Patch _version.py

Courtesy of the ZKAPAuthorizer nix expressions

Author: hacklschorsch

nix/tahoe-lafs.nix

@@ -70,6 +70,24 @@ buildPythonPackage rec {
     };
   };
 
+  postPatch =
+    let
+      versionFileContents = version: ''
+        # This _version.py is generated by tahoe-lafs.nix.
+        # TODO: We can have more metadata after we switch to flakes.
+        # Then the `self` input will have a `sourceInfo` attribute telling
+        __pkgname__ = "tahoe-lafs"
+        real_version = "${version}"
+        full_version = "${version}"
+        branch = ""
+        verstr = "${version}"
+        __version__ = verstr
+      '';
+    in
+      ''
+        cp ${builtins.toFile "_version.py" (versionFileContents version)} src/allmydata/_version.py
+      '';
+
   meta = with lib; {
     homepage = "https://tahoe-lafs.org/";
     description = "secure, decentralized, fault-tolerant file store";