[PATCH v2] exec: run final pipeline command in a subshell in sh mode

zsh typically runs the final command in a pipeline in the main shell instead of a subshell. However, POSIX requires that all commands in a pipeline run in a subshell, but permits zsh's behavior as an extension.

Since zsh may be used as /bin/sh in some cases (such as macOS Catalina), it makes sense to have the POSIX behavior when emulating sh, so do that by checking for being the final item of a multi-item pipeline and creating a subshell in that case.

From the comment above execpline(), we know the following:

  last1 is a flag that this command is the last command in a shell that
  is about to exit, so we can exec instead of forking.  It gets passed
  all the way down to execcmd() which actually makes the decision.  A 0
  is always passed if the command is not the last in the pipeline. […]
  If last1 is zero but the command is at the end of a pipeline, we pass
  2 down to execcmd().

So there are three cases to consider in this code:

• last1 is 0, which means we are not at the end of a pipeline, in which
  case we should not change behavior.
• last1 is 1, which means we are effectively running in a subshell,
  because nothing that happens due to the exec is going to affect the
  actual shell, since it will have been replaced.  So there is nothing
  to do here.
• last1 is 2, which means our command is at the end of the pipeline, so
  in sh mode we should create a subshell by forking.

input is nonzero if the input to this process is a pipe that we've opened. At the end of a multi-stage pipeline, it will necessarily be nonzero.

Note that several of the tests may appear bizarre, since most developers
do not place useless variable assignments directly at the end of a
pipeline.  However, as the function tests demonstrate, there are cases
where assignments may occur when a shell function is used at the end of
a command.  The remaining assignment tests simply test additional cases,
such as the use of local, that would otherwise be untested.
---
 Src/exec.c           | 10 ++++++----
 Test/B07emulate.ztst | 22 ++++++++++++++++++++++
 2 files changed, 28 insertions(+), 4 deletions(-)