download_queue: print fetch heading to stderr when stdout is not a TTY

[carlocab]

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Claude Fable xhigh with manual review and testing.

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Running brew info --installed --json=v2 with a stale cache prints

==> Downloading Homebrew API data

to stdout, which breaks JSON parsing. Let's fix this the same way as #20980.

[MikeMcQuaid]

@carlocab any AI disclosure here?

[Copilot]

Pull request overview

This PR fixes machine-readable command output corruption by ensuring DownloadQueue prints its fetch heading to stderr when stdout is not a TTY (e.g. brew info --json=v2 | jq), while leaving TTY behavior unchanged.

Changes:

• Route DownloadQueue#fetch headings to stderr when @tty is false, keeping parsed stdout clean.
• Add/adjust RSpec coverage to assert heading stream behavior (stderr for non-TTY, stdout for TTY).
• Update cask reinstall specs to expect the “Fetching downloads…” heading on stderr.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/download_queue.rb	Prints the fetch heading to stderr when stdout is non-TTY to avoid corrupting machine-readable stdout.
Library/Homebrew/test/download_queue_spec.rb	Adds specs covering heading output stream behavior for TTY vs non-TTY cases.
Library/Homebrew/test/cask/reinstall_spec.rb	Updates expectations so the fetch heading is asserted on stderr.

Suppressed comments (1)

Library/Homebrew/test/download_queue_spec.rb:174

• This example also depends on stdout being non-TTY at DownloadQueue construction time, but doesn't stub $stdout.tty? before calling enqueue (which instantiates the subject). On a real TTY this will print the heading to stdout, defeating the assertion.

  it "keeps the heading off stdout when stdout is not a TTY" do
    allow(retryable_download).to receive(:fetch).and_return(cached_download)
    download_queue.enqueue(downloadable)

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[carlocab]

@carlocab any AI disclosure here?

Yes. Looks like my sandbox is misconfigured. Hang on.

[carlocab]

@carlocab any AI disclosure here?

Done now!

[MikeMcQuaid]

Thanks! Refactor opportunity I think!

[MikeMcQuaid]

Are there any other headings in this file/around the codebase where we do this (maybe not the same approach but pretty sure there are)? If so, a helper method for this would be nice.