Fix allocator lookup for anonymous WASM modules

[skhaz]

Summary

• Fix cabi_realloc lookup returning nil for core WASM modules instantiated with WithName("") (anonymous)
• Add null pointer guards on allocator return values to prevent passing ptr=0 to WASM modules

Root Cause

When a core WASM module is instantiated anonymously via wazero.NewModuleConfig().WithName(""), wazero's FunctionDefinition.Name() returns an empty string instead of the export name. The allocator caching code was doing:

allocFnDef := instance.ExportedFunctionDefinitions()["cabi_realloc"]  // finds it
wazInst.allocFn = instance.ExportedFunction(allocFnDef.Name())        // Name() = "" → nil

This caused allocFn to be silently nil, making every string parameter encoding fail with "failed to allocate N bytes for string data".

Fix

Use the map key (the export name we searched for) for ExportedFunction lookup instead of FunctionDefinition.Name():

allExports := instance.ExportedFunctionDefinitions()
allocFnDef := allExports[CabiRealloc]
allocName := CabiRealloc
// ...
wazInst.allocFn = instance.ExportedFunction(allocName)

Also added null pointer checks on cabi_realloc return values in both wazeroAllocator.Alloc and AllocatorWrapper.Alloc to surface allocation failures early instead of passing ptr=0 to the WASM guest (which causes NonNull::new_unchecked panics in Rust wit-bindgen bindings).

Reproduction

Any core WASM module (non-component) with string parameters compiled from Rust using wit-bindgen would fail. For example, a module exporting html-to-markdown: func(html: string) -> string built with scraper/html5ever would crash with:

encode params: [encode] invalid_data at param[0]: failed to encode parameter 0:
  [encode] allocation: failed to allocate 4 bytes for string data

Files Changed

• engine/wazero.go — allocator lookup fix + null pointer guards in wazeroAllocator.Alloc
• linker/internal/memory/wrapper.go — null pointer guard in AllocatorWrapper.Alloc

[Copilot]

Pull request overview

Fixes allocator resolution for anonymous core WASM module instances (e.g., instantiated with an empty module name) and adds explicit null-pointer guards to fail fast when cabi_realloc returns 0 for non-zero allocations.

Changes:

• Fix allocator caching to look up cabi_realloc via the export-map key rather than FunctionDefinition.Name() (which can be empty for anonymous modules).
• Add ptr==0 && size>0 guards in allocator wrappers to surface allocation failures early.
• Add regression tests for anonymous-module allocator lookup and null allocator behavior (plus a couple of small builder formatting refactors).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
engine/wazero.go	Fix allocator lookup for anonymous modules; add null-pointer allocation guards.
engine/allocator_lookup_test.go	New regression tests covering anonymous-module allocator lookup and null allocator return handling.
linker/internal/memory/wrapper.go	Add null-pointer guard in AllocatorWrapper.Alloc.
linker/internal/memory/wrapper_test.go	Add test ensuring null allocator return becomes an error for non-zero size.
errors/errors.go	Refactor builder formatting to avoid fmt.Sprintf allocation.
cmd/run/interactive.go	Refactor builder formatting to avoid fmt.Sprintf allocation.
transcoder/internal/types/kind_test.go	Remove //nolint:revive from package declaration.

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[Copilot]

PR description (Summary/Files Changed) doesn't mention the addition of this new engine-level test file (and there are also unrelated formatting changes in other packages). Please update the PR description to reflect all modified files, or split the non-allocator changes into a separate PR to keep the scope focused.