haskell.lib.makePackageSet: all-cabal-hashes can be a directory (#188203)

* all-cabal-hahses can be a directory

Motivating problem: we want to use the flake.lock for all-cabal-hashes
as a github repository as our revision pin, and then import that to pass
as all-cabal-hashes. To do that we need it to accept a directory rather
than a tarball.

* all-cabal-hashes: add some comments

1 files changed, 15 insertions, 2 deletions

diff --git a/pkgs/development/haskell-modules/make-package-set.nix b/pkgs/development/haskell-modules/make-package-set.nix
index 9d35d198825f8..9bbfe44192862 100644
--- a/pkgs/development/haskell-modules/make-package-set.nix
+++ b/pkgs/development/haskell-modules/make-package-set.nix
@@ -21,6 +21,9 @@
 , haskellLib
 
 , # hashes for downloading Hackage packages
+  # This is either a directory or a .tar.gz containing the cabal files and
+  # hashes of Hackage as exemplified by this repository:
+  # https://github.com/commercialhaskell/all-cabal-hashes/tree/hackage
   all-cabal-hashes
 
 , # compiler to use
@@ -136,10 +139,20 @@ let
       cabal2nix --compiler=${self.ghc.haskellCompilerName} --system=${hostPlatform.config} ${sha256Arg} "${src}" ${extraCabal2nixOptions} > "$out/default.nix"
     '';
 
+  # Given a package name and version, e.g. name = "async", version = "2.2.4",
+  # gives its cabal file and hashes (JSON file) as discovered from the
+  # all-cabal-hashes value. If that's a directory, it will copy the relevant
+  # files to $out; if it's a tarball, it will extract and move them to $out.
   all-cabal-hashes-component = name: version: buildPackages.runCommand "all-cabal-hashes-component-${name}-${version}" {} ''
-    tar --wildcards -xzvf ${all-cabal-hashes} \*/${name}/${version}/${name}.{json,cabal}
     mkdir -p $out
-    mv */${name}/${version}/${name}.{json,cabal} $out
+    if [ -d ${all-cabal-hashes} ]
+    then
+      cp ${all-cabal-hashes}/${name}/${version}/${name}.json $out
+      cp ${all-cabal-hashes}/${name}/${version}/${name}.cabal $out
+    else
+      tar --wildcards -xzvf ${all-cabal-hashes} \*/${name}/${version}/${name}.{json,cabal}
+      mv */${name}/${version}/${name}.{json,cabal} $out
+    fi
   '';
 
   hackage2nix = name: version: let component = all-cabal-hashes-component name version; in self.haskellSrc2nix {