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.
      
This fixes several ways file descriptors could be leaked from `subprocess.Popen` constructor during error conditions by opening them later and using a context manager "fds to close" registration scheme to ensure they get closed before returning.
---------
Co-authored-by: Gregory P. Smith [Google] <[email protected]>
      
…GH-96351)
This fixes several ways file descriptors could be leaked from `subprocess.Popen` constructor during error conditions by opening them later and using a context manager "fds to close" registration scheme to ensure they get closed before returning.
---------
(cherry picked from commit 3a4c44b)
Co-authored-by: cptpcrd <[email protected]>
Co-authored-by: Gregory P. Smith [Google] <[email protected]>
gh-87474: Fix file descriptor leaks in subprocess.Popen (GH-96351)
This fixes several ways file descriptors could be leaked from `subprocess.Popen` constructor during error conditions by opening them later and using a context manager "fds to close" registration scheme to ensure they get closed before returning.
---------
(cherry picked from commit 3a4c44b)
Co-authored-by: cptpcrd <[email protected]>
Co-authored-by: Gregory P. Smith [Google] <[email protected]>

      
* main: (26 commits)
  pythonGH-101520: Move tracemalloc functionality into core, leaving interface in Modules. (python#104508)
  typing: Add more tests for TypeVar (python#104571)
  pythongh-104572: Improve error messages for invalid constructs in PEP 695 contexts (python#104573)
  typing: Use PEP 695 syntax in typing.py (python#104553)
  pythongh-102153: Start stripping C0 control and space chars in `urlsplit` (python#102508)
  pythongh-104469: Update README.txt for _testcapi (pythongh-104529)
  pythonGH-103092: isolate `_elementtree` (python#104561)
  pythongh-104050: Add typing to Argument Clinic converters (python#104547)
  pythonGH-103906: Remove immortal refcounting in the interpreter (pythonGH-103909)
  pythongh-87474: Fix file descriptor leaks in subprocess.Popen (python#96351)
  pythonGH-103092: isolate `pyexpat`  (python#104506)
  pythongh-75367: Fix data descriptor detection in inspect.getattr_static (python#104517)
  pythongh-104050: Add more annotations to `Tools/clinic.py` (python#104544)
  pythongh-104555: Fix isinstance() and issubclass() for runtime-checkable protocols that use PEP 695 (python#104556)
  pythongh-103865: add monitoring support to LOAD_SUPER_ATTR (python#103866)
  CODEOWNERS: Assign new PEP 695 files to myself (python#104551)
  pythonGH-104510: Fix refleaks in `_io` base types (python#104516)
  pythongh-104539: Fix indentation error in logging.config.rst (python#104545)
  pythongh-104050: Don't star-import 'types' in Argument Clinic (python#104543)
  pythongh-104050: Add basic typing to CConverter in clinic.py (python#104538)