contents of Map don't survive 2nd snapshot / restore

[dckc]

Steps to Reproduce

in maptest.js:

const progress = [];
const mapTest = new Map();

function proceed() {
  progress.push('moar!');
  mapTest.set(progress.length, 'thing');
  print(progress.length);
  print(progress);
  print(mapTest.size);
  print([...mapTest.entries()]);
}

globalThis.proceed = proceed;

proceed();

in mapcont.js:

proceed();

Then invoke...

$ xsnap -m maptest.js -w s1.xsnap
1
moar!
1
1,thing
$ xsnap -m mapcont.js -r s1.xsnap -w s2.xsnap
2
moar!,moar!
2
1,thing,2,thing
$ xsnap -m mapcont.js -r s2.xsnap -w s3.xsnap
3
moar!,moar!,moar!
1
3,thing

Expected behavior

as above but for the last 2 lines:

3
1,thing,2,thing,3,thing

Build environment: Linux
Target device: xsnap
sdk version 748fda9
xsnap version e8199b1012981d627a77235bb6737f9deef07103

cc @warner @erights @phoddie

[phoddie]

There is a bug in the loading of Maps. The fix is small, so I'll post it here ahead of us updating this repository. Replace these lines:

moddable/xs/sources/xsSnapshot.c

Lines 763 to 765 in 748fda9

	slot = the->firstHeap;
	slot->value.reference = the->freeHeap;
	slot++;

With:

slot = the->firstHeap + 1;

The test isn't quite right though. The second step loads mapcont.js so the third step does nothing as that module is already loaded. mapcont.js is already loaded so nothing happens.

$ xsnap -m maptest.js -w s1.xsnap
1
moar!
1
1,thing
$ xsnap -m mapcont.js -r s1.xsnap -w s2.xsnap
2
moar!,moar!
2
1,thing,2,thing
$ xsnap -m mapcont.js -r s2.xsnap -w s3.xsnap

You need a separate module, such as mapcont2.js:

proceed();

Then the output is as you expect:

$ xsnap -m mapcont2.js -r s2.xsnap -w s3.xsnap
3
moar!,moar!,moar!
3
1,thing,2,thing,3,thing

[dckc]

confirmed. thanks!

(committed locally / temporarily as fb0141f3)