
             }, b.path.raw());