From ac78ae80a43c9ab27a434c3d1c62176cf93b5296 Mon Sep 17 00:00:00 2001 From: Robert Hensing Date: Sun, 19 Sep 2021 20:23:52 +0200 Subject: invalidateFetcherByDrvHash move docs to manual --- .../special/invalidateFetcherByDrvHash.section.md | 31 ++++++++++++++++++++++ 1 file changed, 31 insertions(+) create mode 100644 doc/builders/special/invalidateFetcherByDrvHash.section.md (limited to 'doc/builders/special/invalidateFetcherByDrvHash.section.md') diff --git a/doc/builders/special/invalidateFetcherByDrvHash.section.md b/doc/builders/special/invalidateFetcherByDrvHash.section.md new file mode 100644 index 00000000000..7c2f03a64b7 --- /dev/null +++ b/doc/builders/special/invalidateFetcherByDrvHash.section.md @@ -0,0 +1,31 @@ + +## `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="; + }; -- cgit 1.4.1