add intermediate variables and clarifying comments (#9274)

* add intermediate variables and clarifying comments

Co-authored-by: Alexander Groleau <alex@proof.construction>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libstore/build/local-derivation-goal.cc

@@ -2960,16 +2960,25 @@ bool LocalDerivationGoal::isReadDesc(int fd)
 
 StorePath LocalDerivationGoal::makeFallbackPath(OutputNameView outputName)
 {
+    // This is a bogus path type, constructed this way to ensure that it doesn't collide with any other store path
+    // See doc/manual/src/protocols/store-path.md for details
+    // TODO: We may want to separate the responsibilities of constructing the path fingerprint and of actually doing the hashing
+    auto pathType = "rewrite:" + std::string(drvPath.to_string()) + ":name:" + std::string(outputName);
     return worker.store.makeStorePath(
-        "rewrite:" + std::string(drvPath.to_string()) + ":name:" + std::string(outputName),
+        pathType,
+        // pass an all-zeroes hash
         Hash(HashAlgorithm::SHA256), outputPathName(drv->name, outputName));
 }
 
 
 StorePath LocalDerivationGoal::makeFallbackPath(const StorePath & path)
 {
+    // This is a bogus path type, constructed this way to ensure that it doesn't collide with any other store path
+    // See doc/manual/src/protocols/store-path.md for details
+    auto pathType = "rewrite:" + std::string(drvPath.to_string()) + ":" + std::string(path.to_string());
     return worker.store.makeStorePath(
-        "rewrite:" + std::string(drvPath.to_string()) + ":" + std::string(path.to_string()),
+        pathType,
+        // pass an all-zeroes hash
         Hash(HashAlgorithm::SHA256), path.name());
 }
 

src/libstore/store-api.cc

@@ -131,12 +131,12 @@ StorePath StoreDirConfig::makeFixedOutputPath(std::string_view name, const Fixed
             throw Error("fixed output derivation '%s' is not allowed to refer to other store paths.\nYou may need to use the 'unsafeDiscardReferences' derivation attribute, see the manual for more details.",
                 name);
         }
-        return makeStorePath("output:out",
-            hashString(HashAlgorithm::SHA256,
-                "fixed:out:"
+        // make a unique digest based on the parameters for creating this store object
+        auto payload = "fixed:out:"
                 + makeFileIngestionPrefix(info.method)
-                + info.hash.to_string(HashFormat::Base16, true) + ":"),
-            name);
+                + info.hash.to_string(HashFormat::Base16, true) + ":";
+        auto digest = hashString(HashAlgorithm::SHA256, payload);
+        return makeStorePath("output:out", digest, name);
     }
 }