I am using the XrdCl libraries in a C++ framework which uses libc fork() heavily. When I create a DefaultEnv object, for example to set the log level, the static methods defined there somehow interfere with fork() and cause it to hang. A simple example program to recreate the problem:

> cat fork.cpp
#include <sys/wait.h>
#include <XrdCl/XrdClDefaultEnv.hh>
#include <XrdCl/XrdClLog.hh>

int main () {
  XrdCl::DefaultEnv env;
  XrdCl::Log *log = XrdCl::DefaultEnv::GetLog();
  log->SetLevel(XrdCl::Log::DumpMsg);

  int status;
  pid_t pid;

  pid = fork ();
  if (pid == 0) {
    // child
    sleep(1);
  } else if (pid < 0) {
    // The fork failed.  Report failure.
    status = -1;
  } else {
    // This is the parent process.  Wait for the child to complete.
    if (waitpid (pid, &status, 0) != pid)
      status = -1;
  }
  return status;
}

> g++ -o fork -I /usr/include/xrootd -l XrdCl -l pthread fork.cpp

> ./fork 
[2019-03-04 14:18:58.927546 +0100][Debug  ][Utility           ] In the prepare fork handler for process 1576837
[2019-03-04 14:18:58.927602 +0100][Debug  ][Utility           ] Running the prepare fork handler for process 1576837
[2019-03-04 14:18:58.927609 +0100][Debug  ][Utility           ] Locking File and FileSystem objects for process: 1576837
[2019-03-04 14:18:58.927615 +0100][Debug  ][Utility           ] In the prepare fork handler for process 1576837
[2019-03-04 14:18:58.927620 +0100][Debug  ][Utility           ] Running the prepare fork handler for process 1576837

The process hangs at this point. Removing the XrdCl part, i.e. commenting out the first 3 lines of main(), allows the process to complete normally.

Attaching to the hanging process, one can see that the XrdCl prepare handler is being used:

#0  0x00007ff6914be4ed in __lll_lock_wait () from /lib64/libpthread.so.0
#1  0x00007ff6914b9dcb in _L_lock_883 () from /lib64/libpthread.so.0
#2  0x00007ff6914b9c98 in pthread_mutex_lock () from /lib64/libpthread.so.0
#3  0x00007ff6917834b8 in XrdCl::ForkHandler::Prepare() () from /lib64/libXrdCl.so.2
#4  0x00007ff6917087f0 in prepare () from /lib64/libXrdCl.so.2
#5  0x00007ff690988eac in fork () from /lib64/libc.so.6
#6  0x0000000000400fb7 in main ()

Is there a way to avoid this problem?

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