python3Packages.protobuf: fix derivation version

pkgs/development/python-modules/protobuf/default.nix

···
 , isPyPy
 }:
 
+let
+  versionMajor = lib.versions.major protobuf.version;
+  versionMinor = lib.versions.minor protobuf.version;
+  versionPatch = lib.versions.patch protobuf.version;
+in
 buildPythonPackage {
-  inherit (protobuf) pname src version;
+  inherit (protobuf) pname src;
+
+  # protobuf 3.21 coresponds with its python library 4.21
+  version =
+    if lib.versionAtLeast protobuf.version "3.21"
+    then "${toString (lib.toInt versionMajor + 1)}.${versionMinor}.${versionPatch}"
+    else protobuf.version;
+
   disabled = isPyPy;
 
+  sourceRoot = "source/python";
+
   prePatch = ''
-    while [ ! -d python ]; do
-      cd *
-    done
-    cd python
+    if [[ "$(<../version.json)" != *'"python": "'"$version"'"'* ]]; then
+      echo "Python library version mismatch. Derivation version: $version, actual: $(<../version.json)"
+      exit 1
+    fi
   '';
 
   nativeBuildInputs = [ pyext ];