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author | Robert Hensing <robert@roberthensing.nl> | 2021-09-19 20:23:52 +0200 |
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committer | Robert Hensing <robert@roberthensing.nl> | 2021-09-19 20:27:32 +0200 |
commit | ac78ae80a43c9ab27a434c3d1c62176cf93b5296 (patch) | |
tree | d7ff9495705fe7b96f35ec3222856184c0e8b227 /doc/builders/special/invalidateFetcherByDrvHash.section.md | |
parent | b502de64766757b635ed2bb4aeb9532ceaefe71d (diff) | |
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invalidateFetcherByDrvHash move docs to manual
Diffstat (limited to 'doc/builders/special/invalidateFetcherByDrvHash.section.md')
-rw-r--r-- | doc/builders/special/invalidateFetcherByDrvHash.section.md | 31 |
1 files changed, 31 insertions, 0 deletions
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="; + }; |