Drain command output before waiting for the child

[OskarEichler]

Summary

Read the pipe before waiting for the child, so a full stdout pipe cannot deadlock native pkg-config discovery.

Reproduction and verification

PackageConfig.send(:run_command, RbConfig.ruby, "-e", 'STDOUT.write("x" * 1048576)') does not finish on baseline: the parent waits for exit while the writer waits for pipe space. The isolated reproduction used a three-second deadline and killed/waited for the entire process group. Empty/small output succeeds on both versions. The candidate returns all 1 MiB; a writer that emits 1 MiB then exits 7 returns nil as before.

Four isolated process cases pass; both large-output baseline cases exceeded the deadline.

Existing suite on Ruby 4.0.6 through rbenv: 27 tests, 31 assertions, zero failures/errors, two upstream macOS omissions, baseline and this isolated patch. Combined installed-release consumer also passes the suite, 1,098 focused/model checks, four process cases, five linker-option cases and four path cases. rbenv exec ruby test/run.rb runs the existing suite.

Compatibility and limitations

No API or successful-output change. Nonzero exits still return nil. This fixes the full-pipe deadlock, not arbitrary hanging commands; no command timeout or streaming API is added.

No tests were added or modified under the consuming repository's no-new-tests policy. Verification uses existing tests and external scratch reproductions. No production changes; older Ruby/Linux/Windows matrices await upstream CI. No unrelated version/dependency changes.

[kou]

Do you have any real-world problem by this?

[OskarEichler]

No; this came from the broad dependency audit rather than a real-world failure. I do not have enough project-specific motivation to ask you to carry the change, so I am closing it.