test(xmlgen): cover failure paths with whole-document fixtures (4/4)

[sthanikan2000]

Problem

The golden fixtures in #202 prove the conversions work, but only on the happy path. Nothing shows what a failure looks like on a document of realistic size — and an error that says only "invalid XML" is of no use against a hundred-element declaration.

Changes

Eight deliberately broken templates under testdata/invalid/, each a mistake somebody could plausibly commit, rendered against the real invoice data:

Fixture	Mistake	Expected
two-roots.tmpl	a range at the top level with nothing wrapping it	ErrMultipleRoots
unclosed-element.tmpl	</Header> forgotten	ErrMalformedXML
undeclared-prefix.tmpl	soapp: typed one letter wrong	ErrUndeclaredNamespace
doctype.tmpl	a DOCTYPE Go ignores but a receiving parser will not	ErrDoctypeNotAllowed
unknown-function.tmpl	a resolver never supplied	ErrParseTemplate
syntax-error.tmpl	an if with no end	ErrParseTemplate
raw-bypass.tmpl	raw on a value containing &	ErrMalformedXML
object-as-text.tmpl	a whole object printed where text belongs	ErrUnsupportedValue

Each case asserts the message names what went wrong — soapp, codelist, Header — so it can be found in the template.

For bad data, the real invoice.json is patched at one point rather than duplicated per case: a date in the wrong layout, a non-numeric amount, an object where text belongs, a table that is not a table, and bytes that are not JSON at all. That keeps the broken value beside its assertion and stops these drifting out of step with the fixture.

A second test records which faults Validate catches without data — the parse-time ones — and which can only surface from Generate. That is the difference between a template failing when it is stored and failing when someone presses submit.

Testing

cd xmlgen
go test -race ./...
golangci-lint run -c ../.golangci.yml --timeout=5m ./...

180 subtests cumulative (25 added), race-clean, 0 lint issues. Statement coverage is unchanged at 88.7%, which is the expected result: these paths were already exercised. What is new is proof the errors are legible on a realistic document rather than a one-element toy.

Writing these found a real defect in #202's fixture — under WithStrictKeys the invoice template tripped on a key deliberately left absent, so the fixture could not pass strict mode at all. That fix lives in #202, which is also what keeps this PR purely additive: 9 new files, zero modifications.

Related

Implements #189. Stacked chain — 4 of 4:

#199 engine → #201 helpers and resolvers → #202 golden fixtures → #200 (this)

[coderabbitai]

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