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Host-defined resources: every piece works, the composition crashes
dev/resources/hres.{wit,wat} is a guest that mints a host resource, reads it back through a borrow, and drops it -- the shape every wasi:io interface has. dev/cljwit/spike/host_resource.clj drives it, and the JVM crashes. Committed for how far it gets, which printing from inside each upcall narrows precisely: add_resource, both add_funcs and instantiate succeed mint runs, host_to_any returns no error succeeds peek runs, any_to_host returns the right rep succeeds the destructor upcall fires with that rep succeeds wasmtime_component_func_call crashes in error::source, after the destructor and before it returns So every piece works and the composition does not. Leading suspect: ownership of the `any` that host_to_any produces -- the spike writes it into the result val *and* deletes the host_t it came from, and nothing here has established which of those wasmtime expects. Not a bb task. A task that aborts the JVM is worse than none, so it runs by hand and the docstring says so. Two shapes worth keeping from the attempt: definterface needs ^void for a destructor callback, or findVirtual fails method resolution against a Void/TYPE MethodType; and a resource destructor is a second upcall shape, void (*)(void*, wasmtime_context_t*, uint32_t), distinct from an import's. Claude-Session: https://claude.ai/code/session_01XF5Hfq4Ca2N2XYEzWQQuHt
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‎dev/cljwit/spike/host_resource.clj‎

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(ns cljwit.spike.host-resource
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"Can the host *define* a resource the guest imports?
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Every `wasi:io` interface hands out a handle, so this is the shape `0017` F
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says is inside the host-import unit rather than after it. A map keyed
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`iface#func` cannot express a resource, and minting one needs a second
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upcall shape — a destructor `void (*)(void *, wasmtime_context_t *, u32)`.
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**This does not work yet, and is committed for how far it gets.** Run it
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with
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clojure -M:dev -m cljwit.spike.host-resource target/hres.component.wasm
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and the JVM crashes inside libwasmtime's `error::source`. It is deliberately
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*not* a `bb` task, because a task that aborts the JVM is worse than none.
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What is narrowed, by printing from inside each upcall:
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* `add_resource`, both `add_func`s and `instantiate` all succeed.
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* `mint` runs; `host_to_any` returns no error.
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* `peek` runs; `any_to_host` returns no error and the rep comes back.
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* the destructor upcall runs, with the right rep.
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* the crash is *inside* `wasmtime_component_func_call`, after the
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destructor and before it returns.
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So every piece works and the composition does not. The leading suspect is
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ownership of the `any` that `host_to_any` produces: this code writes it into
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the result val and also deletes the `host_t` it came from, and nothing here
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has established which of those wasmtime expects."
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(:require [clojure.java.io :as io])
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(:import [java.lang.foreign Arena Linker SymbolLookup MemorySegment
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FunctionDescriptor ValueLayout]
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[java.lang.invoke MethodHandle MethodHandles MethodType]))
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(set! *warn-on-reflection* true)
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(def ^:private ^java.lang.foreign.AddressLayout ADDR ValueLayout/ADDRESS)
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(def ^:private ^java.lang.foreign.ValueLayout$OfByte I8 ValueLayout/JAVA_BYTE)
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(def ^:private ^java.lang.foreign.ValueLayout$OfInt I32 ValueLayout/JAVA_INT)
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(def ^:private ^java.lang.foreign.ValueLayout$OfLong I64 ValueLayout/JAVA_LONG)
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(def ^:private VAL 32)
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(def ^:private UNION 8)
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(def ^:private KIND-U32 6)
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(def ^:private KIND-RESOURCE 21)
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(def ^:private TY 7) ; the u32 tag that distinguishes this host type
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(definterface Import
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(^java.lang.foreign.MemorySegment
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call [^java.lang.foreign.MemorySegment data ^java.lang.foreign.MemorySegment ctx
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^java.lang.foreign.MemorySegment ty
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^java.lang.foreign.MemorySegment args ^long nargs
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^java.lang.foreign.MemorySegment res ^long nres]))
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(definterface Dtor
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(^void call [^java.lang.foreign.MemorySegment data ^java.lang.foreign.MemorySegment ctx
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^int rep]))
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(defn -main [& _args]
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(let [linker (Linker/nativeLinker)
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lib (or (System/getenv "CLJWIT_WASMTIME_LIB")
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(throw (ex-info "CLJWIT_WASMTIME_LIB unset" {})))]
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(with-open [arena (Arena/ofShared)]
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(let [lookup (SymbolLookup/libraryLookup ^String lib arena)
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fx (fn [nm ret & args]
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(let [seg ^MemorySegment (.orElseThrow (.find lookup ^String nm))
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ls (into-array java.lang.foreign.MemoryLayout args)]
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(.downcallHandle linker seg
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^FunctionDescriptor
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(if ret (FunctionDescriptor/of ret ls)
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(FunctionDescriptor/ofVoid ls))
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(into-array java.lang.foreign.Linker$Option []))))
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call (fn [^MethodHandle mh & as] (.invokeWithArguments mh ^java.util.List (vec as)))
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emsg (fx "wasmtime_error_message" nil ADDR ADDR)
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ok! (fn [what v]
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(when-not (.equals MemorySegment/NULL ^MemorySegment v)
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(let [nm ^MemorySegment (.allocate arena (long 16))]
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(call emsg v nm)
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(let [n (.get nm I64 (long 0))
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p ^MemorySegment (.get nm ADDR (long 8))
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b (byte-array n)]
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(MemorySegment/copy (.reinterpret p n) I8 0 b 0 (int n))
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(throw (ex-info (str what ": " (String. b "UTF-8")) {})))))
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v)
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cstr (fn [^String t]
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(let [b (.getBytes t "UTF-8")
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s ^MemorySegment (.allocate arena (long (inc (alength b))))]
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(MemorySegment/copy ^bytes b 0 s I8 0 (alength b))
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(.set s I8 (long (alength b)) (byte 0))
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s))
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upcall (fn [obj iface mt desc]
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(.upcallStub linker
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(.bindTo (.findVirtual (MethodHandles/lookup) iface "call" mt) obj)
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desc arena (into-array java.lang.foreign.Linker$Option [])))
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IMPORT-MT (MethodType/methodType MemorySegment ^"[Ljava.lang.Class;"
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(into-array Class [MemorySegment MemorySegment MemorySegment
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MemorySegment Long/TYPE MemorySegment Long/TYPE]))
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IMPORT-FD (FunctionDescriptor/of ADDR (into-array java.lang.foreign.MemoryLayout
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[ADDR ADDR ADDR ADDR I64 ADDR I64]))
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;; reps the host has hidden behind handles, and what happened to them
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table (atom {})
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dropped (atom [])
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next-rep (atom 0)
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hnew (fx "wasmtime_component_resource_host_new" ADDR ValueLayout/JAVA_BOOLEAN I32 I32)
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hrep (fx "wasmtime_component_resource_host_rep" I32 ADDR)
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hdel (fx "wasmtime_component_resource_host_delete" nil ADDR)
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h2a (fx "wasmtime_component_resource_host_to_any" ADDR ADDR ADDR ADDR)
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a2h (fx "wasmtime_component_resource_any_to_host" ADDR ADDR ADDR ADDR)
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engine (call (fx "wasm_engine_new" ADDR))
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store (call (fx "wasmtime_store_new" ADDR ADDR ADDR ADDR)
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engine MemorySegment/NULL MemorySegment/NULL)
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ctx (call (fx "wasmtime_store_context" ADDR ADDR) store)
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mint (reify Import
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(call [_ _d cx _t args _n res _nr]
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(let [v (.get ^MemorySegment (.reinterpret ^MemorySegment args (long VAL))
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I32 (long UNION))
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rep (swap! next-rep inc)
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_ (swap! table assoc rep v)
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hr ^MemorySegment (call hnew true (int rep) (int TY))
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out ^MemorySegment (.allocate arena ^java.lang.foreign.MemoryLayout ADDR)]
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(ok! "host_to_any" (call h2a cx hr out))
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(call hdel hr)
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(let [o ^MemorySegment (.reinterpret ^MemorySegment res (long VAL))]
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(.set o I8 (long 0) (byte KIND-RESOURCE))
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(.set o ADDR (long UNION) ^MemorySegment (.get out ADDR (long 0))))
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MemorySegment/NULL)))
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peek (reify Import
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(call [_ _d cx _t args _n res _nr]
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(let [a ^MemorySegment (.reinterpret ^MemorySegment args (long VAL))
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any ^MemorySegment (.get a ADDR (long UNION))
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out ^MemorySegment (.allocate arena ^java.lang.foreign.MemoryLayout ADDR)]
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(ok! "any_to_host" (call a2h cx any out))
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(let [hr ^MemorySegment (.get out ADDR (long 0))
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rep (call hrep hr)
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o ^MemorySegment (.reinterpret ^MemorySegment res (long VAL))]
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(call hdel hr)
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(.set o I8 (long 0) (byte KIND-U32))
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(.set o I32 (long UNION) (int (get @table rep -1))))
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MemorySegment/NULL)))
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dtor (reify Dtor
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(call [_ _d _cx rep]
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(swap! dropped conj [rep (get @table rep)])
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(swap! table dissoc rep)
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nil))
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bs (.readAllBytes (io/input-stream (or (first *command-line-args*)
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"target/hres.component.wasm")))
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buf ^MemorySegment (.allocate arena (long (alength ^bytes bs)))
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_ (MemorySegment/copy ^bytes bs 0 buf I8 0 (alength ^bytes bs))
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cout ^MemorySegment (.allocate arena ^java.lang.foreign.MemoryLayout ADDR)
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_ (ok! "component_new" (call (fx "wasmtime_component_new" ADDR ADDR ADDR I64 ADDR)
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engine buf (long (alength ^bytes bs)) cout))
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comp (.get cout ADDR (long 0))
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clink (call (fx "wasmtime_component_linker_new" ADDR ADDR) engine)
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root (call (fx "wasmtime_component_linker_root" ADDR ADDR) clink)
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iout ^MemorySegment (.allocate arena ^java.lang.foreign.MemoryLayout ADDR)
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iname "local:hres/host@0.1.0"
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_ (ok! "add_instance" (call (fx "wasmtime_component_linker_instance_add_instance" ADDR ADDR ADDR I64 ADDR)
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root (cstr iname) (long (count (.getBytes iname "UTF-8"))) iout))
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li (.get iout ADDR (long 0))
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rty (call (fx "wasmtime_component_resource_type_new_host" ADDR I32) (int TY))
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_ (ok! "add_resource"
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(call (fx "wasmtime_component_linker_instance_add_resource" ADDR ADDR ADDR I64 ADDR ADDR ADDR ADDR)
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li (cstr "token") (long 5) rty
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(upcall dtor Dtor
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(MethodType/methodType Void/TYPE ^"[Ljava.lang.Class;"
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(into-array Class [MemorySegment MemorySegment Integer/TYPE]))
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(FunctionDescriptor/ofVoid (into-array java.lang.foreign.MemoryLayout [ADDR ADDR I32])))
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MemorySegment/NULL MemorySegment/NULL))
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addf (fx "wasmtime_component_linker_instance_add_func" ADDR ADDR ADDR I64 ADDR ADDR ADDR)
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_ (ok! "add mint" (call addf li (cstr "mint") (long 4)
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(upcall mint Import IMPORT-MT IMPORT-FD)
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MemorySegment/NULL MemorySegment/NULL))
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_ (ok! "add peek" (call addf li (cstr "peek") (long 4)
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(upcall peek Import IMPORT-MT IMPORT-FD)
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MemorySegment/NULL MemorySegment/NULL))
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inst ^MemorySegment (.allocate arena (long 16))
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_ (ok! "instantiate" (call (fx "wasmtime_component_linker_instantiate" ADDR ADDR ADDR ADDR ADDR)
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clink ctx comp inst))
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eidx (call (fx "wasmtime_component_instance_get_export_index" ADDR ADDR ADDR ADDR ADDR I64)
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inst ctx MemorySegment/NULL (cstr "run") (long 3))
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f ^MemorySegment (.allocate arena (long 24))
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_ (when-not (call (fx "wasmtime_component_instance_get_func" ValueLayout/JAVA_BOOLEAN ADDR ADDR ADDR ADDR)
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inst ctx eidx f)
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(throw (ex-info "no export run" {})))
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args ^MemorySegment (.allocate arena (long VAL))
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res ^MemorySegment (.allocate arena (long VAL))]
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(.set args I8 (long 0) (byte KIND-U32))
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(.set args I32 (long UNION) (int 42))
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(ok! "func_call" (call (fx "wasmtime_component_func_call" ADDR ADDR ADDR ADDR I64 ADDR I64)
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f ctx args (long 1) res (long 1)))
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(println "run(42) =" (.get res I32 (long UNION))
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" dropped:" (pr-str @dropped)
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" table now:" (pr-str @table))))))

‎dev/resources/hres.wat‎

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;; The guest half of dev/resources/hres.wit: mint a host resource, read it
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;; back through a borrow, and drop it. The drop is what makes the host's
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;; destructor callback observable.
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(module
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(import "local:hres/host@0.1.0" "mint" (func $mint (param i32) (result i32)))
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(import "local:hres/host@0.1.0" "peek" (func $peek (param i32) (result i32)))
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(import "local:hres/host@0.1.0" "[resource-drop]token" (func $drop (param i32)))
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(memory (export "memory") 1)
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(func (export "cabi_realloc") (param i32 i32 i32 i32) (result i32) (i32.const 0))
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(func (export "run") (param $v i32) (result i32)
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(local $h i32) (local $r i32)
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(local.set $h (call $mint (local.get $v)))
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(local.set $r (call $peek (local.get $h)))
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(call $drop (local.get $h))
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(local.get $r)))

‎dev/resources/hres.wit‎

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// A component that imports a *resource* from the host, which is what every
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// wasi:io interface does and what a map keyed `iface#func` cannot express
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// (`0017` F).
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package local:hres@0.1.0;
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interface host {
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resource token { }
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mint: func(v: u32) -> token;
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peek: func(t: borrow<token>) -> u32;
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}
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world w { import host; export run: func(v: u32) -> u32; }

‎doc/design/0017-host-imports.md‎

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a val — and it panics outside a call scope, which is the landmine already
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recorded above.
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**Tried, 2026-07-30, and it does not work yet.**
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`dev/cljwit/spike/host_resource.clj` drives
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`dev/resources/hres.{wit,wat}` — a guest that mints a host resource, reads it
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through a `borrow`, and drops it. Every individual piece succeeds:
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`add_resource`, both `add_func`s and `instantiate`; `mint` with
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`host_to_any` returning no error; `peek` with `any_to_host` returning the right
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rep; and the destructor upcall firing with that rep. **The crash is inside
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`wasmtime_component_func_call` itself, after the destructor and before it
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returns**, in `error::source`.
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Leading suspect: ownership of the `any` that `host_to_any` produces. The spike
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writes it into the result val *and* deletes the `host_t` it came from, and
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nothing has established which of those wasmtime expects. The spike is not a
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`bb` task, because a task that aborts the JVM is worse than none.
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## Alternatives rejected
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- **Imports on the artifact, sharing one set of host functions.** Wrong the

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