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Extract dummy-ghc, make it cross-aware, and fix plan-nix UnitId stability #2509

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hamishmack merged 1 commit into from
May 23, 2026
+391 −176
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Extract dummy-ghc, make it cross-aware, and fix plan-nix UnitId stability #2509

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197 changes: 32 additions & 165 deletions lib/call-cabal-project-to-nix.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -293,158 +293,11 @@ let

platformString = p: with p.parsed; "${cpu.name}-${vendor.name}-${kernel.name}";

# Dummy `ghc` that uses the captured output
dummy-ghc = evalPackages.writeTextFile {
name = "dummy-" + ghc.name;
executable = true;
destination = "/bin/${ghc.targetPrefix}ghc";
# New versions of cabal pass `-package-env=-`, but dummy-ghc can safely ignore it.
text = ''
#!${evalPackages.runtimeShell}
if [[ "$1" == "-package-env=-" ]]; then
shift
fi
case "$*" in
--version*)
echo "The Glorious Glasgow Haskell Compilation System, version ${ghc.version}"
;;
--numeric-version*)
echo "${ghc.version}"
;;
${pkgs.lib.optionalString (ghc.targetPrefix == "js-unknown-ghcjs-") ''
--numeric-ghc-version*)
echo "${ghc.version}"
;;
--numeric-ghcjs-version*)
echo "${ghc.version}"
;;
''}
--supported-languages*)
cat ${import ./supported-languages.nix { inherit pkgs evalPackages ghc; }}
;;
--print-global-package-db*)
echo "$out/dumby-db"
;;
--info*)
echo '[("target os", "${
if pkgs.stdenv.targetPlatform.isLinux
then "OSLinux"
else if pkgs.stdenv.targetPlatform.isDarwin
then "OSDarwin"
else if pkgs.stdenv.targetPlatform.isWindows
then "OSMinGW32"
else if pkgs.stdenv.targetPlatform.isGhcjs
then "OSGhcjs"
else if pkgs.stdenv.targetPlatform.isWasi
then "OSWasi"
else throw "Unknown target os ${pkgs.stdenv.targetPlatform.config}"
}")'
echo ',("target arch","${
if pkgs.stdenv.targetPlatform.isx86_64
then "ArchX86_64"
else if pkgs.stdenv.targetPlatform.isx86
then "ArchX86"
else if pkgs.stdenv.targetPlatform.isRiscV64
then "ArchRISCV64"
else if pkgs.stdenv.targetPlatform.isAarch64
then "ArchAArch64"
else if pkgs.stdenv.targetPlatform.isAarch32
then "ArchAArch32"
else if pkgs.stdenv.targetPlatform.isJavaScript
then "ArchJavaScript"
else if pkgs.stdenv.targetPlatform.isWasm
then "ArchWasm32"
else throw "Unknown target arch ${pkgs.stdenv.targetPlatform.config}"
}")'
echo ',("target platform string","${platformString pkgs.stdenv.targetPlatform}")'
echo ',("Build platform","${platformString pkgs.stdenv.buildPlatform}")'
echo ',("Host platform","${platformString pkgs.stdenv.hostPlatform}")'
echo ',("Target platform","${platformString pkgs.stdenv.targetPlatform}")'
echo ',("Project Unit Id","ghc-${ghc.version}-inplace")'
${
# Capability fields cabal-install reads to decide what
# configure-args to record in plan.json's per-pkg
# `configure-args` entries. Without these, cabal assumes
# the compiler can't build shared libs / dynamic-too,
# so it records `--disable-shared` /
# `--disable-library-for-ghci` — which feeds into the
# package's UnitId hash. Mirror real GHC's capabilities
# so plan-to-nix's recorded ids match what cabal would
# compute against the actual compiler.
#
# The values are conditioned on the *target* platform
# because cabal reads `ghc --info` to decide things like
# `--enable-shared` vs `--disable-shared`, and those
# decisions feed into the package's UnitId hash.
#
# Reference outputs (verified by running `ghc --info` on
# the actual cross GHC derivation in /nix/store):
#
# * x86_64-w64-mingw32 (Windows mingw): no
# `Support shared libraries` field at all,
# `Support dynamic-too: NO`, `GHC Dynamic: NO`,
# RTS ways without any `_dyn` ways, Stage 1.
# * ghcjs / wasm: built stage-1 with only `v debug` ways
# and no dynamic linking.
# * native Linux/Darwin: dynamic everything, Stage 2.
if pkgs.stdenv.targetPlatform.isGhcjs
|| pkgs.stdenv.targetPlatform.isWasm
then ''
echo ',("Support dynamic-too","NO")'
echo ',("Support shared libraries","NO")'
echo ',("Support reexported-modules","YES")'
echo ',("Support thinning and renaming package flags","YES")'
echo ',("Tables next to code","YES")'
echo ',("Have interpreter","YES")'
echo ',("Use interpreter","YES")'
echo ',("Have native code generator","YES")'
echo ',("target RTS linker only supports shared libraries","NO")'
echo ',("GHC Dynamic","NO")'
echo ',("RTS ways","v debug")'
echo ',("Stage","1")'
''
else if pkgs.stdenv.targetPlatform.isWindows
then ''
echo ',("Support dynamic-too","NO")'
echo ',("Support reexported-modules","YES")'
echo ',("Support thinning and renaming package flags","YES")'
echo ',("Tables next to code","YES")'
echo ',("Have interpreter","YES")'
echo ',("Use interpreter","YES")'
echo ',("Have native code generator","YES")'
echo ',("target RTS linker only supports shared libraries","NO")'
echo ',("GHC Dynamic","NO")'
echo ',("RTS ways","v thr thr_debug thr_debug_p thr_p debug debug_p p")'
echo ',("Stage","1")'
''
else ''
echo ',("Support dynamic-too","YES")'
echo ',("Support shared libraries","YES")'
echo ',("Support reexported-modules","YES")'
echo ',("Support thinning and renaming package flags","YES")'
echo ',("Tables next to code","YES")'
echo ',("Have interpreter","YES")'
echo ',("Use interpreter","YES")'
echo ',("Have native code generator","YES")'
echo ',("target RTS linker only supports shared libraries","NO")'
echo ',("GHC Dynamic","YES")'
echo ',("RTS ways","v thr thr_debug thr_debug_p thr_debug_p_dyn thr_debug_dyn thr_p thr_p_dyn thr_dyn debug debug_p debug_p_dyn debug_dyn p p_dyn dyn")'
echo ',("Stage","2")'
''
}
echo ']'
;;
--print-libdir*)
echo $out/ghc/libdir
;;
*)
echo "Unknown argument '$*'" >&2
exit 1
;;
esac
exit 0
'';
};
# Dummy `ghc` script cabal-install runs against during plan-to-nix.
# See `lib/dummy-ghc.nix` for the implementation and rationale; the
# `--info` output is what cabal uses to elaborate per-unit settings
# that feed into UnitId computation.
dummy-ghc = import ./dummy-ghc.nix { inherit pkgs evalPackages ghc; };

dummy-ghc-pkg-dump = evalPackages.runCommand "dummy-ghc-pkg-dump" {
buildInputs = prebuilt-depends;
Expand Down Expand Up @@ -628,25 +481,31 @@ let
fi
done
${
# A handful of GHC's pre-existing packages have ids that
# are *just* `<name>-<version>` with no `-inplace` suffix
# in their installed package conf:
# GHC ≥ 9.8 registers its pre-existing packages with
# ids that carry an `-inplace` suffix
# (`base-4.19.2.0-inplace`). Two boot packages are
# exceptions even there:
# - `rts`: GHC's runtime, registered as `id: rts-1.0.3`
# - `system-cxx-std-lib`: virtual placeholder for the
# host C++ stdlib, registered as
# `id: system-cxx-std-lib-1.0`
# Every other pre-existing package (base, ghc-prim,
# text, …) uses the `-inplace` convention. Mirror those
# real ids in the dummy `ghc-pkg dump` — both as the
# package's own id and as the dep id used by other
# packages — so the unit-id cabal computes against the
# dummy matches what it computes against the real GHC.
# GHC < 9.8 registers everything with just
# `<name>-<version>`, no `-inplace` suffix at all.
# Mirror those real ids in the dummy `ghc-pkg dump` —
# both as the package's own id and as the dep id used
# by other packages — so the unit-id cabal computes
# against the dummy matches what it computes against
# the real GHC.
""
}suffix() {
case "$1" in
rts|system-cxx-std-lib) echo "" ;;
*) echo "-inplace" ;;
esac
${if pkgs.lib.versionAtLeast ghc.version "9.8" then ''
case "$1" in
rts|system-cxx-std-lib) echo "" ;;
*) echo "-inplace" ;;
esac
'' else ''
echo ""
''}
}
for pkg in $PKGS; do
varname="$(echo $pkg | tr "-" "_")"
Expand Down Expand Up @@ -718,6 +577,14 @@ let
# Needed or stack-to-nix will die on unicode inputs
LOCALE_ARCHIVE = pkgs.lib.optionalString (evalPackages.stdenv.buildPlatform.libc == "glibc") "${evalPackages.glibcLocales}/lib/locale/locale-archive";
LANG = "en_US.UTF-8";
# Pin the unit-id format that `make-install-plan`'s patched
# cabal uses to the *build* platform's OS. Without this the
# format tracks the eval system (which on a Darwin host
# evaluating a Linux derivation gives the `VeryShort` form),
# and plan-nix unit-ids fork from what slice cabal v2-build
# computes on Linux. See
# nix-tools/cabal-install-patches/installed-package-id-os-override.patch.
CABAL_INSTALLED_PACKAGE_ID_OS = pkgs.stdenv.buildPlatform.parsed.kernel.name;
meta.platforms = pkgs.lib.platforms.all;
preferLocalBuild = false;
outputs = [
Expand Down
39 changes: 28 additions & 11 deletions lib/default.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -40,7 +40,11 @@ in {
foldComponents = tys: f: z: conf:
let
comps = conf.components or { };
# ensure that comps.library exists and is not null.
# The comment below used to claim we ensure `comps.library` is
# not null, but the check only looked at presence — null
# libraries (common on exe-only pseudo-pkg entries) would still
# be added to the fold and crash downstream consumers that
# assume non-null (e.g. `allComponent`'s `c.buildable` filter).
libComp = acc:
if (comps.library or null) != null then f comps.library acc else acc;
subComps = acc:
Expand Down Expand Up @@ -613,18 +617,31 @@ in {

# Key used by `uniqueWithName` and `checkUnique` to partition lists.
# Derivations carry a name+version-encoded `.name`
# (e.g. `ghc984-conduit-1.3.5.0`). haskell.nix package values don't --
# they expose `.identifier.{name,version}` -- so we synthesize a key
# with a space separator rather than the derivation `"${name}-${version}"`
# shape, so a derivation key can never collide with a package key derived
# from the same words. Items with neither share a bucket; correctness
# still holds via the inner `lib.unique`, just at a slower path.
# (e.g. `ghc984-conduit-1.3.5.0`). haskell.nix package values
# don't -- they expose `.identifier.{name,version}` -- so we
# synthesize a key with a space separator rather than the
# derivation `"${name}-${version}"` shape, so a derivation key
# can never collide with a package key derived from the same
# words. Items with neither share a bucket; correctness still
# holds via the inner `lib.unique`, just at a slower path.
#
# When an `identifier.unit-id` is present, it is appended to the
# key so two incarnations of `foo-1.2.3` with different inputs
# land in different buckets. Items without a unit-id keep
# exactly the key they had before, so the lexicographic ordering
# `uniqueWithName` produces via `builtins.attrValues
# (groupBy ...)` is byte-stable.
uniqueWithNameKey = x:
if __typeOf x != "set" then "_notset"
else if x ? name then x.name
else if x ? identifier.name
then "${x.identifier.name} ${x.identifier.version or ""}"
else "_noname";
else let
suffix = lib.optionalString
(x ? identifier.unit-id && x.identifier.unit-id != null)
" ${x.identifier.unit-id}";
in
if x ? name then x.name + suffix
else if x ? identifier.name
then "${x.identifier.name} ${x.identifier.version or ""}${suffix}"
else "_noname";

# Like `lib.unique`, but uses `uniqueWithNameKey` as a partition key so
# `lib.unique` only has to scan within each bucket -- the common case
Expand Down
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