fix dynamic builds with liburing

The original implementation of this was really weird, so I restructed it
a lot while debugging, and am just gonna leave the restructured version.

Root cause of the segfault seems to be that upstream nixpkgs liburing
derivation is generating both static and dynamic libraries, causing
rocksdb to statically link liburing in a dynamic build, pulling in some
allocator stuff at the same time. I created a PR[1] to fix this upstream,
but it probably won't be available on nixos-unstable for quite a while,
so we can also patch it locally.

[1]: https://github.com/NixOS/nixpkgs/pull/314945

flake.nix

@@ -42,6 +42,24 @@
             (builtins.fromJSON (builtins.readFile ./flake.lock))
               .nodes.rocksdb.original.ref;
         });
+        # TODO: remove once https://github.com/NixOS/nixpkgs/pull/314945 is available
+        liburing = pkgs.liburing.overrideAttrs (old: {
+          # the configure script doesn't support these, and unconditionally
+          # builds both static and dynamic libraries.
+          configureFlags = pkgs.lib.subtractLists
+            [ "--enable-static" "--disable-shared" ]
+            old.configureFlags;
+
+          postInstall = old.postInstall + ''
+            # we remove the extra outputs
+            #
+            # we need to do this to prevent rocksdb from trying to link the
+            # static library in a dynamic stdenv
+            rm $out/lib/liburing*${
+              if pkgs.stdenv.hostPlatform.isStatic then ".so*" else ".a"
+            }
+          '';
+        });
       });
 
       scopeHost = mkScope pkgsHost;