feature request: export static context through a function #1637

1. 
   purpleKarrot commented at 1:51 pm on November 20, 2024: none

   Some language bindings have issues with exported variables. The static context is currently exported as a variable:

   0SECP256K1_API const secp256k1_context *secp256k1_context_static;
   

   which is implemented as:

   0const secp256k1_context *secp256k1_context_static = &secp256k1_context_static_;
   

   A more portable way would be to provide it through a function:

   0SECP256K1_API const secp256k1_context *secp256k1_context_static(void);
   

   with implementation:

   0const secp256k1_context *secp256k1_context_static(void)
   1{
   2    return &secp256k1_context_static_;
   3}
   

   PS: It is clear that changing the symbol into a function is a breaking change and therefore off the table. However, please consider adding a new symbol with the proposed implementation.

2. real-or-random added the label feature on Nov 20, 2024
3. 
   real-or-random commented at 3:32 pm on November 20, 2024: contributor

   Some language bindings have issues with exported variables.

   Can you elaborate a bit? Which bindings and what are the issues?

4. 
   purpleKarrot commented at 7:25 pm on November 20, 2024: none

   In Swift, when secp256k1_context_static is passed to a function, the compiler complains:

   Reference to var ‘secp256k1_context_static’ is not concurrency-safe because it involves shared mutable state

   This is definitely a false positive, because the instance is const. The workaround is to define a custom function in the ffi wrapper: https://github.com/swift-bitcoin/swift-bitcoin/blob/develop/src/ecc-helper/ecdsa.c#L6-L8

   I remember having had issues with exported variables in the past with other languages. I don’t remember the exact details. It may have been with Javascript, Go, Lua, or Matlab. It is less of an issue when the variable is of a builtin type, which is the case here with a pointer.

   While it is easily possible to find work-arounds for any ffi, providing functions only is just more intuitive. Consider this Python:

    0>>> from ctypes import CDLL
    1>>> lib = CDLL("libsecp256k1.5.dylib")
    2>>> lib
    3<CDLL 'libsecp256k1.5.dylib', handle 6d672be0 at 0x102b3a430>
    4>>> lib.secp256k1_selftest
    5<_FuncPtr object at 0x102b83340>
    6>>> lib.secp256k1_selftest()
    7-1486072369
    8>>> lib.secp256k1_context_static
    9<_FuncPtr object at 0x102b83a00>   # <--- orly?
   10>>> lib.secp256k1_context_static()
   11fish: Job 1, 'python3' terminated by signal SIGBUS (Misaligned address error)
   

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