bug(common/models): cannot apply transform to null context 🐵

[sentry-io]

Sentry Issue: KEYMAN-WEB-FH

TypeError: undefined is not an object (evaluating 'e.left')
  at None (blob:null/2ef37f8a-965f-4741-9ce2-793daf17264b:5:20146)

@jahorton on the analysis here.

Diving into the lm-worker minified source, I have been able to map the error to the following section:

keyman/common/models/templates/src/common.ts

Lines 8 to 11 in baad2e8

	export function applyTransform(transform: Transform, context: Context): Context {
	// First, get the current context
	let fullLeftContext = context.left || '';
	let lLen = fullLeftContext.kmwLength();

Note line 10 in particular - that's the line that's failing. So... something's attempting to apply a transform to a null context. The natural question is, of course... "what?"

---

The section of minified code:

extendString();var SENTINEL_CODE_UNIT="\uFDD0";function applyTransform(r,e){
var t,n,i=e.left||"",a=i.kmwLength(),u=a<r.deleteLeft?a:r.deleteLeft,o=i.kmwSubstr(0,a-u) //...

The line-column identifier matches perfectly to i=e|.left - that is, with the caret between the e and the ..

[sentry-io]

Sentry issue: KEYMAN-WEB-ET

This one appears to line up against a different block, but for the same issue: an unexpected null in place of a context.

My best guess here is this block:

keyman/common/web/lm-worker/src/main/correction/context-tracker.ts

Lines 536 to 544 in d8a6467

	// Using the potential `matchState` + the incoming transform, do the results line up for
	// our observed context? If not, skip it.
	//
	// Necessary to properly handle multitaps, as there are context rewinds that the
	// predictive-text engine is not otherwise warned about.
	const doublecheckContext = applyTransform(transformDistribution[0].sample, priorMatchState.taggedContext);
	if(doublecheckContext.left != context.left) {
	continue;
	}

The context.left reference lines up for the specified column... but this is new code on the gestures line - see #9855. That said, the event was from a local build - thus easily could have been from the gestures line. It'd also make the most sense - that a change introduced on the gestures line is responsible for an error we haven't really been getting elsewhere.

That said, the version with the associated commit is 17.0.190, with the feature-branch merge around 17.0.200. That said, there may have been some rebasing of the commits involved.

[jahorton]

BTW, this is behind the sporadic errors that were noticed when trying out #10103 on an Android phone. The version data & timeline matches quite well, and the "environment" for the first linked event is test.

[jahorton]

At any rate, since this is something we don't seem to be seeing in current stable or alpha builds, it's likely feature-gestures related, and that means that the issue likely arose out of #9855 in some manner.

[jahorton]

Just had a lucky trigger from standalone-Web when testing something else. Here's the log:

common.ts:10 Uncaught TypeError: Cannot read properties of undefined (reading 'left')
    at applyTransform (common.ts:10:33)
    at ContextTracker2.analyzeState (context-tracker.ts:541:38)
    at ModelCompositor2.predict (model-compositor.ts:150:46)
    at Object.handleMessage (index.ts:328:42)
    at LMLayerWorker2.onMessage (index.ts:167:16)

Fortunately, as I was working with full unminified sourcemap access, we got better logs for this event!

My two most-recent keystrokes were BKSP, and I was clearing text. I'd only typed a single "word" to begin with.