invalidateFetcherByDrvHash: init

pkgs/top-level/all-packages.nix

@@ -594,6 +594,42 @@ with pkgs;
 
   installShellFiles = callPackage ../build-support/install-shell-files {};
 
+  /*
+    Use the derivation hash to invalidate the output via name, for testing
+    purposes.
+
+    Type: (a@{ name, ... } -> Derivation) -> a -> Derivation
+
+    Normally, fixed output derivations can and should be cached by their output
+    hash only, but for testing we want to re-fetch everytime the fetcher changes.
+
+    Changes to the fetcher become apparent in the drvPath, which is a hash of
+    how to fetch, rather than a fixed store path.
+    By inserting this hash into the name, we can make sure to re-run the fetcher
+    every time the fetcher changes.
+
+    This relies on the assumption that Nix isn't clever enough to reuse its
+    database of local store contents to optimize fetching.
+
+    Note that the "salt" derives from a different drv than the final fetcher
+    drv. This is necessary, because it can not be defined recursively.
+
+    Example:
+
+      tests.fetchgit = invalidateFetcherByDrvHash fetchgit {
+        name = "nix-source";
+        url = "https://github.com/NixOS/nix";
+        rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
+        sha256 = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";
+      };
+  */
+  invalidateFetcherByDrvHash = f: args:
+    let
+      drvPath = (f args).drvPath;
+      # It's safe to discard the context, because we don't access the path.
+      salt = builtins.unsafeDiscardStringContext (lib.substring 0 12 (baseNameOf drvPath));
+    in f (args // { name = "${args.name or "source"}-salted-${salt}"; });
+
   lazydocker = callPackage ../tools/misc/lazydocker { };
 
   ld-is-cc-hook = makeSetupHook { name = "ld-is-cc-hook"; }