Unexpected error "Сannot finalize the creation of a node that is already dead" while applying snapshot

[dfilatov]

Bug report

• I've checked documentation and searched for existing issues and discussions
• I've made sure my project is based on the latest MST version
• Fork this code sandbox or another minimal reproduction.

Sandbox link or minimal reproduction code

import { types, applySnapshot } from 'mobx-state-tree';

const Model = types.array(
    types.model({
        b: types.string,
        c: types.optional(
            types.model({
                d: types.snapshotProcessor(
                    types.model({ e: '' }),
                    {
                        preProcessor(e: string) {
                            return { e };
                        },

                        postProcessor(snapshot) {
                            return snapshot.e;
                        }
                    }
                )
            }),
            { d: '' }
        )
    })
);

const m = Model.create([{ b: 'b' }]);

applySnapshot(m, []);

Describe the expected behavior

Should work without any errors

Describe the observed behavior

Error: [mobx-state-tree] assertion failed: cannot finalize the creation of a node that is already dead

[coolsoftwaretyler]

I think this may be a docs and TypeScript issue. You're calling preProcessor and postProcessor, but the types.snapshotProcessor actually wants preProcess and postProcess.

You can see it working here: https://codesandbox.io/p/sandbox/issue-2136-7hpyd2?file=%2Fsrc%2Findex.ts%3A21%2C3

But there's TypeScript errors on those actions.

Here's the working code for posterity:

import { types, applySnapshot } from "mobx-state-tree";

const Model = types.array(
  types.model({
    b: types.string,
    c: types.optional(
      types.model({
        d: types.snapshotProcessor(types.model({ e: "" }), {
          // TypeScript errors here, but I think that's incorrect
          preProcess(snapshot: { e: string }) {
            return { e: snapshot.e };
          },

         // TypeScript errors here, but I think that's incorrect
          postProcess(snapshot: { e: string }) {
            return snapshot.e;
          },
        }),
      }),
      { d: { e: "e" } }
    ),
  })
);

const m = Model.create([{ b: "b" }]);

applySnapshot(m, []);

I'll tag this, and please accept our apologies for the bug! We'll close this issue when we fix the types, but please let me know if I've missed anything else.

[dfilatov]

@coolsoftwaretyler In your version preProcess/postProcess aren't even called.
Not sure you're right about naming:
https://github.com/mobxjs/mobx-state-tree/blob/master/src/types/utility-types/snapshotProcessor.ts#L60
https://github.com/mobxjs/mobx-state-tree/blob/master/src/types/utility-types/snapshotProcessor.ts#L75

[coolsoftwaretyler]

Ah, my bad! I had a false positive, was just getting those errors to go away.

There is a documentation issue. We call it preProcess and postProcess here: https://mobx-state-tree.js.org/overview/hooks#snapshot-processing-hooks. That's what threw me and got me here.

I'll remove the TypeScript label here. I've been messing around with the code and I think this is coming from postProcessor. If you remove that action, the error goes away.

[coolsoftwaretyler]

Seems to be related to applying the snapshot as an empty array. I can get the processed snapshot, modify it in place, and apply it correctly:

import { types, applySnapshot, getSnapshot } from "mobx-state-tree";

const Model = types.array(
  types.model({
    b: types.string,
    c: types.optional(
      types.model({
        d: types.snapshotProcessor(types.model({ e: "" }), {
          preProcessor(snapshot: { e: string }) {
            return { e: snapshot.e };
          },

          postProcessor(snapshot: { e: string }, node) {
            return {
              ...snapshot,
              e: "modified",
            };
          },
        }),
      }),
      { d: { e: "e" } }
    ),
  })
);

const m = Model.create([{ b: "b" }]);

const snapshot = getSnapshot(m);

console.log("snapshot", snapshot);

const modifiedSnapshot = [...snapshot];

console.log("modifiedSnapshot", modifiedSnapshot);

modifiedSnapshot[0].c.d.e = "modifiedAgain";

applySnapshot(m, modifiedSnapshot);

console.log("snapshot one last time", getSnapshot(m));

Updated example there:

https://codesandbox.io/p/sandbox/issue-2136-7hpyd2?file=%2Fsrc%2Findex.ts%3A41%2C1

But if you try to apply as an empty array, the error comes back.

[dfilatov]

Sorry, I can't open your example:

[coolsoftwaretyler]

Whoops, getting used to the new CodeSandbox tools. This should work: https://codesandbox.io/p/sandbox/issue-2136-7hpyd2?file=%2Fsrc%2Findex.ts%3A41%2C1

[coolsoftwaretyler]

I've got some pass-by-reference errors in there, but I gotta run at the moment so I can revisit this later. Sorry 'bout that!

[dfilatov]

It's not related with an empty array, following code throws the same error:

const Model = types.array(
      types.model({
        b: types.string,
        c: types.optional(
          types.model({
            d: types.snapshotProcessor(types.model({ e: '' }), {
              preProcessor(e: string) {
                return { e };
              },

              postProcessor(snapshot) {
                return snapshot.e;
              }
            })
          }),
          { d: 'e' }
        )
      })
    );

    const m = Model.create([{ b: 'b' }, { b: 'bb' }]);

    applySnapshot(m, [{ b: 'b' }]); // error

Interestingly, if I remove types.optional in the middle then everything works:

    const Model = types.array(
      types.model({
        b: types.string,
        c:
          types.model({
            d: types.snapshotProcessor(types.model({ e: '' }), {
              preProcessor(e: string) {
                return { e };
              },

              postProcessor(snapshot) {
                return snapshot.e;
              }
            })
          })
      })
    );

    const m = Model.create([{ b: 'b', c: { d: '' } }, { b: 'bb', c: { d: '' } }]);

    applySnapshot(m, [{ b: 'b', c: { d: '' } }]);

[coolsoftwaretyler]

Interesting. Maybe types.optional is having trouble instantiating the default value as a snapshotProcessor.

[coolsoftwaretyler]

Hopefully it's just this: #2137

Once we get a test in there I can merge and deploy a patch, probably early this week.

[dfilatov]

Unfortunately, no (
It just fixes import but exception will be thrown anyway, I've checked it.

[coolsoftwaretyler]

Ah dang. Do you have the full text of the exception with that patched? That might be helpful

[dfilatov]

Ah dang. Do you have the full text of the exception with that patched? That might be helpful

[coolsoftwaretyler]

Thank you! I'll see if I can get anyone to investigate. If not, I can at some point in the coming weeks. Would welcome a PR if you trace the error on your own as well. Apologies for the inconvenience!

[dfilatov]

Hi, @coolsoftwaretyler! Is there any progress?

[coolsoftwaretyler]

Sorry, not yet! Haven't had anyone volunteer to work on it.