brush_core::interp::setup_open_file_with_contents wrote the entire heredoc/here-string body into an anonymous pipe synchronously before handing the reader to the downstream command. Bodies that exceed the OS pipe buffer (~4 KiB on Windows, 16-64 KiB on macOS) deadlocked the writer forever, and the bash tool tripped its 305 s hard timeout without ever launching the consumer. The Linux fast path still uses F_SETPIPE_SZ to grow the pipe inline; every other platform (and Linux bodies that overflow pipe-max-size) now decouples the write onto a fire-and-forget thread that terminates on drain or BrokenPipe.
Adds a 256 KiB regression test that exercises the worst-case shape (: builtin, which never drains stdin), guarded by tokio::time::timeout(10s) so a regression fails CI fast instead of hanging.
oldschoola
ae927b6f81