HowlFrame #90 Phase 2c: mediate read_file on Go and JavaScript

[howlcipher]

Summary

Phase 2c of #90 mediates (read_file) on generated Go and JavaScript under the existing filesystem grant (capability.ForConstruct("read_file") / OpReadFile). The runner grant is HOWLFRAME_ALLOW_CAPS, the same comma-separated list as -allow-caps.

An empty, unset, or non-filesystem grant fails with CAPABILITY_DENIED: capability denied: filesystem before os.ReadFile or readFileSync. The denial does not include the path, and the file is not opened. A filesystem grant reads the file.

Go try_let still binds (bytes, error) from howlFrameReadFile, so a granted miss stays an IO error. let and bytes_to_string use howlFrameReadFileBytes. JavaScript returns UTF-8 text after the same check. The interpreter reads only after its existing filesystem check, so the conformance case can include it. The bytecode VM already gates OpReadFile before the read.

tests/conformance/lowered_hfir_abi_v1.json adds read_file_denied and read_file_granted for the interpreter, the bytecode VM, Go, and JavaScript. The fixture reads /tmp/howlframe-abi-v1-phase2c.txt. The conformance test writes phase2c-read-marker there first, because generated Go runs in its own directory.

write_file and mkdir are unchanged. Production -compile-bc is still runHFIRGate then AST bytecode. #90 stays Partial.

Journal: docs/journals/2026-09-30_lowered_hfir_abi_phase2c_read_file.md.

How to test

go test ./internal/backend/gogen/ ./internal/backend/javascript/ ./internal/vm/ -run 'ReadFile|InterpretReadFile'
go test ./tools/difftest/ -run TestLoweredHFIRABIConformance

Denied hosts exit nonzero with CAPABILITY_DENIED and do not print phase2c-read-marker. With -allow-caps filesystem (or HOWLFRAME_ALLOW_CAPS=filesystem for generated hosts) they print that marker.

go test ./... passed locally.

  

[howlcipher]

COMMENT OK on head e3f14715 (CI build green).

Phase 2c scope check: Go+JS mediate (read_file) under existing filesystem via HOWLFRAME_ALLOW_CAPS / howlFrameGrantHas; CAPABILITY_DENIED before os.ReadFile / readFileSync; denial text has no path. Interpreter reads only after its existing filesystem gate (honest granted path for conformance). Conformance read_file_denied / read_file_granted on interpreter / bytecode VM / Go / JS. write_file / mkdir / fetch untouched. -compile-bc still AST path. #90 stays Partial.

Hard nos clean: no half bytecode linker / HFIR module smuggle / VM module opcodes; no new caps or opcodes; don’t reopen #102–#105/#108; no path flip; no batch of remaining host effects.

Okabe: undraft + squash-merge when ready.