Factor out a new DesugaredEnv from DerivationBuildingGoal

Now we have better separation of the core logic --- an integral part of
the store layer spec even --- from the goal mechanism and other
minutiae.

Co-authored-by: Jeremy Kolb <kjeremy@gmail.com>

src/libstore/build/derivation-env-desugar.cc

@@ -0,0 +1,59 @@
+#include "nix/store/build/derivation-env-desugar.hh"
+#include "nix/store/store-api.hh"
+#include "nix/store/derivations.hh"
+#include "nix/store/derivation-options.hh"
+
+namespace nix {
+
+std::string & DesugaredEnv::atFileEnvPair(std::string_view name, std::string fileName)
+{
+    auto & ret = extraFiles[fileName];
+    variables.insert_or_assign(
+        std::string{name},
+        EnvEntry{
+            .prependBuildDirectory = true,
+            .value = std::move(fileName),
+        });
+    return ret;
+}
+
+DesugaredEnv DesugaredEnv::create(
+    Store & store, const Derivation & drv, const DerivationOptions & drvOptions, const StorePathSet & inputPaths)
+{
+    DesugaredEnv res;
+
+    if (drv.structuredAttrs) {
+        auto json = drv.structuredAttrs->prepareStructuredAttrs(store, drvOptions, inputPaths, drv.outputs);
+        res.atFileEnvPair("NIX_ATTRS_SH_FILE", ".attrs.sh") = StructuredAttrs::writeShell(json);
+        res.atFileEnvPair("NIX_ATTRS_JSON_FILE", ".attrs.json") = json.dump();
+    } else {
+        /* In non-structured mode, set all bindings either directory in the
+           environment or via a file, as specified by
+           `DerivationOptions::passAsFile`. */
+        for (auto & [envName, envValue] : drv.env) {
+            if (!drvOptions.passAsFile.contains(envName)) {
+                res.variables.insert_or_assign(
+                    envName,
+                    EnvEntry{
+                        .value = envValue,
+                    });
+            } else {
+                res.atFileEnvPair(
+                    envName + "Path",
+                    ".attr-" + hashString(HashAlgorithm::SHA256, envName).to_string(HashFormat::Nix32, false)) =
+                    envValue;
+            }
+        }
+
+        /* Handle exportReferencesGraph(), if set. */
+        for (auto & [fileName, storePaths] : drvOptions.getParsedExportReferencesGraph(store)) {
+            /* Write closure info to <fileName>. */
+            res.extraFiles.insert_or_assign(
+                fileName, store.makeValidityRegistration(store.exportReferences(storePaths, inputPaths), false, false));
+        }
+    }
+
+    return res;
+}
+
+} // namespace nix