Initial implementation of lazy JLLs for LinearAlgebra

This alters CompilerSupportLibraries_jll, OpenBLAS_jll and
libblastrampoline_jll to use `LazyLibrary` objects and thereby be loaded
only upon first `dlopen()` or `ccall()` to the individual library
objects.  Note that this is one of the more complicated cases, as
`libblastrampoline` must have OpenBLAS_jll added as a dynamic dependency
(as it does not actually have it listed in its shared object headers)
and also has some on-load callbacks that must be invoked.

This must be paired with the appropriate base Julia changes [0].

[0] JuliaLang/julia#57719

Author: staticfloat

src/LinearAlgebra.jl

@@ -820,25 +820,27 @@ function versioninfo(io::IO=stdout)
     return nothing
 end
 
-function __init__()
-    try
-        verbose = parse(Bool, get(ENV, "LBT_VERBOSE", "false"))
-        BLAS.lbt_forward(OpenBLAS_jll.libopenblas_path; clear=true, verbose)
-        BLAS.check()
-    catch ex
-        Base.showerror_nostdio(ex, "WARNING: Error during initialization of module LinearAlgebra")
-    end
+function lbt_openblas_onload_callback()
+    # We don't use `BLAS.lbt_forward()` here because we don't want to take a lock on the config cache.
+    verbose = parse(Bool, get(ENV, "LBT_VERBOSE", "false"))
+    BLAS.lbt_forward_ccall(OpenBLAS_jll.libopenblas_path; clear=true, verbose)
+    BLAS.check()
+
     # register a hook to disable BLAS threading
     Base.at_disable_library_threading(() -> BLAS.set_num_threads(1))
 
     # https://github.com/xianyi/OpenBLAS/blob/c43ec53bdd00d9423fc609d7b7ecb35e7bf41b85/README.md#setting-the-number-of-threads-using-environment-variables
     if !haskey(ENV, "OPENBLAS_NUM_THREADS") && !haskey(ENV, "GOTO_NUM_THREADS") && !haskey(ENV, "OMP_NUM_THREADS")
         @static if Sys.isapple() && Base.BinaryPlatforms.arch(Base.BinaryPlatforms.HostPlatform()) == "aarch64"
-            BLAS.set_num_threads(max(1, @ccall(jl_effective_threads()::Cint)))
+            nthreads = max(1, @ccall(jl_effective_threads()::Cint))
         else
-            BLAS.set_num_threads(max(1, @ccall(jl_effective_threads()::Cint) ÷ 2))
+            nthreads = max(1, @ccall(jl_effective_threads()::Cint) ÷ 2)
         end
+        BLAS.lbt_set_num_threads(nthreads)
     end
 end
 
+# If users want to lazily load a different BLAS, they'd need to either change this call, or
+# clear the datastructures modified by this call and call it again with their own.
+libblastrampoline_jll.add_dependency!(OpenBLAS_jll, libopenblas, lbt_openblas_onload_callback)
 end # module LinearAlgebra

src/blas.jl

@@ -161,7 +161,9 @@ get_num_threads()::Int = lbt_get_num_threads()
 function check()
     # TODO: once we have bitfields of the BLAS functions that are actually forwarded,
     # ensure that we have a complete set here (warning on an incomplete BLAS implementation)
-    config = get_config()
+    # We don't use `get_config()` here because we are invoked in the onload callback and
+    # we don't want to take any locks.
+    config = LBTConfig(unsafe_load(ccall((:lbt_get_config, libblastrampoline), Ptr{lbt_config_t}, ())))
 
     # Ensure that one of our loaded libraries satisfies our interface requirement
     interface = USE_BLAS64 ? :ilp64 : :lp64

src/lbt.jl

@@ -228,10 +228,14 @@ function lbt_set_num_threads(nthreads)
     return ccall((:lbt_set_num_threads, libblastrampoline), Cvoid, (Int32,), nthreads)
 end
 
+function lbt_forward_ccall(path::AbstractString; clear::Bool = false, verbose::Bool = false, suffix_hint::Union{String,Nothing} = nothing)
+    return ccall((:lbt_forward, libblastrampoline), Int32, (Cstring, Int32, Int32, Cstring),
+                     path, clear ? 1 : 0, verbose ? 1 : 0, something(suffix_hint, C_NULL))
+end
+
 function lbt_forward(path::AbstractString; clear::Bool = false, verbose::Bool = false, suffix_hint::Union{String,Nothing} = nothing)
     _clear_config_with() do
-        return ccall((:lbt_forward, libblastrampoline), Int32, (Cstring, Int32, Int32, Cstring),
-                     path, clear ? 1 : 0, verbose ? 1 : 0, something(suffix_hint, C_NULL))
+        lbt_forward_ccall(path; clear, verbose, suffix_hint)
     end
 end