[Rd] strange behavior in 'inherits' check for loaded S4 object

[Kevin Ushey]

I should add one more item that may be related here -- calling
'methods:::.requirePackage' returns a different result based on
whether the package namespace is already loaded or not.

If the package namespace is not loaded, the package is loaded and
attached, and the package environment is returned:

    > methods:::.requirePackage("digest")
    Loading required package: digest
    <environment: package:digest>
    attr(,"name")
    [1] "package:digest"
    attr(,"path")
    [1] "/Users/kevin/Library/R/3.3/library/digest"
    > "digest" %in% loadedNamespaces()
    [1] TRUE
    > "package:digest" %in% search()
    [1] TRUE

On the other hand, if the package namespace has already been loaded,
the package namespace is returned without attaching the package:

    > requireNamespace("digest")
    Loading required namespace: digest
    > methods:::.requirePackage("digest")
    <environment: namespace:digest>
    > "digest" %in% loadedNamespaces()
    [1] TRUE
    > "package:digest" %in% search()
    [1] FALSE

This may be intentional, but the behavior seems surprising and could
be responsible for the behavior outlined earlier.

Best,
Kevin

On Fri, Jul 29, 2016 at 11:37 AM, Kevin Ushey <kevinushey at gmail.com> wrote:
> I have a small idea as to what's going on now; at least, why exporting
> the class resolves this particular issue.
>
> Firstly, when an S4 class is not exported, the associated
> '.__C__<class>' object is not made part of the package environment.
> For example, I see:
>
>     > getAnywhere(".__C__SubMatrix")
>     A single object matching '.__C__SubMatrix' was found
>     It was found in the following places
>       namespace:s4inherits
>     with value
>     < ... >
>
> Note that the symbol is only discovered in the package namespace. When
> the class is exported (e.g. with 'exportClasses(SubMatrix)' in the
> NAMESPACE file), it's found both in 'package:s4inherits' and
> 'namespace:s4inherits'.
>
> Secondly, when R attempts to resolve the superclasses for an S3 class,
> the function 'methods:::.extendsForS3' is called. Tracing that code
> eventually gets us here:
>
> https://github.com/wch/r-source/blob/trunk/src/library/methods/R/SClasses.R#L255
>
> Note that we reach this code path as the S3 class cache has not been
> populated yet; ie, this code returns NULL:
>
> https://github.com/wch/r-source/blob/trunk/src/library/methods/R/SClasses.R#L238-L240
>
> So, the class hierarchy is looked up using this code:
>
>     if(isTRUE(nzchar(package))) {
>        whereP <- .requirePackage(package)
>        value <- get0(cname, whereP, inherits = inherits)
>     }
>
> However, because the '.__C__SubMatrix' object is only made available
> in the package's namespace, not within the package environment, it is
> not resolved, and so lookup fails. (Presumedly, that lookup is done
> when initially building a cache for S3 dispatch?) So, I wonder if that
> class lookup should occur within the package's namespace instead?
>
> Thanks for your time,
> Kevin
>
> On Sat, Jun 25, 2016 at 12:46 PM, Kevin Ushey <kevinushey at gmail.com> wrote:
>> Hi,
>>
>> (sorry for the wall of text; the issue here appears to be rather complicated)
>>
>> I'm seeing a somewhat strange case where checking whether an S4 object
>> inherits from a parent class defined from another package with
>> 'inherits' fails if that object is materialized through a call to
>> 'load'. That's a mouthful, so I've put together a relatively small
>> reproducible example online here:
>>
>> https://github.com/kevinushey/s4inherits
>>
>> This package, 's4inherits', defines an S4 class, 'SubMatrix', that
>> inherits from the 'dsyMatrix' class defined in the Matrix package.
>> After installing the package, I run some simple tests:
>>
>> $ R -f test-save.R
>>
>>> library(s4inherits)
>>> data <- SubMatrix(1)
>>>
>>> is(data, "SubMatrix")
>> [1] TRUE
>>> inherits(data, "SubMatrix")
>> [1] TRUE
>>>
>>> is(data, "dsyMatrix")
>> [1] TRUE
>>> inherits(data, "dsyMatrix")
>> [1] TRUE
>>>
>>> save(data, file = "test.RData")
>>>
>>
>> All the inheritance checks report as we would expect. I check that the
>> inheritance reports are as expected, then save that object to
>> 'test.RData'. I then load that data file in a new R session and run
>> the same checks:
>>
>> $ R -f test-load.R
>>
>>> library(methods)
>>> load("test.RData")
>>>
>>> inherits(data, "SubMatrix")
>> Loading required package: s4inherits
>> [1] TRUE
>>> is(data, "SubMatrix")
>> [1] TRUE
>>>
>>> inherits(data, "dsyMatrix")
>> [1] FALSE # (??)
>>> is(data, "dsyMatrix")
>> [1] TRUE
>>>
>>
>> Note that R now reports that my loaded object does _not_ inherit from
>> "dsyMatrix", yet this occurs only when checked with 'inherits()' --
>> 'is' produces the expected result.
>>
>> I do not see the behavior if I explicitly load / attach the
>> 's4inherits' package before loading the associated data file; it only
>> occurs if the package namespace is loaded in response to loading the
>> data object hosting a 'SubMatrix' object.
>>
>> More precisely, the package namespace is loaded when the promise
>> hosting the data object is evaluated; that promise being generated by
>> 'load', if I understand correctly. Somehow, evaluation of that promise
>> within the call to 'inherits' in this very special case causes the
>> unexpected behavior -- ie, if the first thing that you do with the
>> loaded object is check its class with 'inherits', then you get this
>> unexpected result.
>>
>> Even more, this behavior seems to go away if the 's4inherits' package
>> explicitly exports the class -- however, you could imagine this class
>> normally being internal to the package, and so it may be undesirable
>> to export it.
>>
>> I checked a bit into the C code, and IIUC the check here looks up the
>> class hierarchy in the R_S4_extends_table object defined in
>> 'attrib.c', so it seems like that mapping is potentially not getting
>> constructed with the full hierarchy (although hopefully someone with
>> more knowledge of the S4 internals + interaction with S3 can
>> elaborate).
>>
>> (FWIW, this was distilled from a case where S3 dispatch on a similar
>> loaded S4 object failed, due to failure to resolve the class hierarchy
>> for the S3 dispatch context.)
>>
>> Thanks,
>> Kevin
>>
>> ---
>>
>> $ R --slave -e "utils::sessionInfo()"
>> R Under development (unstable) (2016-06-13 r70769)
>> Platform: x86_64-apple-darwin15.5.0 (64-bit)
>> Running under: OS X 10.11.5 (El Capitan)