Simon Peyton Jones pushed to branch wip/substitution-via-mapper at Glasgow Haskell Compiler / GHC Commits: 8d49ed42 by Kazu Yamamoto at 2026-10-06T19:37:25-04:00 Do not leave standard file descriptors free at startup (#24450) A process may be started with stdout closed -- `prog >&-` from a shell does exactly that. Nothing then reserves descriptor 1, and open(), pipe() and socket() hand out the lowest free descriptor, so the next descriptor the runtime creates for itself takes it: the ticker's pipe, or a capability's IO manager control pipe. 'GHC.Internal.IO.StdHandles.stdout' is a CAF, built the first time the program writes to stdout, which is long after that has happened. It builds a 'Handle' over descriptor 1, which now belongs to the runtime, and nothing reports an error because the descriptor is perfectly good. The program then writes its output into the runtime's own plumbing. What that looks like from the outside is a hang. The descriptor the Handle has taken over is the read end of a pipe, so poll() for POLLOUT on it never says yes and epoll_ctl() accepts a registration for it; the thread waiting to write blocks for ever. Closing the Handle at exit closes the runtime's descriptor, which is worse: the ticker then fails on every tick. 'hs_init_ghc' now opens /dev/null onto whichever of descriptors 0, 1 and 2 are closed, before the runtime creates any descriptor of its own. Each is opened for the access that descriptor would normally have, read-only for 0 and write-only for 1 and 2, so that writing to 'stdin' or reading from 'stdout' still fails rather than quietly succeeding against /dev/null. See Note [Standard file descriptors must be open]. Moving only the descriptors the runtime opens would not do: the first file the program itself opened would land on the free number instead. The fix has to leave no free number. A program run this way therefore finds stdout connected to /dev/null rather than failing with EBADF on the first write, which is what such a program would get from most other language runtimes. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> - - - - - 65bcf9e9 by Simon Jakobi at 2026-10-06T19:38:09-04:00 changelog: Repair the sentence describing #27477 The description read "programs using could crash", the object of "using" having been lost. The synopsis already says which programs are affected, so drop the dangling word. Assisted-by: Claude Fable 5.1 - - - - - f9ef0775 by Simon Peyton Jones at 2026-10-07T11:00:15+01:00 Refactor type and coercion substitution This MR refactors type and coercion substitution so that they use TyCoMapper, rather than hand-rolled code. Fundamentally it's a simple idea, but I had to do quite a bit of refactoring to make it fly. I got Claude to help me this this refactoring, but I paid careful attention to every line of code. Main changes: * New module GHC.Core.TyCo.Make which defines TyCoMapper, and a bunch of other simple "smart constructors" for types and coercions. See GHC.Core.TyCo.Rep Note [The Type-related module hierarchy], esp (TRM1) * The new TyCoMapper has new fields `tcm_tcapp_ty` and `tcm_tcapp_co` to handle type-constructor applications. These replace `tcm_tycon`. * Rewrite GHC.Core.TyCo.Subst to use TyCoMapper * Rewrite GHC.Core.Type.expandTypeSynonyms to use TyCoMapper. * Implement `seqType` and `seqCo` with TyCoFolder. To to this I had to add a `tcf_lit` field to TyCoFolder. * A new module GHC.Utils.StrictIdentity to define a strict identity monad, which itself is used for substTyCoMapper :: TyCoMapper Subst StrictIdentity Refactoring `seq` functions * In GHC.Core.Seq, redefine `seqOneShot`, `seqRuleInfo`, and friends to return `SeqResult`, a new type defined in GHC.Types.Baseic, rather than `()`. We can define a Monoid instance for `SeqResult` that does the right thing. * This has knock-on effects across the compiler. Refactoring ForAllTyFlag In Note [Comparing visibility] in GHC.Core.TyCo.Compare we read that "we always compare ForAllTyFlags with `eqForAllVis` or `cmpForAllVis`" But alas there was an Eq and Ord instance for ForAllTyFlag, which made it impossible to check this claim. So I made these changes: * Move the definition of ForAllTyFlag and its related functions from L.H.S.Specificity (where it did not belong, since L.H.S didn't use it) into GHC.Types.Var. * Remove the Eq and Ord instances for ForAllTyFlag * Deal with the fallout, by using `eqForAllVis` or related functions. * In interface files we use IfaceType as a key for a de-deduplication map See Note [Deduplication during iface binary serialisation] in GHC.Iface.Binary. For this we need /exact/ comparison, so we use a newtype wrapper for IfaceForAllTyFlag, and give use `cmpForAllTyFlagExactly` to implement its Ord instance. Tiresome but easy. * Rename `toIfaceForAllBndrs` to `toIfaceForAllTyBndrs` - - - - - 64 changed files: - + changelog.d/24450 - changelog.d/fix-parallel-gc-selector-race-27477 - compiler/GHC/Builtin/WiredIn/Prim.hs - compiler/GHC/Core/Coercion.hs - compiler/GHC/Core/Coercion.hs-boot - compiler/GHC/Core/Coercion/Opt.hs - compiler/GHC/Core/DataCon.hs - compiler/GHC/Core/Lint.hs - compiler/GHC/Core/Map/Type.hs - compiler/GHC/Core/Opt/Arity.hs - compiler/GHC/Core/Reduction.hs - compiler/GHC/Core/Seq.hs - compiler/GHC/Core/TyCo/Compare.hs - compiler/GHC/Core/TyCo/FVs.hs - + compiler/GHC/Core/TyCo/Make.hs - + compiler/GHC/Core/TyCo/Make.hs-boot - compiler/GHC/Core/TyCo/Ppr.hs - compiler/GHC/Core/TyCo/Rep.hs - compiler/GHC/Core/TyCo/Rep.hs-boot - compiler/GHC/Core/TyCo/Subst.hs - compiler/GHC/Core/Type.hs - compiler/GHC/Core/Type.hs-boot - compiler/GHC/Core/Unify.hs - compiler/GHC/CoreToIface.hs - compiler/GHC/CoreToIface.hs-boot - compiler/GHC/Driver/Main/Compile.hs - compiler/GHC/Iface/Decl.hs - compiler/GHC/Iface/Ext/Types.hs - compiler/GHC/Iface/Ext/Utils.hs - compiler/GHC/Iface/Syntax.hs - compiler/GHC/Iface/Type.hs - compiler/GHC/Iface/Type.hs-boot - compiler/GHC/IfaceToCore.hs - compiler/GHC/Stg/EnforceEpt/TagSig.hs - compiler/GHC/Tc/Errors/Ppr.hs - compiler/GHC/Tc/Gen/App.hs - compiler/GHC/Tc/TyCl.hs - compiler/GHC/Tc/TyCl/Instance.hs - compiler/GHC/Tc/Utils/Unify.hs - compiler/GHC/Tc/Zonk/TcType.hs - compiler/GHC/Tc/Zonk/Type.hs - compiler/GHC/Types/Arity.hs - compiler/GHC/Types/Basic.hs - compiler/GHC/Types/Cpr.hs - compiler/GHC/Types/Demand.hs - compiler/GHC/Types/Name/Env.hs - compiler/GHC/Types/Name/Occurrence.hs - compiler/GHC/Types/Unique/FM.hs - compiler/GHC/Types/Unique/Map.hs - compiler/GHC/Types/Unique/Set.hs - compiler/GHC/Types/Var.hs - compiler/GHC/Types/Var.hs-boot - compiler/GHC/Types/Var/Set.hs - compiler/GHC/Utils/Binary.hs - compiler/GHC/Utils/Outputable.hs - + compiler/GHC/Utils/StrictIdentity.hs - compiler/Language/Haskell/Syntax/Specificity.hs - compiler/ghc.cabal.in - rts/RtsStartup.c - testsuite/tests/count-deps/CountDepsParser.stdout - + testsuite/tests/rts/T24450.hs - + testsuite/tests/rts/T24450.stderr - testsuite/tests/rts/all.T - utils/haddock/haddock-api/src/Haddock/Backends/Hyperlinker/Utils.hs The diff was not included because it is too large. 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