No-GIL/free-threaded compatibility is not taken into account when checking for Python compatibliity of wheels

[vlaci]

I distribute the lzallright package in pypi which supports free-threaded Python versions as well.

It is distributed as an abi3 wheel for most platforms, and it is distributed as a cp313t wheel for free-threaded builds. Unfortunately pyproject.nix and as a consequence uv2nix treats the cp313t compatible with regular Python 3.13 interpreters, leading to this error:

 error: Failed to determine installation plan
  Caused by: A path dependency is incompatible with the current platform: lzallright-0.2.5-cp313-cp313t-manylinux_2_17_x86_64.manylinux2014_x86_64.whl

I managed to resolve this by applying the following diff:

diff --git i/lib/pypa.nix w/lib/pypa.nix
index ae2e5d8..9b78ce3 100644
--- i/lib/pypa.nix
+++ w/lib/pypa.nix
@@ -24,7 +24,7 @@ let
     take
     splitString
     ;
-  inherit (lib.strings) hasPrefix toInt;
+  inherit (lib.strings) hasPrefix hasSuffix toInt;
 
   matchWheelFileName = match "([^-]+)-([^-]+)(-([[:digit:]][^-]*))?-([^-]+)-([^-]+)-(.+).whl";
 
@@ -384,11 +384,13 @@ lib.fix (self: {
     let
       #   inherit (python.passthru) sourceVersion implementation;
       # in
-      inherit (python.passthru) implementation pythonVersion;
+      inherit (python.passthru) implementation pythonVersion libPrefix;
 
       version' = splitString "." pythonVersion;
       major = elemAt version' 0;
       minor = elemAt version' 1;
+      freeThreadedWheel = hasSuffix pythonVersion "t";
+      freeThreadedPython = hasSuffix libPrefix "t";
     in
     assert length version' >= 2;
     # Python tag
@@ -405,7 +407,9 @@ lib.fix (self: {
       (
         pythonTag.version == null
         || pythonTag.version == major
-        || (hasPrefix major pythonTag.version && ((toInt (major + minor)) >= toInt pythonTag.version))
+        ||
+          (hasPrefix major pythonTag.version && ((toInt (major + minor)) >= toInt pythonTag.version))
+          && freeThreadedWheel == freeThreadedPython
       );
 
   /**

It looks like, the only passthru flag to check is libPrefix. The tag parsing feels very hackish, I don't know if we can do any better though. Also IMO the t suffix cannot appear without specifying the minor version, but I did not find an exact spec.

If you think, this is the correct way to approach this, I am happy to create a PR.

---

I think this is related to the issue mentioned in pyproject-nix/uv2nix#165 as well.

[adisbladis]

Are you sure you're not using an older version of pyproject.nix?

ABI tags were reimplemented in 2db2d95 to fix this exact issue.
Now the inverse is incorrect: When using a python313FreeThreading derivation it will attempt to try and use the non free-threaded wheel as explain in this comment.

There is currently no way to externally check if a Python derivation is a free threaded build or not. This issue needs to be fixed in nixpkgs. The code above will never work because the t suffix on pythonVersion doesn't exist.
Ideally I think that the list of compatible ABI tags should be a passthru attr of the nixpkgs derivation.

[vlaci]

You hunch is right. I've messed up switching between different branches for the repro.
It works as advertised.

Sorry for wasting your time and thank you for this awesome project.

[adisbladis]

Root cause properly fixed in #383.