TL;DR: subprocess.Popen's handling of file descriptors opened for DEVNULL or PIPE inputs/outputs has serious problems, and it can be coerced into leaking file descriptors in several ways. This can cause issues related to resource exhaustion.

# The basic problem

As part of its setup, Popen.__init__() calls Popen._get_handles(), which looks at the given stdin/stdout/stderr arguments and returns a tuple of 6 file descriptors (on Windows, file handles) indicating how stdin/stdout/stderr should be redirected. However, these file descriptors aren't properly closed if exceptions occur in certain cases.

# Variant 1: Bad argument errors (introduced in 3.9)

The first variant of this bug is shockingly easy to reproduce (note that this only works on platforms with /proc/self/fd, like Linux):

import os, subprocess

def show_fds():
    for entry in os.scandir("/proc/self/fd"):
        print(entry.name, "->", os.readlink(entry.path))

print("Before:")
show_fds()

try:
    subprocess.Popen(["ls"], stdin=subprocess.PIPE, user=1.0)
except TypeError as e:  # "User must be a string or an integer"
    print(e)

print("After:")
show_fds()

This produces something like:

Before:
0 -> /dev/pts/1
1 -> /dev/pts/1
2 -> /dev/pts/1
3 -> /proc/12345/fd
User must be a string or an integer
After:
0 -> /dev/pts/1
1 -> /dev/pts/1
2 -> /dev/pts/1
3 -> pipe:[1234567]
3 -> pipe:[1234567]
5 -> /proc/12345/fd

The process never got launched (because of the invalid user argument), but the (unused) pipe created for piping to stdin is left open! Substituting DEVNULL for PIPE instead leaves a single file descriptor open to /dev/null.

This happens because the code that validates the user, group, and extra_groups arguments 1 was added to Popen.init() after the call to Popen._get_handles() 2, and there isn't a try/except that closes the file descriptors if an exception gets raised during validation (which can easily happen).

Variant 2: Error opening file descriptors (seems to have been around in subprocess forever)

Within Popen._get_handles() (on Windows 3 or POSIX 4), previously opened file descriptors are not closed if an error occurs while opening later file descriptors.

For example, take the case where only one more file descriptor can be opened without hitting the limit on the number of file descriptors, and subprocess.Popen(["ls"], stdin=subprocess.DEVNULL, stdout=supbrocess.PIPE) is called. subprocess will be able to open /dev/null for stdin, but trying to creating a pipe() for stdout will fail with EMFILE or ENFILE. Since Popen._get_handles() doesn't handle exceptions from pipe() (or when opening /dev/null), the /dev/null file descriptor opened for stdin will be be left open.

This variant is most easily triggered by file descriptor exhaustion, and it makes that problem worse by leaking even *more* file descriptors.

Here's an example that reproduces this by monkey-patching os to force an error:

import os, subprocess

def show_fds():
    for entry in os.scandir("/proc/self/fd"):
        print(entry.name, "->", os.readlink(entry.path))

print("Before:")
show_fds()

# Trigger an error when trying to open /dev/null
os.devnull = "/NOEXIST"

try:
    subprocess.Popen(["ls"], stdin=subprocess.PIPE, stdout=subprocess.DEVNULL)
except FileNotFoundError as e:  # "User must be a string or an integer"
    print(e)

print("After:")
show_fds()

Output:

Before:
0 -> /dev/pts/1
1 -> /dev/pts/1
2 -> /dev/pts/1
3 -> /proc/12345/fd
[Errno 2] No such file or directory: '/dev/null'
After:
0 -> /dev/pts/1
1 -> /dev/pts/1
2 -> /dev/pts/1
3 -> pipe:[1234567]
4 -> pipe:[1234567]
5 -> /proc/12345/fd

Again, the pipe is left open.

# Paths to fix.

Variant 1 can be fixed by simply reordering code in Popen.__init__() (and leaving comments warning about the importance of maintaining the order!). I've attached a basic patch that does this.

Variant 2 might take some more work -- especially given the shared Popen._devnull file descriptor that needs to be accounted for separately -- and may require significant changes to both Popen.__init__() and Popen._get_handles() to fix.

Thanks for all your work diagnosing and fixing these. It is a good cleanup!