Hang when piping significant output from a built-in command to an external command

[reubeno]

This easily reproduces by running something like declare | wc -l. Here's what's happening:

• We synchronously invoke the declare built-in, with its stdout mapped to the writer end of a pipe we allocate.
• declare writes quite a bit and, presumably, fills up the pipe's internal buffer. This causes declare to block on its next write.
• wc -l hasn't yet been invoked, so nothing is reading from the reader end of the pipe. This means nothing is freeing up space in the pipe's internal buffer and nothing will unblock declare.

In the case above, wc needs to be spawned before the declare built-in has completed.