[PATCH] scripts/check-local-export: avoid 'wait $!' for process substitution

From: Masahiro Yamada <masahiroy@kernel.org>
To: linux-kbuild@vger.kernel.org
Cc: Masahiro Yamada <masahiroy@kernel.org>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Wang Yugui <wangyugui@e16-tech.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Tom Rix <trix@redhat.com>,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev
Subject: [PATCH] scripts/check-local-export: avoid 'wait $!' for process substitution
Date: Tue,  7 Jun 2022 17:43:17 +0900	[thread overview]
Message-ID: <20220607084317.211785-1-masahiroy@kernel.org> (raw)

Bash>=4.4 supports 'wait $!' to check the exit status of a process
substitution, but some people using older bash versions reported an
error like this:

  ./scripts/check-local-export: line 54: wait: pid 17328 is not a child of this shell

I used the process substitution because a pipeline executes each command
in a subshell; variables modified within the while-loop in the subshell
context would be lost after the subshell terminates.

Fortunately, Bash>=4.2 supports the 'lastpipe' option, which runs the
last element of a pipeline in the current shell process.

Also, set 'pipefail' to catch errors from ${NM}.

Bash 4.2, released in 2011, is 5 years older than Bash 4.4.

Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 scripts/check-local-export | 35 ++++++++++++++++++++---------------
 1 file changed, 20 insertions(+), 15 deletions(-)

diff --git a/scripts/check-local-export b/scripts/check-local-export
index da745e2743b7..e21c7b54885d 100755
--- a/scripts/check-local-export
+++ b/scripts/check-local-export
@@ -8,11 +8,30 @@
 
 set -e
 
+# catch errors from ${NM}
+set -o pipefail
+
+# Run the last element of a pipeline in the current shell.
+# Without this, the while-loop would be executed in a subshell, and
+# the changes made to 'symbol_types' and 'export_symbols' would be lost.
+shopt -s lastpipe
+
 declare -A symbol_types
 declare -a export_symbols
 
 exit_code=0
 
+# If there is no symbol in the object, ${NM} (both GNU nm and llvm-nm) shows
+# 'no symbols' diagnostic (but exits with 0). It is harmless and hidden by
+# '2>/dev/null'. However, it suppresses real error messages as well. Add a
+# hand-crafted error message here.
+#
+# Use --quiet instead of 2>/dev/null when we upgrade the minimum version of
+# binutils to 2.37, llvm to 13.0.0.
+#
+# Then, the following line will be really simple:
+#   ${NM} --quiet ${1} |
+{ ${NM} ${1} 2>/dev/null || { echo "${0}: ${NM} failed" >&2; false; } } |
 while read value type name
 do
 	# Skip the line if the number of fields is less than 3.
@@ -37,21 +56,7 @@ do
 	if [[ ${name} == __ksymtab_* ]]; then
 		export_symbols+=(${name#__ksymtab_})
 	fi
-
-	# If there is no symbol in the object, ${NM} (both GNU nm and llvm-nm)
-	# shows 'no symbols' diagnostic (but exits with 0). It is harmless and
-	# hidden by '2>/dev/null'. However, it suppresses real error messages
-	# as well. Add a hand-crafted error message here.
-	#
-	# Use --quiet instead of 2>/dev/null when we upgrade the minimum version
-	# of binutils to 2.37, llvm to 13.0.0.
-	#
-	# Then, the following line will be really simple:
-	#   done < <(${NM} --quiet ${1})
-done < <(${NM} ${1} 2>/dev/null || { echo "${0}: ${NM} failed" >&2; false; } )
-
-# Catch error in the process substitution
-wait $!
+done
 
 for name in "${export_symbols[@]}"
 do
-- 
2.32.0

