syntax-check: Introduce sc_prohibit_local_with_subshell rule

In shell, the following function doesn't echo '1' but '0':

  func() {
      local var=$(false)
      echo $?
  }

This is because '$?' does not refer to 'false' but 'local'. The
bash_builtins(1) manpage explains it well. And it also mentions
other commands behaving the same: export, declare and readonly.
Since it is really easy to miss this pattern, introduce a
syntax-check rule. Mind you, the following pattern (which passes
the rule) does check for the subshell exit code:

  func() {
      local var
      var=$(false)
      echo $?
  }

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

diff --git a/build-aux/syntax-check.mk b/build-aux/syntax-check.mk
index f605c9b0e3c59a0019adb3193fb46414ba12f081..022e8e6550c5367bdec8cbdcd55a651ee50c2ecd 100644 (file)
--- a/build-aux/syntax-check.mk
+++ b/build-aux/syntax-check.mk
@@ -394,6 +394,17 @@ sc_prohibit_g_autofree_const:
        halt='‘g_autofree’ discards ‘const’ qualifier from pointer target type' \
          $(_sc_search_regexp)
 
+sc_prohibit_local_with_subshell:
+       @err=0; for f in $$($(VC_LIST_EXCEPT) | $(GREP) -E '\.sh(\.in)?$$'); do \
+         lines=$$( $(AWK) \
+         '/^\s*(local|export|declare|readonly)\s+.*=/ { nr=NR; c=1; next } \
+         /^\s*$$/ { if (c) next } \
+         /\$$\?/ { if (c) { print nr; c=0; } next } \
+         { c=0 }' $$f ) ; \
+         for l in $$lines; do echo $$f:$$l 1>&2; err=1; done ; \
+       done; \
+       test $$err -eq 0 || { msg="Declare and assign separately to avoid masking return values" $(_sc_say_and_exit) }
+
 
 # Many of the function names below came from this filter:
 # git grep -B2 '\<_('|grep -E '\.c- *[[:alpha:]_][[:alnum:]_]* ?\(.*[,;]$' \