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text/template: panics on method on nil interface value [1.12 backport] #30464

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gopherbot opened this issue Feb 28, 2019 · 4 comments
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text/template: panics on method on nil interface value [1.12 backport] #30464

gopherbot opened this issue Feb 28, 2019 · 4 comments
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CherryPickCandidate Used during the release process for point releases FrozenDueToAge
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Go1.12.1

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@gopherbot
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@mvdan requested issue #30143 to be considered for backport to the next 1.12 minor release.

@gopherbot please backport to 1.12
@gopherbot gopherbot added the CherryPickCandidate Used during the release process for point releases label Feb 28, 2019
@gopherbot gopherbot mentioned this issue Feb 28, 2019
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@gopherbot gopherbot added this to the Go1.12.1 milestone Feb 28, 2019
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Change https://golang.org/cl/164457 mentions this issue: [release-branch.go1.12] text/template: error on method calls on nil interfaces

@bep bep mentioned this issue Mar 1, 2019
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@julieqiu
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@mvdan - there isn't a reason provided in the gopherbot message. Would you mind providing one for this backport?

@mvdan
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mvdan commented Mar 13, 2019

We fixed one set of panics when making template calls, but forgot some others. This makes 1.12 more consistent in handling panics during template calls.

Is there a convention to add the reason for the backport in the backport issue directly?

gopherbot pushed a commit that referenced this issue Mar 13, 2019
@mvdan @bradfitz
[release-branch.go1.12] text/template: error on method calls on nil i…
ad8ebb9 
…nterfaces

Trying to call a method on a nil interface is a panic in Go. For
example:

	var stringer fmt.Stringer
	println(stringer.String()) // nil pointer dereference

In https://golang.org/cl/143097 we started recovering panics encountered
during function and method calls. However, we didn't handle this case,
as text/template panics before evalCall is ever run.

In particular, reflect's MethodByName will panic if the receiver is of
interface kind and nil:

	panic: reflect: Method on nil interface value

Simply add a check for that edge case, and have Template.Execute return
a helpful error. Note that Execute shouldn't just error if the interface
contains a typed nil, since we're able to find a method to call in that
case.

Finally, add regression tests for both the nil and typed nil interface
cases.

Fixes #30464.

Change-Id: Iffb21b40e14ba5fea0fcdd179cd80d1f23cabbab
Reviewed-on: https://go-review.googlesource.com/c/161761
Run-TryBot: Daniel Martí <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>
(cherry picked from commit 15b4c71)
Reviewed-on: https://go-review.googlesource.com/c/go/+/164457
Reviewed-by: Brad Fitzpatrick <[email protected]>
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Closed by merging ad8ebb9 to release-branch.go1.12.

@bep bep mentioned this issue Mar 14, 2019
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