Close stdin immediately when using popen2e#9540
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May 12, 2026
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Pull request overview
This PR updates Gem::Ext::Builder.run to proactively close the child process STDIN pipe created by Open3.popen2e, preventing subprocess trees from blocking on STDIN reads during native extension builds (e.g., extconf.rb → cargo build → credential helper).
Changes:
- Close the
stdinIO immediately inside theOpen3.popen2eblock inGem::Ext::Builder.run. - Add a regression test asserting that
Gem::Ext::Builder.runprovides an immediately-EOF STDIN to the child process.
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Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| lib/rubygems/ext/builder.rb | Closes stdin immediately after spawning the build command to avoid hangs from downstream STDIN reads. |
| test/rubygems/test_gem_ext_builder.rb | Adds a test that verifies STDIN is closed (EOF) for commands run via Gem::Ext::Builder.run. |
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| Gem::Ext::Builder.run([Gem.ruby, "-e", check_stdin_script], results) | ||
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| command_output = results.last | ||
| assert_equal "STDIN: \"\"\n", command_output | ||
| end |
hsbt
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May 14, 2026
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@rwstauner Thanks always. I'm not sure why |
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It's good hygiene to close the stdin pipe as soon as you are done writing to it. This can happen for example if "ruby extconf.rb" spawns another process (for example "cargo build", which may spawn arbitrary commands to fetch credentials) and any of those subprocesses attempt to read STDIN until it is closed. We can close it as soon as it's created since we aren't writing to it at all.
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I rebased and it's green now. |
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It's good hygiene to close the stdin pipe as soon as you are done writing to it.
This can happen for example if "ruby extconf.rb" spawns another process (for example "cargo build", which may spawn arbitrary commands to fetch credentials) and any of those subprocesses attempt to read STDIN until it is closed.
We can close it as soon as it's created since we aren't writing to it at all.
What was the end-user or developer problem that led to this PR?
Our CI was hanging and timing out at the "bundle install" step because a gem's extconf.rb calls
cargo buildwhich calls a custom command to fetch credentials and that command tries to read STDIN when it detects that it is given a pipe.What is your fix for the problem, implemented in this PR?
Close the STDIN pipe created by Open3 so that if a target command tries to read the pipe it will finish.
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