Helper function to set the close-on-exec bit in a bit mask which specifies option flags for a system call that obtains a new file descriptor, such as open(2). On recent Linux all such system/library functions support enabling the close-on-exec option; this is a Linux extension to POSIX. Some of these flag accepting functions were added more recently with a name extension, for example accept4(2) or inotify_init1(2).
If true then all ocean functions obtaining a file descriptor from the system set the close-on-exec option; if false they don't. Changing the value of this variable does not change the state of previously obtained file descriptors.
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Global configuration variable to define whether the close-on-exec option should be set on system calls that create a file descriptor in ocean modules.
On a POSIX system, if a program calls exec(2) (or an equivalent function from the exec family) to execute another program, then the devices referred to by the file descriptors held by the original program stay open, and the executed program inherits all of them. This has the consequence that, if the executed program is unaware of the original program's file descriptors so it doesn't close those it doesn't need, then all devices will stay open until the process exits.
Most of the time an exec call is preceded by a fork call to execute a program in a new process. As with exec the child process created by fork inherits all file descriptors from its parent process that called fork. However, each device referred to by one of these file descriptors stays open until _both_ the parent and child process close them or exit. This means that the parent process has no way of closing its devices any more unless the child process cooperates and does so as well, which is in practice very unlikely.
This can cause the following situation: - A program opens a device (file, socket, timer or event fd). - The program registers the device with epoll. - The program uses a third-party library which starts a task in a separate process using fork + exec. - The original program (parent process) closes the device. - The executed program (child process) is, as libraries are, unaware of its parent's business so it doesn't close any of the inherited file descriptors; it doesn't even know which it inherited. So the device stays opened. - Because the device is opened it stays registered with epoll. The parent process cannot unregister it any more because it has closed the file descriptor. - Until the child process exits epoll_wait(2) keeps reporting events for the device in the parent process.
This is a problem because it can cause sporadic erratic behaviour in a program. The error may be hard to reproduce and track down.
To prevent this from happening POSIX provides a close-on-exec option for each file descriptor. By default it is disabled. If enabled, exec will close the file descriptor so that the executed program won't inherit it. But it works only if the parent process explicitly enables this option for every single file descriptor it obtains from the system.
The global open_with_close_on_exec variable in this module is read by all functions and class constructors in ocean that obtain a file descriptor from the system. If it is true then they set the close-on-exec option.
This does not affect stdin, stdout and stderr, which are opened before the start of the program.
The flag is true by default, which is in general recommended to avoid the aforementioned problem unless file descriptor inheriting is needed, which is not a feature that is currently relevant to Sociomantic's use cases.
IF YOU HAVE A USE CASE FOR INHERITING FILE DESCRIPTORS, PLEASE CONTACT THE OCEAN MAINTAINERS.
The following ocean functions obtain a file descriptor and use this flag:
- ocean.sys.Epoll: Epoll.create - ocean.sys.EventFD: EventFD constructor - ocean.sys.Inotify: Inotify constructor - ocean.sys.SignalFD: SignalFD.register - ocean.sys.TimerFD: TimerFD constructor - ocean.sys.socket: The socket and accept methods in all *Socket classes - ocean.io.device.File: File.open