Deduplicate "export reference graph" logic a bit

The first part on `drvOptions.exportReferencesGraph` is the same in both
cases. It is just how the information is finally rendered that is
different.

src/libstore/derivation-options.cc

@@ -256,6 +256,24 @@ DerivationOptions::fromStructuredAttrs(const StringMap & env, const StructuredAt
     };
 }
 
+std::map<std::string, StorePathSet>
+DerivationOptions::getParsedExportReferencesGraph(const StoreDirConfig & store) const
+{
+    std::map<std::string, StorePathSet> res;
+
+    for (auto & [fileName, ss] : exportReferencesGraph) {
+        StorePathSet storePaths;
+        for (auto & storePathS : ss) {
+            if (!store.isInStore(storePathS))
+                throw BuildError("'exportReferencesGraph' contains a non-store path '%1%'", storePathS);
+            storePaths.insert(store.toStorePath(storePathS).first);
+        }
+        res.insert_or_assign(fileName, storePaths);
+    }
+
+    return res;
+}
+
 StringSet DerivationOptions::getRequiredSystemFeatures(const BasicDerivation & drv) const
 {
     // FIXME: cache this?

src/libstore/include/nix/store/derivation-options.hh

@@ -8,10 +8,12 @@
 
 #include "nix/util/types.hh"
 #include "nix/util/json-impls.hh"
+#include "nix/store/path.hh"
 
 namespace nix {
 
 class Store;
+struct StoreDirConfig;
 struct BasicDerivation;
 struct StructuredAttrs;
 
@@ -116,6 +118,22 @@ struct DerivationOptions
      */
     std::map<std::string, StringSet> exportReferencesGraph;
 
+    /**
+     * Once a derivations is resolved, the strings in in
+     * `exportReferencesGraph` should all be store paths (with possible
+     * suffix paths, but those are discarded).
+     *
+     * @return The parsed path set for for each key in the map.
+     *
+     * @todo Ideally, `exportReferencesGraph` would just store
+     * `StorePath`s for this, but we can't just do that, because for CA
+     * derivations they is actually in general `DerivedPath`s (via
+     * placeholder strings) until the derivation is resolved and exact
+     * inputs store paths are known. We can use better types for that
+     * too, but that is a longer project.
+     */
+    std::map<std::string, StorePathSet> getParsedExportReferencesGraph(const StoreDirConfig & store) const;
+
     /**
      * env: __sandboxProfile
      *

src/libstore/parsed-derivations.cc

@@ -113,10 +113,7 @@ nlohmann::json StructuredAttrs::prepareStructuredAttrs(
     json["outputs"] = std::move(outputsJson);
 
     /* Handle exportReferencesGraph. */
-    for (auto & [key, inputPaths] : drvOptions.exportReferencesGraph) {
-        StorePathSet storePaths;
-        for (auto & p : inputPaths)
-            storePaths.insert(store.toStorePath(p).first);
+    for (auto & [key, storePaths] : drvOptions.getParsedExportReferencesGraph(store)) {
         json[key] = pathInfoToJSON(store, store.exportReferences(storePaths, storePaths));
     }
 

src/libstore/unix/build/derivation-builder.cc

@@ -785,17 +785,11 @@ void DerivationBuilderImpl::startBuilder()
 
     /* Handle exportReferencesGraph(), if set. */
     if (!drv.structuredAttrs) {
-        for (auto & [fileName, ss] : drvOptions.exportReferencesGraph) {
-            StorePathSet storePathSet;
-            for (auto & storePathS : ss) {
-                if (!store.isInStore(storePathS))
-                    throw BuildError("'exportReferencesGraph' contains a non-store path '%1%'", storePathS);
-                storePathSet.insert(store.toStorePath(storePathS).first);
-            }
+        for (auto & [fileName, storePaths] : drvOptions.getParsedExportReferencesGraph(store)) {
             /* Write closure info to <fileName>. */
             writeFile(
                 tmpDir + "/" + fileName,
-                store.makeValidityRegistration(store.exportReferences(storePathSet, inputPaths), false, false));
+                store.makeValidityRegistration(store.exportReferences(storePaths, inputPaths), false, false));
         }
     }