BLAS: avoid calling dlclose (#27509)

We do not want libblas to get unloaded again here,
but we call this early enough, this is potentially
the only reference to it. That may lead to race
conditions where we try to teardown the library
while it is trying to spawn threads
(spawning threads is not generally permitted on Windows,
but is down anyways by libopenblas.dll).

So instead, we just cache our single reference to it,
and thereby avoid the need to call `dlclose`
to maintain the proper refcount.

stdlib/LinearAlgebra/src/blas.jl

@@ -69,8 +69,9 @@ import LinearAlgebra: BlasReal, BlasComplex, BlasFloat, BlasInt, DimensionMismat
 import Libdl
 
 # utility routines
-function vendor()
- lib = Libdl.dlopen_e(Base.libblas_name)
+let lib = C_NULL
+global function vendor()
+ lib == C_NULL && (lib = Libdl.dlopen_e(Base.libblas_name))
  vend = :unknown
  if lib != C_NULL
  if Libdl.dlsym_e(lib, :openblas_set_num_threads) != C_NULL
@@ -80,23 +81,23 @@ function vendor()
  elseif Libdl.dlsym_e(lib, :MKL_Set_Num_Threads) != C_NULL
  vend = :mkl
  end
- Libdl.dlclose(lib)
  end
  return vend
 end
+end
 
 if vendor() == :openblas64
  macro blasfunc(x)
  return Expr(:quote, Symbol(x, "64_"))
  end
- openblas_get_config() = strip(unsafe_string(ccall((:openblas_get_config64_, Base.libblas_name), Ptr{UInt8}, () )))
 else
  macro blasfunc(x)
  return Expr(:quote, x)
  end
- openblas_get_config() = strip(unsafe_string(ccall((:openblas_get_config, Base.libblas_name), Ptr{UInt8}, () )))
 end
 
+openblas_get_config() = strip(unsafe_string(ccall((@blasfunc(openblas_get_config), Base.libblas_name), Ptr{UInt8}, () )))
+
 """
  set_num_threads(n)