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+
+## `invalidateFetcherByDrvHash` {#sec-pkgs-invalidateFetcherByDrvHash}
+
+Use the derivation hash to invalidate the output via name, for testing.
+
+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.
+
+You might notice that the "salted" name derives from the normal invocation,
+not the final derivation. `invalidateFetcherByDrvHash` has to invoke the fetcher
+function twice: once to get a derivation hash, and again to produce the final
+fixed output derivation.
+
+Example:
+
+    tests.fetchgit = invalidateFetcherByDrvHash fetchgit {
+      name = "nix-source";
+      url = "https://github.com/NixOS/nix";
+      rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
+      sha256 = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";
+    };