R devel - Jan 2022 - isNamespaceLoaded() while the namespace is loading

We ran into a bug in our package that seems to boil down to
isNamespaceLoaded() returning TRUE for namespaces that R is currently
loading.

We had something like this in an .onLoad function:

if (isNamespaceLoaded("upstream")) {
  upstream::upstream_function()
}

Which seems OK, unless upstream (recursively) imports the package
having this code. Should that happen, the loading of upstream triggers
the loading of this package as well and isNamespaceLoaded() seems to
return TRUE, even though the namespace of upstream is not fully loaded
yet, and we get an error that looks like this:

Error : .onLoad failed in loadNamespace() for 'foo', details:
     call: NULL
     error: 'upstream_function' is not an exported object from
'namespace:upstream'

I wonder if isNamespaceLoaded() returning TRUE is correct in this
case, or returning FALSE would be better. Or maybe it would make sense
to have a way to query the packages that are being loaded currently?

AFAICT this works, but it does use some implementation details from
loadNamespace(), so it does not seem like a proper solution:

dynGet("__NameSpacesLoading__", NULL)

Another workaround is something like this:

  is_loaded <- function(pkg) {
    if (!isNamespaceLoaded(pkg)) return(FALSE)
    tryCatch({
      loadNamespace(pkg)
      TRUE
    }, error = function(err) FALSE)
  }

which forces an error for currently loading namespaces by triggering a
(fake) recursive dependency.

Thanks,
Gabor

Hello,

We've run into this issue multiple times and it's often a head
scratcher when it happens. We are using workarounds but it would be
great to fix this for R 4.3. Would an R core member have time to
review the patch that we supplied in
https://bugs.r-project.org/show_bug.cgi?id=18489 ?

Best,
Lionel


On 1/21/22, G?bor Cs?rdi <csardi.gabor at gmail.com>
wrote:
> We ran into a bug in our package that seems to boil down to
> isNamespaceLoaded() returning TRUE for namespaces that R is currently
> loading.
>
> We had something like this in an .onLoad function:
>
> if (isNamespaceLoaded("upstream")) {
>   upstream::upstream_function()
> }
>
> Which seems OK, unless upstream (recursively) imports the package
> having this code. Should that happen, the loading of upstream triggers
> the loading of this package as well and isNamespaceLoaded() seems to
> return TRUE, even though the namespace of upstream is not fully loaded
> yet, and we get an error that looks like this:
>
> Error : .onLoad failed in loadNamespace() for 'foo', details:
>      call: NULL
>      error: 'upstream_function' is not an exported object from
> 'namespace:upstream'
>
> I wonder if isNamespaceLoaded() returning TRUE is correct in this
> case, or returning FALSE would be better. Or maybe it would make sense
> to have a way to query the packages that are being loaded currently?
>
> AFAICT this works, but it does use some implementation details from
> loadNamespace(), so it does not seem like a proper solution:
>
> dynGet("__NameSpacesLoading__", NULL)
>
> Another workaround is something like this:
>
>   is_loaded <- function(pkg) {
>     if (!isNamespaceLoaded(pkg)) return(FALSE)
>     tryCatch({
>       loadNamespace(pkg)
>       TRUE
>     }, error = function(err) FALSE)
>   }
>
> which forces an error for currently loading namespaces by triggering a
> (fake) recursive dependency.
>
> Thanks,
> Gabor
>
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