Ideas for runtime loading of shared libraries.

[Martin Nowak]

On Tue, 03 Jan 2012 18:27:56 +0100, Sean Kelly <sean at invisibleduck.org>  
wrote:

> The trick seems to be mapping in TLS (on OSX anyway) and running static  
> crore at the right time. Are there other issues as well?
>
I was hoping to hook thread local module ctors to TLS initialization
which is already done lazily, but the semantics of 'static this()'
allow to run arbitrary code, so the right time currently is before any
code/data from that library can be accessed by this particular thread.
This necessitates to initialize all library dependencies as well.

Implementing dynamic TLS support for OSX might lead to some useful  
findings.

> Sent from my iPhone
>
> On Jan 3, 2012, at 8:53 AM, "Martin Nowak" <dawg at dawgfoto.de> wrote:
>
>> On Tue, 03 Jan 2012 08:20:38 +0100, Jacob Carlborg <doob at me.com> wrote:
>>
>>> On 2012-01-02 21:57, Martin Nowak wrote:
>>>> On Mon, 02 Jan 2012 20:38:50 +0100, Jacob Carlborg <doob at me.com>  
>>>> wrote:
>>>>
>>>>> On 2012-01-02 20:20, Martin Nowak wrote:
>>>>>> I think that I'll defer the support for runtime loading of shared
>>>>>> library (plugins)
>>>>>> in favor of getting linked shared library support done now.
>>>>>> There are several issues that require more thoughts.
>>>>>>
>>>>>> - Per-thread initialization of modules is somewhat tricky.
>>>>>> Doing it in Runtime.loadLibrary requires knowledge of shared library
>>>>>> dependencies
>>>>>> because different threads might share dependencies but this is not
>>>>>> provided by libc/libdl.
>>>>>>
>>>>>> - Libraries might not be unloaded as long as GC collected class
>>>>>> instances still exist because
>>>>>> finalization fails otherwise.
>>>>>>
>>>>>> - Getting symbols through mangled names is difficult/unstable.
>>>>>>
>>>>>> - D libraries used by a C library should provide proper runtime
>>>>>> initialization
>>>>>> even if the C library is used by a D application.
>>>>>>
>>>>>> Any ideas or use-cases for plugins are welcome.
>>>>>>
>>>>>> martin
>>>>>
>>>>>
>>>>> - Initializing module infos
>>>>> - Initializing exception handling tables
>>>>> - Running module constructors
>>>>> - Initializing TLS
>>>>>
>>>>> Then also unload all this when the library is unloaded.
>>>>>
>>>> It seems that libraries can't be unloaded deterministically,
>>>> because GC finalization still references them.
>>>>
>>>>> On Mac OS X, can't "_dyld_register_func_for_add_image" be used? Then
>>>>> it will work, hopefully, transparently for the user. D libraries used
>>>>> by C wouldn't need any different handling. Because they will be  
>>>>> linked
>>>>> with druntime it can initializing everything with the help of
>>>>> "_dyld_register_func_for_add_image".
>>>>>
>>>>
>>>> That was the approach I took and it is partly a dead-end.
>>>>
>>>> I have a mechanism similar to _dyld_register_func_for_add_image
>>>> but runtime loaders have no notion of per-thread initialization,
>>>> i.e. when two threads load the same library only the first one will
>>>> actually cause the image to be loaded.
>>>> This implies that the second thread would need to check all
>>>> dependencies of the loaded library to do the initialization.
>>>> I've written something along this line but it requires to
>>>> exploit/rewrite part of the runtime linker.
>>>> Using dlmopen on linux would be a terrible inefficient hack
>>>> around this issue, it allows to load libraries multiple times.
>>>
>>> I'm not quite sure I understand. Most of the things that should be  
>>> done, initializing module infos and so on, should only be done once.
>>>
>> Yes most, but not all.
>> The core issue here is that C++'s __thread doesn't allow dynamic  
>> initializers,
>> thus there is no infrastructure to do such things. And really a clean  
>> approach
>> would be to extend libc/ld.so.