Prefix async job output to prevent external corruption

If an external process interferes with the async worker (e.g. writes its
pty), the output can become corrupt. To minimize this occurance we now
prefix all job output with the NULL character, instead of just separate
on NULLs (suffix).

async.zsh

@@ -36,7 +36,7 @@ _async_job() {
 		read -r -k 1 -p tok || exit 1
 
 		# Return output (<job_name> <return_code> <stdout> <duration> <stderr>).
-		print -r -n - ${(q)1} $ret ${(q)stdout} $duration
+		print -r -n - $'\0'${(q)1} $ret ${(q)stdout} $duration
 	} 2> >(stderr=$(cat) && print -r -n - " "${(q)stderr}$'\0')
 
 	# Unlock mutex by inserting a token.
@@ -245,13 +245,15 @@ async_process_results() {
 			if (( $#items == 5 )); then
 				items+=($has_next)
 				$callback "${(@)items}"  # Send all parsed items to the callback.
+				(( num_processed++ ))
+			elif [[ -z $items ]]; then
+				# Empty items occur between results due to double-null ($'\0\0')
+				# caused by commands being both pre and suffixed with null.
 			else
 				# In case of corrupt data, invoke callback with *async* as job
 				# name, non-zero exit status and an error message on stderr.
 				$callback "async" 1 "" 0 "$0:$LINENO: error: bad format, got ${#items} items (${(q)items})" $has_next
 			fi
-
-			(( num_processed++ ))
 		done
 	done
 

async_test.zsh

@@ -7,8 +7,8 @@ test__async_job_print_hi() {
 	local line
 	local -a out
 	line=$(_async_job print hi)
-	# Remove trailing null, parse, unquote and interpret as array.
-	line=$line[1,$#line-1]
+	# Remove leading/trailing null, parse, unquote and interpret as array.
+	line=$line[2,$#line-1]
 	out=("${(@Q)${(z)line}}")
 
 	coproc exit
@@ -396,8 +396,10 @@ test_async_flush_jobs() {
 	# TODO: Confirm that they no longer exist in the process tree.
 	local output
 	output="${(Q)$(ASYNC_DEBUG=1 async_flush_jobs test)}"
-	[[ $output = *'print_four 0 4'* ]] || {
-		t_error "want discarded output 'print_four 0 4' when ASYNC_DEBUG=1, got ${(Vq-)output}"
+	# NOTE(mafredri): First 'p' in print_four is lost when null-prefixing
+	# _async_job output.
+	[[ $output = *'rint_four 0 4'* ]] || {
+		t_error "want discarded output 'rint_four 0 4' when ASYNC_DEBUG=1, got ${(Vq-)output}"
 	}
 
 	# Check that the killed job did not produce output.