wheel-axle-runtime 0.0.6

Description:

wheelaxleruntime 0.0.6

Axle-Runtime - Python Wheel enhancement library








This is a companion project to Wheel Axle/bdist_axle
Problem

Python wheels do not support symlinks.
PIP installation procedure is not locally extensible and does not allow adding post-install hooks.

Solution
WARNING: THIS IS EXPERIMENTAL BETA SOFTWARE. THERE ARE NO WARRANTIES OF ANY KIND. USE AT YOUR OWN RISK. ADDITIONAL
INCLUDED DISCLAIMERS ALSO APPLY.
Wheel Axle Runtime library utilizes a little-known trick used in site.py's .pth files that allows executing
arbitrary code while the site packages are being added. Thus, specially-crafted wheels can silently execute installed
code on Python interpreter startup, facilitating the "post-install hook" functionality.
Python Invariants
The core functionality relies on the following Python behaviors:

site.py processes .pth files
site.py executes .pth import lines
.pth file's line is executed with a local
variable fullname denoting the .pth file path

These invariants have not changed for 18 years.
Implementation
Once the distribution-specific .pth is executed by the Python interpreter, the Wheel Axle Runtime behaves as follows:

The library checks whether a file .dist-info/axle.done exists. If it does it is the indication that the
post-install hook has executed successfully and nothing more is to be done, terminating all further processing.
A process-wide inter-thread lock is acquired.
An OS-wide inter-process file lock is acquired on a file .dist-info/axle.lck.
Once the locks are acquired the .dist-info/axle.done existence is rechecked (double-checked locking optimization).
Now that in-process and inter-process race conditions are excluded the post-install work can begin.
Registered installers are run in sequence. Installers should be idempotent. The following installers are
currently implemented:

LibPython installer checks for the presence of .dist-info/require-libpython.

The installer determines the location of the installation: venv, user or other.
The list of all libpython library files is located from the sys.base_exec_prefix.
If the installation is either venv or user and the link to the libpython library doesn't exist the symlink
is created.


Symlinks installer processes .dist-info/symlinks.txt, if any.

Based on the location of the .pth file being executed the current installation schema and its paths are
determined. Currently, installation into a virtual environment or user location is supported and tested.
For each symlink the target path is resolved and realpath is used to determine the final target path.
If the symlink path and symlink target path are within one of the permitted schema locations the symlink is
created. Otherwise, an exception is raised and the processing is aborted.
After all symlinks are created, the .dist-info/RECORD file is updated to reflect the created symlinks.


Axle installer finalizes the installation. This installer is always executed last.

The .dist-info/RECORD is updated with .dist-info/axle.done file record.
.dist-info/axle.done is created.
<distribution name and version>.pth is then removed. If the file cannot be removed it is left in place.
This can happen on Windows, since the .pth file in question is likely opened for exclusive reading on
Windows.




Any failure anywhere in the above process will result in an abort, an error message, and a retry the next time
the .pth will be activated.

Security
There are several security requirements and implications of having post-install hooks implemented this way.

The installation requires write permissions to the distribution. This will be a problem if the package is installed
as root in locations such as /usr or /usr/local, or is otherwise not write-permitted, unless the post-install
hook is also ran with the sufficient privileges. This is generally acceptable as the primary use is considered to be
installation into virtual envs and user locations. That said, simply running python -c pass or any other python
invocation that does activate site.py under the required privileges will finalize post-install procedures.
There is an attempt to ensure that that axle wheels symlinks and targets don't extend beyond the allowed schema
locations. Those attempts are superficial and have not been formally verified. For example, it may be
possible to escape the path validation/confinement by:

hacking symlink creation order
hacking symlink directory targets
exploiting OS-specific realpath implementation idiosyncrasies (i.e. strict vs not, and what is considered
strict)



TODOs

Support schema detection for prefix installations.
Validate and verify Windows support.