Don't return typed nil interface values #46
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Methods that return interfaces shouldn't implicitly return nil values, they should return nil explicitly. Otherwise the result is a typed nil, which will compare != nil but still trigger a nil reference panic if any methods are called on it. https://golang.org/doc/faq#nil_error (About errors, but applies to any interface.) https://bugs.launchpad.net/juju-core/+bug/1573659 This crash was caused by a typed nil link - the line immediately before the panic checked for a nil, but because the type pointer is set on the Link it wasn't equal to nil.
| @@ -19,6 +19,11 @@ type deviceSuite struct { | |||
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| var _ = gc.Suite(&deviceSuite{}) | |||
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| func (*deviceSuite) TestNilZone(c *gc.C) { | |||
| var empty device | |||
| c.Check(empty.Zone() == nil, jc.IsTrue) | |||
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Or alternatively:
var zeroDevice device
c.Check(zeroDevice.Zone(), gc.IsNil)There was a problem hiding this comment.
I tried that first, but it uses reflection so didn't fail before I fixed the problem.
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LGTM, apart from a few suggestions for tests simplifications. |
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Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju-gomaasapi |
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Fix crash in Juju caused by typed nil Link.
https://bugs.launchpad.net/juju-core/+bug/1573659
The line immediately before the panic checked for nil, but because the type pointer is set on the Link it wasn't equal to nil.
From reading about this it seems like methods that return interfaces shouldn't implicitly return nil values, they should return nil explicitly, otherwise the result will be a typed nil. Checking for a typed nil on the client side is awkward - if the underlying type is known and accessible you could try a type assertion, otherwise it requires reflection.
https://golang.org/doc/faq#nil_error
(About errors, but applies to any interface.)