Scope compiler selection to the cached Hydra build context

flake.nix

@@ -37,6 +37,7 @@
       }; in
       # `appendOverlays` with a singleton is identical to `extend`.
       let pkgs = haskellNix.legacyPackages.${system}.appendOverlays [android26]; in
+      let compilerSelection = import ./scripts/nix/compiler-selection.nix { inherit haskellNix; }; in
       let drv' = { extra-modules, pkgs', package-flags ? {}, package-ghc-options ? {}, ... }:
       let packageFlags = pkgs.lib.recursiveUpdate {
         direct-sqlcipher = { openssl = true; commoncrypto = false; tomcrypt = false; };
@@ -47,6 +48,7 @@
         }) package-ghc-options;
       in pkgs'.haskell-nix.project {
         compiler-nix-name = "ghc914-sh";
+        inherit compilerSelection;
         # We need this, to specify we want the cabal project.
         # If the stack.yaml was dropped, this would not be necessary.
         projectFileName = "cabal.project";

scripts/nix/README.md

@@ -5,6 +5,10 @@ The compiler and all cross-compilers come from the haskell.nix input pinned in
 GHC 9.14 release from GHCup. Use the same source checkout and lock file for every
 platform; Android ARMv7 does not require a separate branch or tag.
 
+`compiler-selection.nix` matches Hydra's compiler build context so the pinned
+compiler can use its existing cache. That context is scoped to the compiler and
+its build tools; application dependencies keep the project's current packages.
+
 Run these commands from the repository root:
 
 ```sh

scripts/nix/compiler-selection.nix

@@ -0,0 +1,7 @@
+{ haskellNix }:
+pkgs:
+# Hydra's compiler builds use this overlay. Its pkg-config metadata changes
+# the Cabal build tool's hash, and therefore every compiler slice's hash.
+# Keep that context inside compiler selection: application dependencies still
+# use the project's package set and its current libsodium mapping.
+(pkgs.appendOverlays [ (import (haskellNix + "/test/overlay.nix")) ]).haskell-nix.compiler