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.

ChangeLog

@@ -5,6 +5,10 @@
 
 2021-04-10  dana  <dana@dana.is>
 
+	* brian m. carlson: 47794: README, Src/exec.c,
+	Test/B07emulate.ztst: exec: run final pipeline command in a
+	subshell in sh mode
+
 	* unposted (see 48415): README: Document incompatibility
 	caused by workers/43928
 

README

@@ -95,6 +95,10 @@ right thing, but this should be viewed as an unsupported hack.)
 The XTRACE option is now disabled while running user-defined completion
 widgets.  See NEWS.
 
+emulate sh: When zsh emulates sh, the final command in a pipeline is now run in
+a subshell.  This differs from the behavior in the native (zsh) mode, but is
+consistent with most other sh implementations.
+
 Incompatibilities between 5.7.1 and 5.8
 ---------------------------------------
 

Src/exec.c

@@ -2901,11 +2901,13 @@ execcmd_exec(Estate state, Execcmd_params eparams,
 	    pushnode(args, dupstring("fg"));
     }
 
-    if ((how & Z_ASYNC) || output) {
+    if ((how & Z_ASYNC) || output ||
+	(last1 == 2 && input && EMULATION(EMULATE_SH))) {
 	/*
-	 * If running in the background, or not the last command in a
-	 * pipeline, we don't need any of the rest of this function to
-	 * affect the state in the main shell, so fork immediately.
+	 * If running in the background, not the last command in a
+	 * pipeline, or the last command in a multi-stage pipeline
+	 * in sh mode, we don't need any of the rest of this function
+	 * to affect the state in the main shell, so fork immediately.
 	 *
 	 * In other cases we may need to process the command line
 	 * a bit further before we make the decision.

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