Always fallback to the Pypi API when fetching sources & wheels

Previously we randomly encountered issues where the Pypi mirror URLs
wouldn't be correct with packages whos first character is frequently
written in upper case (e.g. SqlAlchemy, MarkupSafe, ...). The Pypi
mirrors are aparently not consistent in the naming of the file locations
which lead to random errors after package version bumps.

By falling back to the API lookup (that we already did for wheels) we
can workaround that situation.

fetch-from-pypi.sh

@@ -14,7 +14,7 @@ curl="curl            \
  $curlOpts            \
  $NIX_CURL_FLAGS"
 
-echo "Trying to fetch wheel with predicted URL: $predictedURL"
+echo "Trying to fetch with predicted URL: $predictedURL"
 
 $curl $predictedURL --output $out && exit 0
 

lib.nix

@@ -93,17 +93,19 @@ let
   );
 
 
-  # Fetch the wheels from the PyPI index.
-  # We need to first get the proper URL to the wheel.
+  # Fetch from the PyPI index.
+  # At first we try to fetch the predicated URL but if that fails we
+  # will use the Pypi API to determine the correct URL.
   # Args:
   #   pname: package name
   #   file: filename including extension
+  #   version: the version string of the dependency
   #   hash: SRI hash
   #   kind: Language implementation and version tag
-  fetchWheelFromPypi = lib.makeOverridable (
-    { pname, file, hash, kind, curlOpts ? "" }:
+  fetchFromPypi = lib.makeOverridable (
+    { pname, file, version, hash, kind, curlOpts ? "" }:
     let
-      version = builtins.elemAt (builtins.split "-" file) 2;
+      predictedURL = predictURLFromPypi { inherit pname file hash kind; };
     in
     (pkgs.stdenvNoCC.mkDerivation {
       name = file;
@@ -111,7 +113,7 @@ let
         pkgs.curl
         pkgs.jq
       ];
-      isWheel = true;
+      isWheel = lib.strings.hasSuffix "whl" file;
       system = "builtin";
 
       preferLocalBuild = true;
@@ -119,36 +121,20 @@ let
         "NIX_CURL_FLAGS"
       ];
 
-      predictedURL = predictURLFromPypi { inherit pname file hash kind; };
-      inherit pname file version curlOpts;
+      inherit pname file version curlOpts predictedURL;
 
-      builder = ./fetch-wheel.sh;
+      builder = ./fetch-from-pypi.sh;
 
       outputHashMode = "flat";
       outputHashAlgo = "sha256";
       outputHash = hash;
+
+      passthru = {
+        urls = [ predictedURL ]; # retain compatibility with nixpkgs' fetchurl
+      };
     })
   );
 
-  # Fetch the artifacts from the PyPI index. Since we get all
-  # info we need from the lock file we don't use nixpkgs' fetchPyPi
-  # as it modifies casing while not providing anything we don't already
-  # have.
-  #
-  # Args:
-  #   pname: package name
-  #   file: filename including extension
-  #   hash: SRI hash
-  #   kind: Language implementation and version tag https://www.python.org/dev/peps/pep-0427/#file-name-convention
-  fetchFromPypi = lib.makeOverridable (
-    { pname, file, hash, kind }:
-    if lib.strings.hasSuffix "whl" file then fetchWheelFromPypi { inherit pname file hash kind; }
-    else
-      pkgs.fetchurl {
-        url = predictURLFromPypi { inherit pname file hash kind; };
-        inherit hash;
-      }
-  );
   getBuildSystemPkgs =
     { pythonPackages
     , pyProject
@@ -215,7 +201,6 @@ in
 {
   inherit
     fetchFromPypi
-    fetchWheelFromPypi
     getManyLinuxDeps
     isCompatible
     readTOML

mk-poetry-dep.nix

@@ -175,6 +175,7 @@ pythonPackages.callPackage
           fetchFromPypi {
             pname = name;
             inherit (fileInfo) file hash kind;
+            inherit version;
           };
     }
   )