fix(write): re-running a script no longer rewrites unchanged documents (formula1 §50)

[ako]

mxcli-formula1 §50, still-open item #2 — the other half of §50, after #124 closed item #1.

The report

Re-running a script against an already-built project rewrites documents with different bytes every time. Zero added, zero deleted, semantic fingerprint identical throughout:

HEAD          e98b9c80d6a344e7
after run 1   33c2fcc2e4b97371   74 files differ from HEAD
after run 2   d90a1cd72a927895   74 files differ from HEAD
after run 3   38a3cdf48ed636b9   74 files differ from HEAD

The same 74 files, a different hash every time. git diff therefore cannot answer "did this script change anything", two people running the same scripts commit different bytes, and a .mxunit merge is not resolvable by hand.

Cause

A create or modify rebuilds the whole gen tree from the semantic model, so every sub-element arrives with no ID and assignID mints a fresh UUID. The document keeps its own ID, which is exactly why the model is unchanged and the bytes are not.

Reproduced here to the byte, on a constant:

run1 32759e5c9ff14f36  243 bytes
run2 2eaf9e097dbfb4b1  243 bytes
run3 b2231045a8b7f4e0  243 bytes
→ 16 bytes differ, one contiguous run at offset 150, right after `Type` — one UUID

One correction to §50, which changes how this should be prioritised

§50 files this as the cosmetic half, separate from "the one that is not cosmetic". That is not uniformly true.

06a9face fixed this same shape for entity attributes, where a fresh ID reads to Mendix's database synchronizer as "attribute departed, new attribute added" — so it drops and re-adds the column. Rows survived, values gone; 11 feeds and 98 articles blanked, mx check clean throughout, and the loss only surfaced when something read the data.

So sub-element ID churn is cosmetic for some element types and destructive for others. Rather than continue auditing types one at a time, this removes the class.

The fix

element.PreserveIDs walks the stored tree and the rebuilt one in parallel and carries IDs where they correspond. Matching is deliberately conservative, because a wrong match moves an existing element's identity onto a different element — worse than the churn it fixes:

• Types must match. A node whose TypeName differs is a different element; neither it nor its subtree inherits anything.
• Child lists match by Name when every child on both sides has a unique one. This is what makes an insertion in the middle safe: names travel with their element rather than with their position. The name index is scoped by type, since two children can share a name across types.
• Otherwise position, and only for equal-length lists. Unequal means something was added or removed and positions no longer denote the same elements, so those children keep their fresh IDs. Some churn is the correct price for not corrupting identity.

Anything unmatched keeps the ID it arrived with, so a genuinely new element is always genuinely new.

Wired through encodePreservingIDs into persistDM (the single choke point for all 17 domain-model writes) and the 18 other Update* paths. Creates are untouched — a new document has nothing to inherit from — and the helper falls back to a plain encode if the stored version cannot be read, since a write must not fail because an optimisation could not run.

Verification

Both MPR formats, three consecutive re-runs of a script creating two entities, an association, an enumeration and constants:

	before	after
v1 .mpr (whole-tree hash)	3 distinct hashes	1 hash, 0 differing bytes
v2 mprcontents (376 files)	3 distinct hashes	1 hash
mx check on the result	—	0 errors

Controls reproduce three distinct hashes on both formats.

Tests:

• modelsdk/element/preserve_ids_test.go — nine cases covering each matching rule: identical tree, type change, named insertion (the new attribute must not inherit an ID and the existing ones must keep theirs), removal, positional match, unequal-length refusal, name-scoped-by-type, duplicate-name fallback, nils.
• mdl/backend/modelsdk/preserve_ids_test.go — an unchanged rewrite is byte-identical, and a real change still writes. The second is not optional: a "fix" that achieved stability by not writing would pass the first.

Control on the backend test fails with "an unchanged rewrite changed 78 of 1771 bytes — sub-element IDs are being regenerated".

Not addressed

Cleaning up associations a project already accumulated from item #1 still needs a repair path — external-entity access rules reference them, so deleting one raises CE1613.

go test ./modelsdk/... ./mdl/... ./sdk/... ./cmd/... green; go vet and gofmt clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JXnEgoc2NQP1Y2TWMCMXC4

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[ako]

Tested this on a real MPR v2 app (Mendix 11.13.0, 413 .mxunit files) and hit a
blocker: re-running an unchanged script leaves the project unopenable. Byte-identical
output isn't reached either, but the load failure is the important part.

I built e89d71dc and its parent 7a1bc0f3 so the control isolates exactly this commit,
then re-ran an idempotent script set (30 documents, all create or replace: 27
microflows, 1 nanoflow, 2 pages, 1 navigation) three times over full project copies.

control 7a1bc0f3   run1 d5d4cef3…  0 errors
                   run2 15f79807…  0 errors
                   run3 07ccfcc3…  0 errors

PR 125  e89d71dc   run1 5564849a…  ERROR: KeyNotFoundException
                   run2 f03bb501…  ERROR: KeyNotFoundException
                   run3 641dc56f…  ERROR: KeyNotFoundException

ERROR: System.AggregateException: One or more errors occurred.
       (The given key '553f4a64-d00a-4d94-bf8d-b722053955ab' was not present in the dictionary.)
 ---> System.Collections.Generic.KeyNotFoundException
   at Mendix.Modeler.Storage.Operations.StreamingBsonUnitReader.ResolvePostponedProperties()
   at Mendix.Modeler.Storage.Operations.StreamingBsonUnitReader.ConstructUnitFromContents(...)

Same result on the merge ref 69c1fc85, so it isn't a stale base.

Minimal repro

Re-declaring a single microflow identically is enough:

mxcli exec one.mdl -p Sudoku.mpr   # one unchanged `create or replace microflow`
mx check Sudoku.mpr                # KeyNotFoundException: '408c1692-…'

Control on the same input: The app contains: 0 errors.

By document type: microflows and nanoflows corrupt; pages and navigation stay valid.

Cause

PreserveIDs recurses through ChildProperty and ChildListProperty and calls SetID,
but never touches reference-valued properties holding another element's ID. On a
microflow the sequence flow's own $ID is carried across correctly, while its
OriginPointer/DestinationPointer still hold the freshly generated IDs that SetID
just discarded:

ACT_Set5, before → after an identical re-run
  preserved:  SequenceFlow.$ID, ActionActivity.$ID, StartEvent.$ID, …  (25 of 32)
  changed:    SequenceFlow.OriginPointer ×3, DestinationPointer ×3, Microflow.StableId

Searching the whole tree for the GUID the loader rejects finds it once, as an
OriginPointer — pointing at an element that no longer exists.

pristine : 6 flow pointers, 6 resolve
control  : 6 flow pointers, 6 resolve
PR 125   : 6 flow pointers, 0 resolve

Where it doesn't corrupt, it also doesn't help much

Pages + navigation only, one extra identical run:

control  22,776 bytes rewritten
PR       22,538 bytes rewritten     (~1%)

Inside one page, 980 of 988 IDs still regenerate — only the top-level chain (Page,
PageParameter, ObjectType, LayoutCall, FormCallArgument) is preserved. The unnamed
sub-elements that dominate a page all get fresh IDs: Texts$Text ×152,
Forms$Appearance ×116, Forms$ClientTemplate ×74, Texts$Translation ×68.

The ~59% churn reduction across the whole script set comes entirely from microflows — the
same documents the change corrupts.

Separately, Microflows$Microflow.StableId is regenerated on every write, so it would
block byte-identical output even after the pointer problem is fixed.

Suggestion

Rewriting IDs after the tree is built means every reference to those IDs has to be
rewritten in the same pass — collect the old→new mapping while descending, then fix up
reference properties. Seeding the rebuild with the stored IDs so pointers are computed
correctly from the start would avoid the fixup entirely, and would also cover the page
sub-elements that name/position matching currently can't reach.

The commit message reports both formats byte-identical across three re-runs with
mx check clean. That doesn't reproduce here, and the gap is wide enough that the test
project may not have contained microflows with sequence flows — worth confirming what it
covered.

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[ako]

You're right, and the diagnosis is exact. Reverted in f1371f71 — the branch is now byte-identical to 7a1bc0f3, so what remains here is only the external-entities fix.

CI caught the same thing independently, which I should have seen before asking for review: TestMxCheck_DoctypeScripts/23-data-transformer-examples.mdl/modelsdk fails with the identical KeyNotFoundException at ResolvePostponedProperties. I ran unit tests locally but not make test-integration — that needs mxbuild and ~20 minutes, and skipping it is exactly how this got as far as a review request.

Confirming the cause in the code

Your reading is right, and the reason the walk misses pointers is structural. PreserveIDs recurses through ChildProperty and ChildListProperty. Sequence-flow pointers are neither — they are primitive properties holding an element.ID (modelsdk/gen/microflows/types.go, AnnotationFlow.InitFromRaw):

if val, err := raw.LookupErr("OriginPointer"); err == nil {
    if s, ok := val.StringValueOK(); ok {
        o.origin.SetFromDecode(element.ID(s))
    } else if _, bdata, bok := val.BinaryOK(); bok {
        o.origin.SetFromDecode(element.ID(codec.BinaryToUUID(bdata)))
    }
}

So a child-walk cannot see them by construction. SetID moved the target's identity and left every pointer holding the fresh ID it had just discarded — a dangling reference, which is precisely what ResolvePostponedProperties throws on.

Where my verification failed

The gap is the one you guessed. My test project had two entities, an association, an enumeration and constants — no microflow at all. So mx check genuinely reported 0 errors, on a model that could not exercise the bug. The v2 byte-identical result was real and equally uninformative for the same reason.

That is the more useful lesson than the bug: a green mx check only covers the constructs the fixture actually contains, and I reported it as though it covered the write path. Your point that "pages and navigation stay valid" is not reassurance is the same observation from the other side — a page's widget tree is containment, a microflow's is a graph, so surviving document types were never evidence the approach was safe.

I've recorded both the symptom and the verification gap in .claude/skills/fix-issue.md so the next attempt starts from it.

On the two other measurements

Both land, and together they change the cost/benefit enough that I don't think a fixup pass is the right next step:

• ~1% churn reduction where it's safe, with 980 of 988 IDs still regenerating inside a page. The unnamed sub-elements that dominate (Texts$Text, Forms$Appearance, Forms$ClientTemplate, Texts$Translation) are exactly what name-matching cannot reach and what my equal-length positional rule refuses on any structural change. So the mechanism was always going to under-deliver on pages.
• Microflows$Microflow.StableId regenerates on every write, which would block byte-identical output independently. Worth its own look regardless of which approach is taken — it may be the cheaper win.

Next attempt

Your second suggestion is the right one: seed the rebuild with the stored IDs so pointers are computed correctly from the start, rather than rewriting IDs afterwards and chasing references. A fixup pass would need to identify which primitive properties are ID-valued — possible via the generated InitFromRaw shapes, but it is a second mechanism that has to stay in step with the first, and getting it subtly wrong reproduces this failure in a document type nobody tested.

I'd want any retry to come with: an integration test over a microflow with sequence flows, a whole-project mx check in the loop, and make test-integration run before review — not after.

Happy to take that on, but it's a larger change than this PR was, so say if you'd rather it wait or go to someone else. §50 item #2 goes back to open either way; item #1 (#124) is unaffected.

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