feat(vm): IO — byte/char reads, gets limit/chomp, pread/pwrite, half-close - #347

Horodate: 2026-08-10 10:41 CEST

Advances the IO / File (as IO) / StringIO read+write surface toward MRI 3.4/4.0 on the existing buffer-backed IOObj model, with per-branch tests and a ruby/spec ratchet bump.

What's implemented

Shared (IO, File, StringIO):

• getbyte / readbyte / readchar (EOFError at EOF)
• ungetbyte / ungetc (Integer + String pushback, nil no-op, TypeError)
• each_byte / each (return self)
• sysread (length + output-buffer arg, EOFError, negative → ArgumentError) / syswrite
• gets / readline / readlines / each_line now honour (sep, limit, chomp:), with universal-newline chomp
• #lineno / #lineno= (advanced by each successful line read)
• close_read / close_write / closed_read? / closed_write? (both halves shut ⇒ fully closed; close_write flushes)
• Readable/writable guards: IOError "not opened for reading|writing", "closed stream"

IO + File only (not StringIO):

• pread / pwrite (absolute offset, cursor untouched; Errno::EINVAL, EOFError, FrozenError on a frozen buffer)
• sysseek (returns the resulting position)
• binmode? / autoclose? / autoclose= / fdatasync

Correctness fix: readlines / each_line / each / IO.foreach / IO.readlines now raise ArgumentError "invalid limit: 0 for <meth>" for an explicit 0 limit (a 0-byte line never advances the cursor — the limit support otherwise looped forever on core/io/{each_line,foreach}_spec). A single #gets(0) still returns "". The foreach/readlines class methods also gained (sep, limit) pass-through.

Deferred

• #fileno (no real fd in the buffer-backed model; StringIO returns nil, IO an Integer — fabricating an fd was judged worse than omitting)
• Non-duplex close_read/close_write raise ("closing non-duplex IO")
• IO.select blocking/threaded corners, tty/ioctl

Verification

• go test ./...: PASS (8 packages)
• Whole-package CI-exact coverage gate: only the pre-existing tolerated partials, none new (registerIOClassMethods reached 100%)
• New code 100% incl error branches; gofmt + go vet clean; no prelude.rb change
• ruby/spec ratchet: 12517 passing (baseline 12408, +109); FLOOR bumped 12378 → 12487 (N−30 margin). core/io/{each_line,foreach,readlines} + StringIO specs that previously hung now complete with higher pass counts.

🤖 Generated with Claude Code