Checking a process is alive¶

Sometimes sharedbox has to answer a simple-sounding question: is the process recorded in a segment still running? The answer decides whether a crashed waiter's slot can be freed, and what an error message tells you about who holds a name. This page explains how it answers, and why the obvious check isn't enough.

A pid and a start time¶

The obvious check would be "is there a process with this process id (pid)?", but that isn't enough: once a process exits, the operating system can give its pid to a new one. So sharedbox.hpp names a process by its pid and its start time, the way the psutil library tells a process from a later one with the same pid.¹ On Windows the start time is the creation time from GetProcessTimes. On Linux it is field 22 of /proc/<pid>/stat.² process_alive() reads the start time of whatever process has the pid now and decides like this:

A recorded start time that couldn't be read is treated the same way: the process counts as alive.²

On Linux there's one more catch: containers give their processes their own set of pids, called a pid namespace, so the same pid can mean different processes in different containers. That's why waiter slots and the header's creator fields also record the namespace, and a process in another namespace, or one whose namespace is unknown, is never judged dead.

Where the check is used¶

• Freeing the waiter slots of dead processes, as Waiting for changes describes.
• Naming the creator in SegmentExistsError. When a create finds the name taken, the extension reads the existing header with sharedbox::inspect() and checks the creator it records. The creator writes the header before it publishes the box, so a name that holds no published box is checked the same way. The docstring of SegmentExistsError lists the messages that result.

Even when the creator has exited, nothing is removed automatically, because other processes may still be using a segment whose creator died. Removing it is your call; How to clean up segments shows how.

Sources¶