Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

[Ninja Ikta]

I encountered this tricky bug today. After going through all the playground 
examples, I finally understood what was happening.

Similar to Jochen, I was including encoding.MarshalBinary() and 
encoding.UnmarshalBinary() in the interface definition.

A question though. Why does this bug resolve by simply eliminating the 
above two method signatures from the interface definition?

1) My basic understanding was that including these method signatures just 
informs that the underlying concrete type will support these methods. How 
does their inclusion or exclusion change anything?
2) Even if the method signatures are not part of the interface definition, 
the problem of dereferencing a nil-Duck still exists. How does that get 
resolved of methods are excluded in interface definition?

Thanks!

On Friday, August 16, 2019 at 5:45:44 PM UTC+5:30 Jochen Voss wrote:

> Hi Axel,
>
> Thanks a lot for looking into this.  Your post makes things much clearer 
> for me.  In particular, I now think that I probably should not have 
> (Un)MarshalBinary() methods on the interface type, but have them only on 
> the concrete type instead.  For example, this works: 
> https://play.golang.org/p/oBgHqN74ZeM .
>
>
> On Thursday, 15 August 2019 11:16:45 UTC+1, Axel Wagner wrote:
>>
>> The panic, I think, comes from the fact that you pass a *Duck, which then 
>> gets dereferenced, pointing at a nil Duck. Even a nil-Duck *does* have a 
>> DecodeBinary method though, so gob thinks it should use that. It then 
>> type-asserts to BinaryDecoder, which panics, because it's a nil-interface, 
>> just like this example: https://play.golang.org/p/EEwOt0FQunh.
>>
>
> I had forgotten that type assertions don't allow nil values, but indeed 
> they don't.  Thank you for reminding me.
>
> Many thanks,
> Jochen
>

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Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

[Jochen Voss]

Hi Axel,

Thanks a lot for looking into this.  Your post makes things much clearer 
for me.  In particular, I now think that I probably should not have 
(Un)MarshalBinary() methods on the interface type, but have them only on 
the concrete type instead.  For example, this 
works: https://play.golang.org/p/oBgHqN74ZeM .

On Thursday, 15 August 2019 11:16:45 UTC+1, Axel Wagner wrote:
>
> The panic, I think, comes from the fact that you pass a *Duck, which then 
> gets dereferenced, pointing at a nil Duck. Even a nil-Duck *does* have a 
> DecodeBinary method though, so gob thinks it should use that. It then 
> type-asserts to BinaryDecoder, which panics, because it's a nil-interface, 
> just like this example: https://play.golang.org/p/EEwOt0FQunh.
>

I had forgotten that type assertions don't allow nil values, but indeed 
they don't.  Thank you for reminding me.

Many thanks,
Jochen

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Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

['Axel Wagner' via golang-nuts]

Please see upthread - gob does that for you.

On Thu, Aug 15, 2019 at 5:59 PM Robert Engels  wrote:

> I think you will need to encode a struct that contains the type name and
> the struct. Then you decode using reflection.
>
> -Original Message-
> From: 'Axel Wagner' via golang-nuts
> Sent: Aug 15, 2019 3:12 AM
> To: Jochen Voss
> Cc: golang-nuts
> Subject: Re: [go-nuts] panic: interface conversion: interface is nil, not
> encoding.BinaryUnmarshaler
>
> I haven't really used gob much, so unfortunately I can't *really* help
> you. But one thing to note is that you can dump the content of the buffer
> and see that it doesn't actually contain the name of the type you are
> encoding: https://play.golang.org/p/R8HB6RP8kS0
> I agree that from the documentation, it seems to me that it *should* and I
> can't really see what this is doing wrong. And I agree that this probably
> shouldn't panic, but instead return an error. But given that the encoded
> bytes don't contain the name of the concrete struct, there is no way the
> decoder can know where you want to put it, without giving it the correct
> type.
>
> On Thu, Aug 15, 2019 at 9:22 AM Jochen Voss  wrote:
>
>> Dear Marcin,
>>
>> My aim is to unmarshal into an interface variable, without having to
>> know in advance which concrete type I'm receiving (the actual interface has
>> several possible implementations, and gob data comes in over a network
>> connection). So, while your code avoids the panic, it does not solve my
>> problem.
>>
>> Many thanks,
>> Jochen
>>
>> On Wednesday, 14 August 2019 22:12:54 UTC+1, Marcin Romaszewicz wrote:
>>>
>>> Here you go:
>>> https://play.golang.org/p/_cPmaSxRatC
>>>
>>> You want to unmarshal into , not into 
>>>
>>> This means:
>>> var duck2 A
>>>
>>> not:
>>> var duck2 Duck
>>>
>>> On Wed, Aug 14, 2019 at 8:46 AM Jochen Voss  wrote:
>>>
>>>> Hello,
>>>>
>>>> I'm trying to read gob-encoded data into a variable of interface type.
>>>> In simple cases, this works for me, but when I use a custom encoding
>>>> via MarshalBinary() and UnmarshalBinary() methods, my code keeps crashing,
>>>> with the error message
>>>>
>>>> panic: interface conversion: interface is nil, not
>>>> encoding.BinaryUnmarshaler
>>>>
>>>> Example:
>>>>
>>>> - The code at https://play.golang.org/p/y8nWNhObUwb, letting gob do
>>>> the en-/decoding works.
>>>>
>>>> - The very similar code at https://play.golang.org/p/-zS7QjEJg9x,
>>>> which uses MarshalBinary() and UnmarshalBinary() crashes with the panic
>>>> shown above.
>>>>
>>>> What am I doing wrong?
>>>>
>>>> [I asked the same question at
>>>> https://stackoverflow.com/questions/57485698/, with no answers so far]
>>>>
>>>> Many thanks,
>>>> Jochen
>>>>
>>>> --
>>>> You received this message because you are subscribed to the Google
>>>> Groups "golang-nuts" group.
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>>>> an email to golan...@googlegroups.com.
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>>>> .
>>>>
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Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

['Axel Wagner' via golang-nuts]

Hm, I looked a bit at the code. ISTM that if you implement
BinaryMarshaler/Unmarshaler (or GobEncoder/Decoder), the values are sent as
a []byte, not an interface, so the concrete type information is lost. You
can kind of circumvent this, by instead passing the values as interface{}:
https://play.golang.org/p/n5oxq9g2Q-9 -- As you can see, gob still uses the
methods, but because you more specifically say that it should en/decode
interfaces, it's passing the type-info along too. This will require that
you do your own type-assertions after decoding, but maybe that's acceptable?

The panic, I think, comes from the fact that you pass a *Duck, which then
gets dereferenced, pointing at a nil Duck. Even a nil-Duck *does* have a
DecodeBinary method though, so gob thinks it should use that. It then
type-asserts to BinaryDecoder, which panics, because it's a nil-interface,
just like this example: https://play.golang.org/p/EEwOt0FQunh. Arguably,
the language should allow this (returning nil from the type-assertion), but
either way, it won't help you, because it would panic when calling the
method anyway. So, in a way, gob behaves correctly. It says that if the
value you decode into implements BinaryUnmarshaler, it will call it's
UnmarshalBinary method. And the BinaryUnmarshaler you pass will panic if
you actually call that method. It's still not a great failure mode.

I hope this kinda helps. Or that someone can give a more authoritative
answer :)

On Thu, Aug 15, 2019 at 11:54 AM Axel Wagner 
wrote:

> On Thu, Aug 15, 2019 at 11:14 AM Jochen Voss 
> wrote:
>
>> You are right.  But the type is somehow represented by some index or so,
>> instead of the name? Since the type needs to be registered using
>> gob.Register() beforehand, this seems not totally impossible to me.
>>
>
> `Register` only gets the type-name though. It somehow would have to
> reproducibly map from type-names to indices and back for this to work - the
> process doing the *decoding* isn't necessarily the same process doing the
> encoding (that's kind of the point :) ). So AIUI, the name still needs to
> be present. The documentation
>  says so too:
>
> Interface values are transmitted as a string identifying the concrete type
>> being sent (a name that must be pre-defined by calling Register), followed
>> by a byte count of the length of the following data (so the value can be
>> skipped if it cannot be stored), followed by the usual encoding of concrete
>> (dynamic) value stored in the interface value. (A nil interface value is
>> identified by the empty string and transmits no value.) Upon receipt, the
>> decoder verifies that the unpacked concrete item satisfies the interface of
>> the receiving variable.
>
>
> I wish I could help more :) I'd be interested to see how you get this
> resolved though (and maybe someone who used gob more can chime in). AIUI it
> *should* be possible to do what you want, so once we figured out how, it
> would probably be super helpful to contribute it as a runnable example.
>
>   But then, when I change your code to let gob do the encoding itself, the
>> name is suddenly there: https://play.golang.org/p/Qz7zDKL047z, so
>> probably the absence of the type name is part of the problem.
>>
>
>>
>>> And I agree that this probably shouldn't panic, but instead return an
>>> error. But given that the encoded bytes don't contain the name of the
>>> concrete struct, there is no way the decoder can know where you want to put
>>> it, without giving it the correct type.
>>>
>>
>> Yes, an informative error instead of the hard to interpret panic would be
>> nice.  I wonder whether this is indeed intended behaviour, or whether this
>> is some kind of bug in the encoding/gob package.
>>
>> --
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>> 
>> .
>>
>

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Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

['Axel Wagner' via golang-nuts]

On Thu, Aug 15, 2019 at 11:14 AM Jochen Voss  wrote:

> You are right.  But the type is somehow represented by some index or so,
> instead of the name? Since the type needs to be registered using
> gob.Register() beforehand, this seems not totally impossible to me.
>

`Register` only gets the type-name though. It somehow would have to
reproducibly map from type-names to indices and back for this to work - the
process doing the *decoding* isn't necessarily the same process doing the
encoding (that's kind of the point :) ). So AIUI, the name still needs to
be present. The documentation
 says so too:

Interface values are transmitted as a string identifying the concrete type
> being sent (a name that must be pre-defined by calling Register), followed
> by a byte count of the length of the following data (so the value can be
> skipped if it cannot be stored), followed by the usual encoding of concrete
> (dynamic) value stored in the interface value. (A nil interface value is
> identified by the empty string and transmits no value.) Upon receipt, the
> decoder verifies that the unpacked concrete item satisfies the interface of
> the receiving variable.


I wish I could help more :) I'd be interested to see how you get this
resolved though (and maybe someone who used gob more can chime in). AIUI it
*should* be possible to do what you want, so once we figured out how, it
would probably be super helpful to contribute it as a runnable example.

  But then, when I change your code to let gob do the encoding itself, the
> name is suddenly there: https://play.golang.org/p/Qz7zDKL047z, so
> probably the absence of the type name is part of the problem.
>

>
>> And I agree that this probably shouldn't panic, but instead return an
>> error. But given that the encoded bytes don't contain the name of the
>> concrete struct, there is no way the decoder can know where you want to put
>> it, without giving it the correct type.
>>
>
> Yes, an informative error instead of the hard to interpret panic would be
> nice.  I wonder whether this is indeed intended behaviour, or whether this
> is some kind of bug in the encoding/gob package.
>
> --
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Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

[Jochen Voss]

Hi Axel,

On Thursday, 15 August 2019 09:13:17 UTC+1, Axel Wagner wrote:
>
> I haven't really used gob much, so unfortunately I can't *really* help 
> you. But one thing to note is that you can dump the content of the buffer 
> and see that it doesn't actually contain the name of the type you are 
> encoding: https://play.golang.org/p/R8HB6RP8kS0
>

You are right.  But the type is somehow represented by some index or so, 
instead of the name?  Since the type needs to be registered using 
gob.Register() beforehand, this seems not totally impossible to me.  But 
then, when I change your code to let gob do the encoding itself, the name 
is suddenly there: https://play.golang.org/p/Qz7zDKL047z, so probably the 
absence of the type name is part of the problem.
 

> And I agree that this probably shouldn't panic, but instead return an 
> error. But given that the encoded bytes don't contain the name of the 
> concrete struct, there is no way the decoder can know where you want to put 
> it, without giving it the correct type.
>

Yes, an informative error instead of the hard to interpret panic would be 
nice.  I wonder whether this is indeed intended behaviour, or whether this 
is some kind of bug in the encoding/gob package.

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Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

['Axel Wagner' via golang-nuts]

I haven't really used gob much, so unfortunately I can't *really* help you.
But one thing to note is that you can dump the content of the buffer and
see that it doesn't actually contain the name of the type you are encoding:
https://play.golang.org/p/R8HB6RP8kS0
I agree that from the documentation, it seems to me that it *should* and I
can't really see what this is doing wrong. And I agree that this probably
shouldn't panic, but instead return an error. But given that the encoded
bytes don't contain the name of the concrete struct, there is no way the
decoder can know where you want to put it, without giving it the correct
type.

On Thu, Aug 15, 2019 at 9:22 AM Jochen Voss  wrote:

> Dear Marcin,
>
> My aim is to unmarshal into an interface variable, without having to know
> in advance which concrete type I'm receiving (the actual interface has
> several possible implementations, and gob data comes in over a network
> connection). So, while your code avoids the panic, it does not solve my
> problem.
>
> Many thanks,
> Jochen
>
> On Wednesday, 14 August 2019 22:12:54 UTC+1, Marcin Romaszewicz wrote:
>>
>> Here you go:
>> https://play.golang.org/p/_cPmaSxRatC
>>
>> You want to unmarshal into , not into 
>>
>> This means:
>> var duck2 A
>>
>> not:
>> var duck2 Duck
>>
>> On Wed, Aug 14, 2019 at 8:46 AM Jochen Voss  wrote:
>>
>>> Hello,
>>>
>>> I'm trying to read gob-encoded data into a variable of interface type.
>>> In simple cases, this works for me, but when I use a custom encoding
>>> via MarshalBinary() and UnmarshalBinary() methods, my code keeps crashing,
>>> with the error message
>>>
>>> panic: interface conversion: interface is nil, not
>>> encoding.BinaryUnmarshaler
>>>
>>> Example:
>>>
>>> - The code at https://play.golang.org/p/y8nWNhObUwb, letting gob do the
>>> en-/decoding works.
>>>
>>> - The very similar code at https://play.golang.org/p/-zS7QjEJg9x, which
>>> uses MarshalBinary() and UnmarshalBinary() crashes with the panic shown
>>> above.
>>>
>>> What am I doing wrong?
>>>
>>> [I asked the same question at
>>> https://stackoverflow.com/questions/57485698/, with no answers so far]
>>>
>>> Many thanks,
>>> Jochen
>>>
>>> --
>>> You received this message because you are subscribed to the Google
>>> Groups "golang-nuts" group.
>>> To unsubscribe from this group and stop receiving emails from it, send
>>> an email to golan...@googlegroups.com.
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>>> https://groups.google.com/d/msgid/golang-nuts/b19c51bc-524c-4292-915b-fc00e9289052%40googlegroups.com
>>> 
>>> .
>>>
>> --
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Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

[Jochen Voss]

Dear Marcin,

My aim is to unmarshal into an interface variable, without having to know 
in advance which concrete type I'm receiving (the actual interface has 
several possible implementations, and gob data comes in over a network 
connection). So, while your code avoids the panic, it does not solve my 
problem.

Many thanks,
Jochen

On Wednesday, 14 August 2019 22:12:54 UTC+1, Marcin Romaszewicz wrote:
>
> Here you go:
> https://play.golang.org/p/_cPmaSxRatC
>
> You want to unmarshal into , not into 
>
> This means:
> var duck2 A
>
> not:
> var duck2 Duck
>
> On Wed, Aug 14, 2019 at 8:46 AM Jochen Voss  > wrote:
>
>> Hello,
>>
>> I'm trying to read gob-encoded data into a variable of interface type.  
>> In simple cases, this works for me, but when I use a custom encoding 
>> via MarshalBinary() and UnmarshalBinary() methods, my code keeps crashing, 
>> with the error message
>>
>> panic: interface conversion: interface is nil, not 
>> encoding.BinaryUnmarshaler
>>
>> Example:
>>
>> - The code at https://play.golang.org/p/y8nWNhObUwb, letting gob do the 
>> en-/decoding works.
>>
>> - The very similar code at https://play.golang.org/p/-zS7QjEJg9x, which 
>> uses MarshalBinary() and UnmarshalBinary() crashes with the panic shown 
>> above.
>>
>> What am I doing wrong?
>>
>> [I asked the same question at 
>> https://stackoverflow.com/questions/57485698/, with no answers so far]
>>
>> Many thanks,
>> Jochen
>>
>> -- 
>> You received this message because you are subscribed to the Google Groups 
>> "golang-nuts" group.
>> To unsubscribe from this group and stop receiving emails from it, send an 
>> email to golan...@googlegroups.com .
>> To view this discussion on the web visit 
>> https://groups.google.com/d/msgid/golang-nuts/b19c51bc-524c-4292-915b-fc00e9289052%40googlegroups.com
>>  
>> 
>> .
>>
>

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Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

[Marcin Romaszewicz]

Here you go:
https://play.golang.org/p/_cPmaSxRatC

You want to unmarshal into , not into 

This means:
var duck2 A

not:
var duck2 Duck

On Wed, Aug 14, 2019 at 8:46 AM Jochen Voss  wrote:

> Hello,
>
> I'm trying to read gob-encoded data into a variable of interface type.  In
> simple cases, this works for me, but when I use a custom encoding
> via MarshalBinary() and UnmarshalBinary() methods, my code keeps crashing,
> with the error message
>
> panic: interface conversion: interface is nil, not
> encoding.BinaryUnmarshaler
>
> Example:
>
> - The code at https://play.golang.org/p/y8nWNhObUwb, letting gob do the
> en-/decoding works.
>
> - The very similar code at https://play.golang.org/p/-zS7QjEJg9x, which
> uses MarshalBinary() and UnmarshalBinary() crashes with the panic shown
> above.
>
> What am I doing wrong?
>
> [I asked the same question at
> https://stackoverflow.com/questions/57485698/, with no answers so far]
>
> Many thanks,
> Jochen
>
> --
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> 
> .
>

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[go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

[Jochen Voss]

Hello,

I'm trying to read gob-encoded data into a variable of interface type.  In 
simple cases, this works for me, but when I use a custom encoding 
via MarshalBinary() and UnmarshalBinary() methods, my code keeps crashing, 
with the error message

panic: interface conversion: interface is nil, not 
encoding.BinaryUnmarshaler

Example:

- The code at https://play.golang.org/p/y8nWNhObUwb, letting gob do the 
en-/decoding works.

- The very similar code at https://play.golang.org/p/-zS7QjEJg9x, which 
uses MarshalBinary() and UnmarshalBinary() crashes with the panic shown 
above.

What am I doing wrong?

[I asked the same question at https://stackoverflow.com/questions/57485698/, 
with no answers so far]

Many thanks,
Jochen

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