Use flock if available, otherwise use perl(!)

pre-commit-hadolint

@@ -12,18 +12,25 @@ HADOLINT_URL="${HADOLINT_PREFIX_URL}/${HADOLINT_VERSION}/${HADOLINT_BINARY}"
 DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
 LOCAL_BINARY="${DIR}/${HADOLINT_BINARY}-${HADOLINT_VERSION}"
 
+function mylock() {
+    # *sigh*
+    # macOS does not have flock so fall back to perl(!) if it's not
+    # available
+    if command -v flock &>/dev/null; then
+        flock -x 200
+    else
+        perl -e 'use Fcntl qw(:flock); open($fh, "<&=", 200) || die "Cannot open"; flock($fh, LOCK_EX) || die "Cannot lock"'
+    fi
+}
+
 # Download the binary if it doesn't already exist
-if [[ ! -x ${LOCAL_BINARY} ]]; then
-    # use mkdir as a lock so only single download happens
-    # can't use flock as that doesn't exist on macOS
-    if mkdir "${LOCAL_BINARY}.lock" 2>/dev/null; then
+(
+    mylock
+    if [[ ! -x ${LOCAL_BINARY} ]]; then
         curl -o "${LOCAL_BINARY}" -L -sSf "${HADOLINT_URL}"
         chmod 755 "${LOCAL_BINARY}"
-    else
-        # another process is downloading, give it time to complete
-        sleep 5
     fi
-fi
+) 200> "${LOCAL_BINARY}.lock"
 
 files=()