popen: add ability to keep child on deletion

Currently popen_delete kills all children process.
Moreover we use popen_delete on tarantool exit.

Alexander pointed out that keep children running
even if tarantool is exited is still needed.

Part of #4031

Reported-by: Alexander Turenko <alexander.turenko@tarantool.org>
Acked-by: Alexander Turenko <alexander.turenko@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

src/lib/core/popen.c

@@ -585,12 +585,15 @@ popen_delete(struct popen_handle *handle)
 
 	assert(handle != NULL);
 
-	/*
-	 * Unable to kill the process -- give an error.
-	 * The process is not exist already -- pass over.
-	 */
-	if (popen_send_signal(handle, SIGKILL) != 0 && errno != ESRCH)
-		return -1;
+	if ((handle->flags & POPEN_FLAG_KEEP_CHILD) == 0) {
+		/*
+		 * Unable to kill the process -- give an error.
+		 * The process is not exist already -- pass over.
+		 */
+		if (popen_send_signal(handle, SIGKILL) != 0 &&
+		    errno != ESRCH)
+			return -1;
+	}
 
 	for (i = 0; i < lengthof(handle->ios); i++) {
 		if (handle->ios[i].fd != -1)

src/lib/core/popen.h

@@ -102,6 +102,12 @@ enum popen_flag_bits {
 	 */
 	POPEN_FLAG_GROUP_SIGNAL_BIT	= 16,
 	POPEN_FLAG_GROUP_SIGNAL		= (1 << POPEN_FLAG_GROUP_SIGNAL_BIT),
+
+	/*
+	 * Keep child running on delete.
+	 */
+	POPEN_FLAG_KEEP_CHILD_BIT	= 17,
+	POPEN_FLAG_KEEP_CHILD		= (1 << POPEN_FLAG_KEEP_CHILD_BIT),
 };
 
 /**