Do not stop the zstream loop for an interrupt

[bruno-costanzo]

#121 fixed the Zlib::BufError from #57, but its retry condition lets Zlib::Deflate#finish return before the stream ends. This is the iteration @ioquatix mentioned in #74: the same idea as #74 (@ianks), taken one step further.

Problem

In zlib 3.2.4 (and in Ruby 3.4.9 and later, which ship #121 as zlib "3.2.3"), when a thread is interrupted during a deflate, the output can be truncated:

• Zlib.deflate returns truncated output and raises nothing.
• Zlib::Deflate#finish returns truncated output. A later close raises Zlib::DataError, because deflateEnd sees an unfinished stream.
• Zlib::GzipWriter#close writes truncated output, and then usually raises Zlib::DataError.

An interrupt is Thread#wakeup, or a signal such as SIGCHLD from a backtick in another thread. This partly brings back Bug #10961 ("Zlib corrupts data when receive signal"), which #22 fixed with the retry.

Reproduction (ruby 4.0.6, arm64-darwin, libz 1.3.1):

require "zlib"
require "securerandom"

content = SecureRandom.random_bytes(256 * 1024)
outputs = []

thr = Thread.new do
  100.times do
    z = Zlib::Deflate.new
    outputs << (z.deflate(content) << z.finish)
    z.close
  end
end

while thr.alive?
  begin
    thr.wakeup
  rescue ThreadError
  end
  Thread.pass
end
thr.join

p outputs.count { |output| Zlib::Inflate.inflate(output) == content }

• zlib 3.2.3 gem: Zlib::BufError from deflate (Thread#wakeup leads to Zlib::BufError #57), 20 of 20 runs.
• zlib 3.2.4 and master: Zlib::DataError from close, 20 of 20 runs. Same on Ruby 3.4.11's bundled zlib.
• This branch: prints 100, 20 of 20 runs.

Cause

Since #88, each deflate() and inflate() call runs in its own rb_nogvl, and rb_nogvl handles pending interrupts when the call returns. zstream_run_func still stops its loop when the unblock function sets args->interrupt, and zstream_run_try then guesses whether to resume:

• Before Fix buf error on interrupt. #121 it always resumed. A resume after a call that already finished raised Z_BUF_ERROR (Thread#wakeup leads to Zlib::BufError #57).
• Since Fix buf error on interrupt. #121 it resumes only when avail_in > 0 && avail_out > 0. Under Z_FINISH, deflate can still have pending output with avail_in == 0. Then finish returns before Z_STREAM_END.

Fix

The loop no longer stops for an interrupt, and the resume block is removed. Interrupts still reach Ruby after each call through rb_nogvl. The unblock function still sets args->interrupt, but nothing reads it now. I kept the field to make the diff small and can remove it if you prefer.

Checked on this branch:

• Thread#raise, Thread#kill, Timeout.timeout and a raising trap still stop deflate, inflate, Zlib.gzip and GzipWriter within about 60 ms.
• With a Thread#wakeup storm or about 1,700 SIGUSR1 with a non-raising trap, every API gives correct output, with no slowdown: Deflate with each flush mode, Zlib.deflate, Zlib.gzip, GzipWriter, Zlib.inflate, chunked Inflate, Zlib.gunzip, and GzipReader#read and #readpartial.
• The SIGINT/fork reproduction from Thread#wakeup leads to Zlib::BufError #57 passes 30 of 30 runs.

Test

TestZlibDeflate#test_thread_wakeup_interrupt deflates 256 KiB of random data 100 times in a thread that is woken up in a loop, then inflates every output. It runs in less than 0.5 s and is pended on TruffleRuby, like the existing interrupt tests.

• Without the fix it fails on every run: Zlib::DataError from close. I checked this on macOS and on Linux (ruby 3.4.11, aarch64).
• With the fix, bundle exec rake compile test passes on Ruby 4.0.6, 3.4.8, 3.3.10 and 3.2.9 (99 tests, 639 assertions on macOS). The existing test_thread_wakeup_interrupt tests in TestZlib and TestZlibGzipReader still pass.

Ruby's ruby_3_4 branch has #121, so it needs this change too.

[Copilot]

🔵 Needs a closer look

The C-level change affects Ruby VM interrupt handling across all deflate and inflate paths and warrants final human review.

0 open findings

What changed in this PR

Fixes truncated deflate output when non-raising interrupts occur during stream processing.

Changes:

• Continues zstream processing after interrupts until normal termination.
• Adds regression coverage for repeated Thread#wakeup calls.

File	Description
ext/​zlib/​zlib.c	Removes interrupt-based loop termination and retry logic.
test/​zlib/​test_zlib.rb	Verifies interrupted deflate streams remain complete.

🧠 Review effort: Balanced

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