java: Get stdout/stderr from fdtransfer

[Jongy]

fdtransfer works by fork()ing, returning control to its parent while the child process waits until
async-profiler connects. The child inherits the standard stream fds, preventing subprocess's communicate
from returning when the process exits. I added close() calls and now it works.

The rationale - I'm seeing errors in _run_fdtransfer but I'm unable to debug because all I get is:

Traceback (most recent call last):
  File "/app/gprofiler/profilers/profiler_base.py", line 139, in snapshot
    result = future.result()
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 437, in result
    return self.__get_result()
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 389, in __get_result
    raise self._exception
  File "/usr/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/app/gprofiler/profilers/java.py", line 953, in _profile_process
    stackcollapse = self._profile_ap_process(ap_proc, comm)
  File "/app/gprofiler/profilers/java.py", line 958, in _profile_ap_process
    started = ap_proc.start_async_profiler(self._interval, ap_timeout=self._ap_timeout)
  File "/app/gprofiler/profilers/java.py", line 519, in start_async_profiler
    self._run_fdtransfer()
  File "/app/gprofiler/profilers/java.py", line 506, in _run_fdtransfer
    run_process(
  File "/app/gprofiler/utils/__init__.py", line 265, in run_process
    raise CalledProcessError(retcode, process.args, output=stdout, stderr=stderr)
gprofiler.exceptions.CalledProcessError: Command '['/app/gprofiler/resources/java/fdtransfer', '54327']' returned non-zero exit status 1. 
stdout: None
stderr: None

with this PR, and this small patch on fdtransfer:

diff --git a/src/fdtransfer/fdtransferServer.cpp b/src/fdtransfer/fdtransferServer.cpp
index fad31b6..bf5a7b5 100644
--- a/src/fdtransfer/fdtransferServer.cpp
+++ b/src/fdtransfer/fdtransferServer.cpp
@@ -53,7 +53,7 @@ int FdTransferServer::_peer;
 
 bool FdTransferServer::bindServer(struct sockaddr_un *sun, socklen_t addrlen, int accept_timeout) {
     _server = socket(AF_UNIX, SOCK_SEQPACKET, 0);
-    if (_server == -1) {
+    if (_server == -1 || 1) {
         perror("FdTransfer socket()");
         return false;

I'm now getting:

Traceback (most recent call last):
  File "/app/gprofiler/profilers/profiler_base.py", line 119, in _wait_for_profiles
    result = future.result()
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 437, in result
    return self.__get_result()
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 389, in __get_result
    raise self._exception
  File "/usr/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/app/gprofiler/profilers/java.py", line 953, in _profile_process
    stackcollapse = self._profile_ap_process(ap_proc, comm, duration)
  File "/app/gprofiler/profilers/java.py", line 958, in _profile_ap_process
    started = ap_proc.start_async_profiler(self._interval, ap_timeout=self._ap_timeout)
  File "/app/gprofiler/profilers/java.py", line 518, in start_async_profiler
    self._run_fdtransfer()
  File "/app/gprofiler/profilers/java.py", line 506, in _run_fdtransfer
    run_process(
  File "/app/gprofiler/utils/__init__.py", line 265, in run_process
    raise CalledProcessError(retcode, process.args, output=stdout, stderr=stderr)
gprofiler.exceptions.CalledProcessError: Command '['/app/gprofiler/resources/java/fdtransfer', '2580351']' returned non-zero exit status 1. 
stdout: b''
stderr: b'FdTransfer socket(): Success\n'

so I will be able to debug real problems later on.

See commit Granulate/async-profiler@83fd518.