Show a build progress spinner

[HaraldNordgren]

Without --verbose, build commands only write to their logs, so brew shows nothing while they run. A long build such as llvm prints ==> cmake --build build and then nothing for a long time, so it looks stuck. Unpacking a large source tarball and configure steps can be silent for a while too. The only way to tell it is still working is to find and tail the logs in ~/Library/Logs/Homebrew/<formula>.

This keeps a spinner below the build output for the whole build, so it never looks stuck. During a step without progress markers, e.g. CMake's configure step:

==> Upgrading ccache dependency: llvm (22.1.8 -> 23.1.3_1)
==> Patching
==> Applying llvm/pull-111397.diff
patching file clang/lib/Driver/Driver.cpp
Hunk #1 succeeded at 1461 (offset -30 lines).
Hunk #2 succeeded at 1508 (offset -30 lines).
Hunk #3 succeeded at 1568 (offset -30 lines).
patching file clang/test/Driver/Inputs/config-cl/config-arm64ec-arg.cfg
patching file clang/test/Driver/config-file.c
==> cmake -S /private/tmp/s-E9aY5M2F/llvm-20261014-36487-a8o0ru/llvm-project-23.1.3.src/llvm -B build -G Ninja -DLLVM_ENABLE_PROJECTS=clang;clang-tools-extra;mlir;polly -DLLVM_ENABLE_RUNT
⠦

And when the current command prints Ninja ([12/345]) or CMake ([ 42%]) progress markers:

==> cmake -S /private/tmp/s-E9aY5M2F/llvm-20261014-36487-a8o0ru/llvm-project-23.1.3.src/llvm -B build -G Ninja -DLLVM_ENABLE_PROJECTS=clang;clang-tools-extra;mlir;polly -DLLVM_ENABLE_RUNT
==> cmake --build build
⠓ 63/9135 (0%)

Sandboxed builds already run in a pseudo-terminal whose output the parent brew process relays in Sandbox#copy_pty_output. For builds, BuildProgress#forward takes over that relay: it clears the status line before each chunk of output and redraws it afterwards, so the ==> lines, patch output and warnings always print above it. It reuses the download queue's spinner.

The step progress is read from the tail of the newest log in formula.logs, so Formula#system is unchanged and build commands still write straight to their logs. Compiler wrapper (*.cc.log) and sandbox logs are skipped, as are logs from before the build started.

While a build runs, Ctrl-C goes to the build's pseudo-terminal rather than to brew, so the parent only sees the ^C it echoes. Once that appears the status line stops being drawn, as the progress no longer matters while the build is being interrupted.

Nothing changes with --verbose, --debug or --interactive, when output isn't a terminal or TERM=dumb, or for builds that don't run in the sandbox.

To see it, build any formula from source that uses CMake or Ninja, e.g. brew install --build-from-source llvm.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Code (Claude Opus 5.5) wrote the change, its tests and this description. Verified with brew typecheck, brew style, the affected specs and a source build of llvm on macOS 12 for the examples above.

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[MikeMcQuaid]

Sorry, passing on this. Building from source is not a supported user configuration and don't want to add dedicated code here when we already have e.g. --verbose for more explicit output.

[HaraldNordgren]

@MikeMcQuaid Fair enough.