[PATCH v2] scripts/check-local-export: avoid 'wait $!' for process substitution

[Masahiro Yamada]

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@xxxxxxxxxxxxxxxxxxx>
Reported-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Reported-by: Wang Yugui <wangyugui@xxxxxxxxxxxx>
Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Tested-by: Jon Hunter <jonathanh@xxxxxxxxxx>
Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
---

Changes in v2:
  - Add bash to Documentation/process/changes.rst

 Documentation/process/changes.rst | 12 +++++++++++
 scripts/check-local-export        | 35 ++++++++++++++++++-------------
 2 files changed, 32 insertions(+), 15 deletions(-)

diff --git a/Documentation/process/changes.rst b/Documentation/process/changes.rst
index 34415ae1af1b..19c286c23786 100644
--- a/Documentation/process/changes.rst
+++ b/Documentation/process/changes.rst
@@ -32,6 +32,7 @@ you probably needn't concern yourself with pcmciautils.
 GNU C                  5.1              gcc --version
 Clang/LLVM (optional)  11.0.0           clang --version
 GNU make               3.81             make --version
+bash                   4.2              bash --version
 binutils               2.23             ld -v
 flex                   2.5.35           flex --version
 bison                  2.0              bison --version
@@ -84,6 +85,12 @@ Make
 
 You will need GNU make 3.81 or later to build the kernel.
 
+Bash
+----
+
+Some bash scripts are used for the kernel build.
+Bash 4.2 or newer is needed.
+
 Binutils
 --------
 
@@ -362,6 +369,11 @@ Make
 
 - <ftp://ftp.gnu.org/gnu/make/>
 
+Bash
+----
+
+- <ftp://ftp.gnu.org/gnu/bash/>
+
 Binutils
 --------
 
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