[wrapper] Make cc_wrapper.sh POSIX-compliant and use /bin/sh

Some Linux distributions, such as NixOS, do not provide /bin/bash. This
commit aims to make the LLVM toolchain compatible with these platforms
by only using POSIX-compatible features in the shell script and
replacing the shebang with #!/bin/sh .

This commit does not update the corresponding MacOS script, as all MacOS
builds should have /bin/bash.

Supercedes #543.

Author: Ryan Torok

toolchain/cc_wrapper.sh.tpl

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 #
 # Copyright 2021 The Bazel Authors. All rights reserved.
 #
@@ -16,14 +16,12 @@
 
 # shellcheck disable=SC1083
 
-set -euo pipefail
+set -euo
 
-CLEANUP_FILES=()
-
-function cleanup() {
-  if [[ ${#CLEANUP_FILES[@]} -gt 0 ]]; then
-    rm -f "${CLEANUP_FILES[@]}"
-  fi
+cleanup() {
+    while read -r f; do
+        rm -f "${f}";
+    done < "${CLEANUP_FILES}"
 }
 
 trap cleanup EXIT
@@ -50,13 +48,13 @@ trap cleanup EXIT
 #
 
 dirname_shim() {
-  local path="$1"
+  path="$1"
 
   # Remove trailing slashes
   path="${path%/}"
 
   # If there's no slash, return "."
-  if [[ "${path}" != */* ]]; then
+  if [ "${path}" != "*/*" ]; then
     echo "."
     return
   fi
@@ -68,74 +66,85 @@ dirname_shim() {
   echo "${path:-/}"
 }
 
-script_dir=$(dirname_shim "${BASH_SOURCE[0]}")
+script_dir=$(dirname_shim "$0")
 toolchain_path_prefix="%{toolchain_path_prefix}"
 
 # Sometimes this path may be an absolute path in which case we dont do anything because
 # This is using the host toolchain to build.
-if [[ ${toolchain_path_prefix} != /* ]]; then
+if [ "${toolchain_path_prefix}" != "/*" ]; then
   toolchain_path_prefix="${script_dir}/../../${toolchain_path_prefix#external/}"
 fi
 
-if [[ ! -f ${toolchain_path_prefix}bin/clang ]]; then
+if [ ! -f "${toolchain_path_prefix}bin/clang" ]; then
   echo >&2 "ERROR: could not find clang; PWD=\"${PWD}\"; PATH=\"${PATH}\"; toolchain_path_prefix=${toolchain_path_prefix}."
   exit 5
 fi
 
 OUTPUT=
 
-function parse_option() {
-  local -r opt="$1"
-  if [[ "${OUTPUT}" = "1" ]]; then
-    OUTPUT=${opt}
-  elif [[ "${opt}" = "-o" ]]; then
+parse_option() {
+  po_opt="$1"
+  if [ "${OUTPUT}" = "1" ]; then
+    OUTPUT=${po_opt}
+  elif [ "${po_opt}" = "-o" ]; then
     # output is coming
     OUTPUT=1
   fi
 }
 
-function sanitize_option() {
-  local -r opt=$1
-  if [[ ${opt} == */cc_wrapper.sh ]]; then
+sanitize_option() {
+  so_opt="$1"
+  if [ "${so_opt}" = "*/cc_wrapper.sh" ]; then
     printf "%s" "${toolchain_path_prefix}bin/clang"
-  elif [[ ${opt} =~ ^-fsanitize-(ignore|black)list=[^/] ]] && [[ ${script_dir} == /* ]]; then
+  elif eval "case $so_opt in *^-fsanitize-(ignore|black)list=[^/]*) [ $script_dir == /* ] ;; esac"; then
+    # Split flag name and value.
+    #
     # shellcheck disable=SC2206
-    parts=(${opt/=/ }) # Split flag name and value into array.
-    printf "%s" "${parts[0]}=${script_dir}/../../../${parts[1]}"
+    part0=$(echo "$so_opt" | cut -d '=' -f 1)
+    part1=$(echo "$so_opt" | cut -d '=' -f 2)
+    printf "%s" "${part0}=${script_dir}/../../../${part1}"
   else
-    printf "%s" "${opt}"
+    printf "%s" "${so_opt}"
   fi
 }
 
-cmd=()
-for ((i = 0; i <= $#; i++)); do
-  if [[ ${!i} == @* && -r "${i:1}" ]]; then
+COUNT=$#
+i=0
+while [ $i -le $COUNT ]; do
+  temp=""
+  eval "temp=\${$i}"
+  substr="${temp#?}"
+  if eval "case $temp in @*) [ -r $substr ] ;; esac"; then
     # Create a new, sanitized file.
     tmpfile=$(mktemp)
-    CLEANUP_FILES+=("${tmpfile}")
+    # POSIX shell does not support arrays, so we write the cleanup files as an
+    # array-separated list. We do not need to worry about spaces in filenames,
+    # because `mktemp` cannot use them when using the default template.
+    CLEANUP_FILES="${CLEANUP_FILES} ${tmpfile}"
     while IFS= read -r opt; do
       opt="$(
         set -e
         sanitize_option "${opt}"
       )"
       parse_option "${opt}"
       echo "${opt}" >>"${tmpfile}"
-    done <"${!i:1}"
-    cmd+=("@${tmpfile}")
+    done <"$substr"
+    cmd="${cmd} ${tmpfile}"
   else
     opt="$(
       set -e
-      sanitize_option "${!i}"
+      sanitize_option "${temp}"
     )"
     parse_option "${opt}"
-    cmd+=("${opt}")
+    cmd="$cmd ${opt}"
   fi
+  i=$(( i + 1 ))
 done
 
 # Call the C++ compiler.
-"${cmd[@]}"
+${cmd}
 
 # Generate an empty file if header processing succeeded.
-if [[ "${OUTPUT}" == *.h.processed ]]; then
-  echo -n >"${OUTPUT}"
+if [ "${OUTPUT}" = "*.h.processed" ]; then
+  true >"${OUTPUT}"
 fi