Detach Elasticsearch on startup (#859)

With this commit we detach Elasticsearch on startup from its controlling
process via the
[setpgrp](http://man7.org/linux/man-pages/man3/setpgrp.3p.html) system
call. Although we start Elasticsearch with the `-d` flag, this is
necessary to ensure it does not receive signals like `SIGHUP` from its
original parent process (Rally).

esrally/mechanic/launcher.py

@@ -194,7 +194,7 @@ def _start_process(binary_path, env):
         os.chdir(binary_path)
         cmd = [io.escape_path(os.path.join(".", "bin", "elasticsearch"))]
         cmd.extend(["-d", "-p", "pid"])
-        ret = process.run_subprocess_with_logging(command_line=" ".join(cmd), env=env)
+        ret = process.run_subprocess_with_logging(command_line=" ".join(cmd), env=env, detach=True)
         if ret != 0:
             msg = "Daemon startup failed with exit code [{}]".format(ret)
             logging.error(msg)

esrally/utils/process.py

@@ -60,22 +60,29 @@ def exit_status_as_bool(runnable, quiet=False):
         return False
 
 
-def run_subprocess_with_logging(command_line, header=None, level=logging.INFO, env=None):
+def run_subprocess_with_logging(command_line, header=None, level=logging.INFO, env=None, detach=False):
     """
     Runs the provided command line in a subprocess. All output will be captured by a logger.
 
     :param command_line: The command line of the subprocess to launch.
     :param header: An optional header line that should be logged (this will be logged on info level, regardless of the defined log level).
     :param level: The log level to use for output (default: logging.INFO).
     :param env: Use specific environment variables (default: None).
+    :param detach: Whether to detach this process from its parent process (default: False).
     :return: The process exit code as an int.
     """
     logger = logging.getLogger(__name__)
     logger.debug("Running subprocess [%s] with logging.", command_line)
     command_line_args = shlex.split(command_line)
+    pre_exec = os.setpgrp if detach else None
     if header is not None:
         logger.info(header)
-    with subprocess.Popen(command_line_args, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, env=env) as command_line_process:
+    # pylint: disable=subprocess-popen-preexec-fn
+    with subprocess.Popen(command_line_args,
+                          stdout=subprocess.PIPE,
+                          stderr=subprocess.STDOUT,
+                          env=env,
+                          preexec_fn=pre_exec) as command_line_process:
         has_output = True
         while has_output:
             line = command_line_process.stdout.readline()