Support request about FFI Panama Implementation

[Jorn Vernee]

I want to caution a bit that, while the FFM API does allow loading the 
same library in multiple class loaders, the underlying OS library loader 
will _not_ load the same library twice, but instead keeps a reference 
count for each library, and increments it when it is loaded (again), and 
decrements it when it is unload. This can be demonstrated with the 
following example:

lib.c:

     int x;

Main.java:

     SymbolLookup s1 = SymbolLookup.libraryLookup("lib.so", Arena.global());
     MemorySegment x1 = s1.findOrThrow("x")
             .reinterpret(ValueLayout.JAVA_INT.byteSize());
     SymbolLookup s2 = SymbolLookup.libraryLookup("lib.so", Arena.global());
     MemorySegment x2 = s2.findOrThrow("x")
             .reinterpret(ValueLayout.JAVA_INT.byteSize());

     x1.set(ValueLayout.JAVA_INT, 0L, 42);
     System.out.println(x2.get(ValueLayout.JAVA_INT, 0L));

This program will print '42'. Note that this is a limitation of the 
operating system's library loader, combined with how the library is 
implemented.

 > The goal is that each object/tread parallely instantiate the same 
library but having isolated states, configurations and variables stored 
in a dedicated memory slot for each tread, so even if the C++ library 
maybe does not completely support cuncurrency itself it should attend 
multiple treads as it is virtually responding at once having one 
specific configuration per user.

In other words, what you seem to want here is not possible without 
either: 1. implementing a custom library loader that creates separate 
instances of the library in memory, or 2. changing the library so that 
each client can have a separate library state (this is typically done by 
the library returning some kind of 'handle', and then that handle is 
used in subsequent operations).

Jorn

On 5-2-2025 10:50, Maurizio Cimadamore wrote:
>
> Hi Roberto,
> by "singleton limitation" in JNA, I assume you refer to the fact that, 
> in JNA (but also JNI) you can only load a native library once, by only 
> one classloader. Correct?
>
> If that's the limitation you are trying to workaround, then yes, FFM 
> allows you to load the same native library multiple times using its 
> SymbolLookup abstraction. Make sure to use the correct factory -- the 
> one that does not rely on an association with the current classloader:
>
> https://docs.oracle.com/en/java/javase/23/docs//api/java.base/java/lang/foreign/SymbolLookup.html#libraryLookup(java.lang.String,java.lang.foreign.Arena)
>
> This is just a very thin wrapper around dlopen, so it will take 
> whatever library name dlopen can resolve, and will also allow you to 
> open the library multiple times. Note that the library will be 
> unloaded when the provided arena is closed -- so ideally you could use 
> one arena per client.
>
> Examples of Arena.ofConfined, or custom arenas can be found in the 
> javadoc:
>
> * confined arena - 
> https://docs.oracle.com/en/java/javase/23/docs//api/java.base/java/lang/foreign/package-summary.html
>
> * custom arena - 
> https://docs.oracle.com/en/java/javase/22/docs//api/java.base/java/lang/foreign/SymbolLookup.html#libraryLookup(java.lang.String,java.lang.foreign.Arena)
>
> We also have some narrative documents on how to use memory segment and 
> linker API here:
>
> https://github.com/openjdk/panama-foreign/blob/foreign-memaccess%2Babi/doc/panama_memaccess.md
>
> https://github.com/openjdk/panama-foreign/blob/foreign-memaccess%2Babi/doc/panama_ffi.md
>
> And, if you want to know more on jextract, we have a comprehensive 
> guide here:
>
> https://github.com/openjdk/jextract/blob/master/doc/GUIDE.md
>
>
> Cheers
> Maurizio
>
> On 05/02/2025 00:03, roberto.minoletti at trusthub.cloud wrote:
>>
>> Hello Maurizio,
>>
>> Thank you to provide the right contact of the Panama project (I 
>> reached to you in LinkedIn some days ago)
>>
>> I am writing to you on behalf of a backend development team that is 
>> trying to accomplish our requirements.
>>
>> We need to interact from our java code (JDK21 that we can easely 
>> update to JDK23 if required and Spring Web-Flux) to a C++ library 
>> that allow to create Slots, generate Keypairs store Certificates 
>> simulating a real HSM (Hardware Security Module). This is named 
>> libsofthsm2.so (SoftHSM2).
>>
>> As I mentioned we customized this library as we need to store 
>> certificates and keys in a different path for each user. One 
>> important requirement is that we need to be able to process many 
>> requests coming from many different users to the same library that 
>> should operate in different paths at the same time.
>>
>> We preliminary resolved a POC using JNA but when tried to execute 
>> many parallel treads we faced with his singleton limitation and after 
>> a deep reasearch we founded Panama project.
>>
>> Does Panama FFI support multiple parallel treads? Does it have any 
>> Singleton limitations?
>>
>> What we need is to get many objects/treads at the same time 
>> (paralley) loading its own different instance of the same library, 
>> getting its own memory context, variables and states, all completely 
>> isolated but working at the same time. This was not possible for us 
>> because JNA uses the Singleton pattern to load the library directly 
>> into the process, making it only possible to have a single instance 
>> of the library for the entire process.
>>
>> The goal is that each object/tread parallely instantiate the same 
>> library but having isolated states, configurations and variables 
>> stored in a dedicated memory slot for each tread, so even if the C++ 
>> library maybe does not completely support cuncurrency itself it 
>> should attend multiple treads as it is virtually responding at once 
>> having one specific configuration per user.
>>
>> What we are looking for is that each ARENA has access to 
>> libsofthsm2.so and its internal variables using that dedicated memory 
>> space to each tread to calculate the operations it must perform.
>>
>> Does Panama FFI allow all this? This is the first basic question we 
>> have. After this we will be able to perform more specific questions.
>>
>> Also if you have any sample of code that use Arena.ofConfined() or 
>> any Custom Arena it would be really fantastic.
>>
>> Thank you in advance for any information and/or support impelmenting 
>> Panama in our Project.
>>
>> Roberto Minoletti
>>
>> Trusthub LLC
>>
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