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encoding/json: clarify what happens when multiple items match the same field #43664

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robpike opened this issue Jan 13, 2021 · 5 comments
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@robpike
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robpike commented Jan 13, 2021

This test I added to encoding/json

func TestUnmarshalCase(t *testing.T) {
	var v struct {
		Dist string `json:"dist"`
	}

	Unmarshal([]byte(`{"dist": "ok", "Dist": "KO"}`), &v)
	t.Logf("%+v", v)

	Unmarshal([]byte(`{"Dist": "ok", "dist": "KO"}`), &v)
	t.Logf("%+v", v)
}

prints

    x_test.go:13: {Dist:KO}
    x_test.go:16: {Dist:KO}

The comment says Unmarshal "prefers" exact matches, but if there are no words about what happens if multiple fields match.

It's clearly an odd case, but clarity would be nice. I expected the exact match to win always.

@cagedmantis cagedmantis added this to the Backlog milestone Jan 13, 2021
@cagedmantis cagedmantis added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jan 13, 2021
@cagedmantis
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cagedmantis commented Jan 13, 2021

/cc @rsc @mvdan

@mvdan
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mvdan commented Jan 14, 2021

I'm well aware of this problem, unfortunately. See #14750, which this issue is likely a duplicate of. My attempt at fixing this without incurring a performance or memory cost is https://go-review.googlesource.com/c/go/+/224079, which has been stuck in review limbo for a little while :)

@dsnet
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dsnet commented Jan 23, 2021

At the core of this problem is whether we want to favor a streaming decode or whether we want to buffer all the members of a JSON object in order to perform an exact match. The former has significantly better performance, while the latter probably matches user expectation better.

@cristaloleg
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cristaloleg commented Apr 22, 2021

Kindly ping on this. Looks like this will not be available in 1.17 too already.

@mvdan looks like PR is ready, there is 1 question is 1% perf acceptable, am I right?

@mvdan
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mvdan commented Apr 22, 2021

The PR isn't ready - there are conflicts, and some edge cases that Joe brought up require extra work.

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