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encoding/json: incorrect object key unmarshaling when using custom TextUnmarshaler as Key with string values #39555

mrwonko opened this issue Jun 12, 2020 · 6 comments


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@mrwonko mrwonko commented Jun 12, 2020

What version of Go are you using (go version)?

$ go version

Does this issue reproduce with the latest release?


What operating system and processor architecture are you using (go env)?

go env Output
$ go env

What did you do?

I have a custom type with MarshalText and UnmarshalText methods. I use it as a key in a map, and the value of the map is string (not reproducible with struct{}), and I json.Marshal that map and then json.Unmarshal it again.

What did you expect to see?

UnmarshalText should receive the data that was returned by MarshalText.

What did you see instead?

The data passed to UnmarshalText is an escaped version of the data returned by MarshalText, for example " turns into \" and \ turns into \\.

At first I thought I was running into #38105, but the issue still exists in Go 1.14.4.

@toothrot toothrot added this to the Backlog milestone Jun 12, 2020
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@toothrot toothrot commented Jun 12, 2020

/cc @rsc

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@networkimprov networkimprov commented Jun 13, 2020

cc @mvdan

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@mvdan mvdan commented Jun 14, 2020

Thank you for the report. This is indeed a different regression caused by the same original performance optimization, The previous fix was good as far as I can tell, but it evidently wasn't a complete fix.

It's pretty obvious that the original optimization was a mistake, and applying more best-effort fixes seems too little and too late at this point. I'll send a CL to revert the entire thing, and add more tests. We can backport this into 1.14.5.

@gopherbot please consider this for backport to 1.14, it's a regression.

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@gopherbot gopherbot commented Jun 14, 2020

Backport issue(s) opened: #39585 (for 1.14).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to

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@gopherbot gopherbot commented Jun 14, 2020

Change mentions this issue: encoding/json: revert "avoid work when unquoting strings, take 2"

@gopherbot gopherbot closed this in 11389ba Jun 15, 2020
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@gopherbot gopherbot commented Jul 6, 2020

Change mentions this issue: [release-branch.go1.14] encoding/json: revert "avoid work when unquoting strings, take 2"

gopherbot pushed a commit that referenced this issue Jul 9, 2020
…ing strings, take 2"

This reverts and, minus the
regression tests in the latter.

The original work happened in, which was reverted
in due to a crash found by fuzzing.

We tried a second time in, which shipped with Go
1.14. A bug was found, where strings would be mangled in certain edge
cases. The fix for that was, which was backported
into Go 1.14.4.

Unfortunately, a second regression was just reported in #39555, which is
a similar case of strings getting mangled when decoding under certain
conditions. It would be possible to come up with another small patch to
fix that edge case, but instead, let's just revert the entire
optimization, as it has proved to do more harm than good. Moreover, it's
hard to argue or prove that there will be no more such regressions.

However, all the work wasn't for nothing. First, we learned that the way
the decoder unquotes tokenized strings isn't simple; initially, we had
wrongly assumed that each string was unquoted exactly once and in order.
Second, we have gained a number of regression tests which will be useful
to prevent the same mistakes in the future, including the test cases we
add in this CL.

For #39555.
Fixes #39585.

Change-Id: I66a6919c2dd6d9789232482ba6cf3814eaa70f61
Run-TryBot: Daniel Martí <>
TryBot-Result: Gobot Gobot <>
Reviewed-by: Andrew Bonventre <>
(cherry picked from commit 11389ba)
Reviewed-by: Daniel Martí <>
Run-TryBot: Dmitri Shuralyov <>
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