Feature gate DownstreamPlaceholder::unknownCaOutput

This is a part of CA derivations that we forgot to put behind the
experimental feature.

This was caught by @fricklerhandwerk in
https://github.com/NixOS/nix/pull/8369#discussion_r1258133719

src/libexpr/eval.cc

@@ -1068,14 +1068,15 @@ void EvalState::mkOutputString(
     Value & value,
     const StorePath & drvPath,
     const std::string outputName,
-    std::optional<StorePath> optOutputPath)
+    std::optional<StorePath> optOutputPath,
+    const ExperimentalFeatureSettings & xpSettings)
 {
     value.mkString(
         optOutputPath
             ? store->printStorePath(*std::move(optOutputPath))
             /* Downstream we would substitute this for an actual path once
                we build the floating CA derivation */
-            : DownstreamPlaceholder::unknownCaOutput(drvPath, outputName).render(),
+            : DownstreamPlaceholder::unknownCaOutput(drvPath, outputName, xpSettings).render(),
         NixStringContext {
             NixStringContextElem::Built {
                 .drvPath = drvPath,

src/libexpr/eval.hh

@@ -689,12 +689,15 @@ public:
      * be passed if and only if output store object is input-addressed.
      * Will be printed to form string if passed, otherwise a placeholder
      * will be used (see `DownstreamPlaceholder`).
+     *
+     * @param xpSettings Stop-gap to avoid globals during unit tests.
      */
     void mkOutputString(
         Value & value,
         const StorePath & drvPath,
         const std::string outputName,
-        std::optional<StorePath> optOutputPath);
+        std::optional<StorePath> optOutputPath,
+        const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 
     void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx);
 

src/libexpr/primops.cc

@@ -83,22 +83,21 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
     for (auto & d : drvs) buildReqs.emplace_back(DerivedPath { d });
     store->buildPaths(buildReqs);
 
-    /* Get all the output paths corresponding to the placeholders we had */
     for (auto & drv : drvs) {
         auto outputs = resolveDerivedPath(*store, drv);
         for (auto & [outputName, outputPath] : outputs) {
-            res.insert_or_assign(
-                DownstreamPlaceholder::unknownCaOutput(drv.drvPath, outputName).render(),
-                store->printStorePath(outputPath)
-            );
-        }
-    }
-
-    /* Add the output of this derivations to the allowed
-       paths. */
-    if (allowedPaths) {
-        for (auto & [_placeholder, outputPath] : res) {
-            allowPath(store->toRealPath(outputPath));
+            /* Add the output of this derivations to the allowed
+               paths. */
+            if (allowedPaths) {
+                allowPath(outputPath);
+            }
+            /* Get all the output paths corresponding to the placeholders we had */
+            if (experimentalFeatureSettings.isEnabled(Xp::CaDerivations)) {
+                res.insert_or_assign(
+                    DownstreamPlaceholder::unknownCaOutput(drv.drvPath, outputName).render(),
+                    store->printStorePath(outputPath)
+                );
+            }
         }
     }
 

src/libexpr/tests/derived-path.cc

@@ -37,8 +37,15 @@ RC_GTEST_FIXTURE_PROP(
     prop_built_path_placeholder_round_trip,
     (const StorePath & drvPath, const StorePathName & outputName))
 {
+    /**
+     * We set these in tests rather than the regular globals so we don't have
+     * to worry about race conditions if the tests run concurrently.
+     */
+    ExperimentalFeatureSettings mockXpSettings;
+    mockXpSettings.set("experimental-features", "ca-derivations");
+
     auto * v = state.allocValue();
-    state.mkOutputString(*v, drvPath, outputName.name, std::nullopt);
+    state.mkOutputString(*v, drvPath, outputName.name, std::nullopt, mockXpSettings);
     auto [d, _] = state.coerceToDerivedPathUnchecked(noPos, *v, "");
     DerivedPath::Built b {
         .drvPath = drvPath,