Resolve the program path in nix run

Fix #4768

src/nix/app.cc

@@ -3,9 +3,61 @@
 #include "eval-inline.hh"
 #include "eval-cache.hh"
 #include "names.hh"
+#include "command.hh"
 
 namespace nix {
 
+struct InstallableDerivedPath : Installable
+{
+    ref<Store> store;
+    const DerivedPath derivedPath;
+
+    InstallableDerivedPath(ref<Store> store, const DerivedPath & derivedPath)
+        : store(store)
+        , derivedPath(derivedPath)
+    {
+    }
+
+
+    std::string what() override { return derivedPath.to_string(*store); }
+
+    DerivedPaths toDerivedPaths() override
+    {
+        return {derivedPath};
+    }
+
+    std::optional<StorePath> getStorePath() override
+    {
+        return std::nullopt;
+    }
+};
+
+/**
+ * Return the rewrites that are needed to resolve a string whose context is
+ * included in `dependencies`
+ */
+StringPairs resolveRewrites(Store & store, const BuiltPaths dependencies)
+{
+    StringPairs res;
+    for (auto & dep : dependencies)
+        if (auto drvDep = std::get_if<BuiltPathBuilt>(&dep))
+            for (auto & [ outputName, outputPath ] : drvDep->outputs)
+                res.emplace(
+                    downstreamPlaceholder(store, drvDep->drvPath, outputName),
+                    store.printStorePath(outputPath)
+                );
+    return res;
+}
+
+/**
+ * Resolve the given string assuming the given context
+ */
+std::string resolveString(Store & store, const std::string & toResolve, const BuiltPaths dependencies)
+{
+    auto rewrites = resolveRewrites(store, dependencies);
+    return rewriteStrings(toResolve, rewrites);
+}
+
 App Installable::toApp(EvalState & state)
 {
     auto [cursor, attrPath] = getCursor(state);
@@ -18,19 +70,21 @@ App Installable::toApp(EvalState & state)
             throw Error("app program '%s' is not in the Nix store", program);
     };
 
+    std::vector<std::shared_ptr<Installable>> context;
+    std::string unresolvedProgram;
+
+
     if (type == "app") {
-        auto [program, context] = cursor->getAttr("program")->getStringWithContext();
+        auto [program, context_] = cursor->getAttr("program")->getStringWithContext();
+        unresolvedProgram = program;
 
-        checkProgram(program);
+        for (auto & [path, name] : context_)
-
+            context.push_back(std::make_shared<InstallableDerivedPath>(
-        std::vector<StorePathWithOutputs> context2;
+                state.store,
-        for (auto & [path, name] : context)
+                DerivedPathBuilt{
-            context2.push_back({state.store->parseStorePath(path), {name}});
+                    .drvPath = state.store->parseStorePath(path),
-
+                    .outputs = {name},
-        return App {
+                }));
-            .context = std::move(context2),
-            .program = program,
-        };
     }
 
     else if (type == "derivation") {
@@ -44,16 +98,24 @@ App Installable::toApp(EvalState & state)
             aMainProgram
             ? aMainProgram->getString()
             : DrvName(name).name;
-        auto program = outPath + "/bin/" + mainProgram;
+        unresolvedProgram = outPath + "/bin/" + mainProgram;
-        checkProgram(program);
+        context = {std::make_shared<InstallableDerivedPath>(
-        return App {
+            state.store,
-            .context = { { drvPath, {outputName} } },
+            DerivedPathBuilt{
-            .program = program,
+                .drvPath = drvPath,
-        };
+                .outputs = {outputName},
+            })};
     }
 
     else
         throw Error("attribute '%s' has unsupported type '%s'", attrPath, type);
+
+    auto builtContext = build(state.store, Realise::Outputs, context);
+    auto program = resolveString(*state.store, unresolvedProgram, builtContext);
+    checkProgram(program);
+    return App {
+        .program = program,
+    };
 }
 
 }