47794: 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 specifies that all commands in a
pipeline run in a subshell, but permits zsh's behavior as an extension.
The default /bin/sh implementations on various Linux distros and the
BSDs always use a subshell for all components of a pipeline.

Since zsh may be used as /bin/sh in some cases (such as macOS Catalina),
it makes sense to have the common sh 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.

1 files changed, 22 insertions, 0 deletions

diff --git a/Test/B07emulate.ztst b/Test/B07emulate.ztst
index 7b1592fa9..45c39b51d 100644
--- a/Test/B07emulate.ztst
+++ b/Test/B07emulate.ztst
@@ -276,3 +276,25 @@ F:Some reserved tokens are handled in alias expansion
 0:--emulate followed by other options
 >yes
 >no
+
+  emulate sh -c '
+  foo () {
+    VAR=foo &&
+    echo $VAR | bar &&
+    echo "$VAR"
+  }
+  bar () {
+    tr f b &&
+    VAR="$(echo bar | tr r z)" &&
+    echo "$VAR"
+  }
+  foo
+  '
+  emulate sh -c 'func() { echo | local def="abc"; echo $def;}; func'
+  emulate sh -c 'abc="def"; echo | abc="ghi"; echo $abc'
+0:emulate sh uses subshell for last pipe entry
+>boo
+>baz
+>foo
+>
+>def