[LinearAlgebra] flesh out LBT API a bit more

This adds `lbt_find_backing_library()`, which is a useful debugging
routine to allow advanced users/package authors to query LBT to
determine which backing BLAS library will service a particular BLAS
call.  It also exposes the "footgun API", which allows users to directly
set/get forwarding on a per-function basis.  Because this has the
ability to generate truly bizarre setups, we do not advertise this
capability broadly (simply using `lbt_forward()` should be enough for
most usecases) however it's nice to have wrapped.

stdlib/LinearAlgebra/src/blas.jl

@@ -79,7 +79,7 @@ import LinearAlgebra: BlasReal, BlasComplex, BlasFloat, BlasInt, DimensionMismat
 include("lbt.jl")
 
 """
-get_config()
+    get_config()
 
 Return an object representing the current `libblastrampoline` configuration.
 

stdlib/LinearAlgebra/src/lbt.jl

@@ -17,6 +17,7 @@ const LBT_INTERFACE_MAP = Dict(
     LBT_INTERFACE_ILP64   => :ilp64,
     LBT_INTERFACE_UNKNOWN => :unknown,
 )
+const LBT_INV_INTERFACE_MAP = Dict(v => k for (k, v) in LBT_INTERFACE_MAP)
 
 const LBT_F2C_PLAIN         =  0
 const LBT_F2C_REQUIRED      =  1
@@ -26,6 +27,7 @@ const LBT_F2C_MAP = Dict(
     LBT_F2C_REQUIRED => :required,
     LBT_F2C_UNKNOWN  => :unknown,
 )
+const LBT_INV_F2C_MAP = Dict(v => k for (k, v) in LBT_F2C_MAP)
 
 struct LBTLibraryInfo
     libname::String
@@ -164,14 +166,74 @@ function lbt_get_default_func()
     return ccall((:lbt_get_default_func, libblastrampoline), Ptr{Cvoid}, ())
 end
 
-#=
-Don't define footgun API (yet)
+"""
+    lbt_find_backing_library(symbol_name, interface; config::LBTConfig = lbt_get_config())
 
-function lbt_get_forward(symbol_name, interface, f2c = LBT_F2C_PLAIN)
-    return ccall((:lbt_get_forward, libblastrampoline), Ptr{Cvoid}, (Cstring, Int32, Int32), symbol_name, interface, f2c)
+Return the `LBTLibraryInfo` that represents the backing library for the given symbol
+exported from libblastrampoline.  This allows us to discover which library will service
+a particular BLAS call from Julia code.  This method returns `nothing` if either of the
+following conditions are met:
+
+ * No loaded library exports the desired symbol (the default function will be called)
+ * The symbol was set via `lbt_set_forward()`, which does not track library provenance.
+
+If the given `symbol_name` is not contained within the list of exported symbols, an
+`ArgumentError` will be thrown.
+"""
+function lbt_find_backing_library(symbol_name, interface::Symbol;
+                                  config::LBTConfig = lbt_get_config())
+    if interface ∉ (:ilp64, :lp64)
+        throw(Argument("Invalid interface specification: '$(interface)'"))
+    end
+    symbol_idx = findfirst(s -> s == symbol_name, config.exported_symbols)
+    if symbol_idx === nothing
+        throw(ArgumentError("Invalid exported symbol name '$(symbol_name)'"))
+    end
+    # Convert to zero-indexed
+    symbol_idx -= 1
+
+    forward_byte_offset = div(symbol_idx, 8)
+    forward_byte_mask = 1 << mod(symbol_idx, 8)
+    for lib in filter(l -> l.interface == interface, config.loaded_libs)
+        if lib.active_forwards[forward_byte_offset+1] & forward_byte_mask != 0x00
+            return lib
+        end
+    end
+
+    # No backing library was found
+    return nothing
 end
 
+
+## NOTE: Manually setting forwards is referred to as the 'footgun API'.  It allows truly
+## bizarre and complex setups to be created.  If you run into strange errors while using
+## it, the first thing you should ask yourself is whether it's truly 
 function lbt_set_forward(symbol_name, addr, interface, f2c = LBT_F2C_PLAIN; verbose::Bool = false)
-    return ccall((:lbt_set_forward, libblastrampoline), Int32, (Cstring, Ptr{Cvoid}, Int32, Int32, Int32), symbol_name, addr, interface, f2c, verbose ? 1 : 0)
+    return ccall(
+        (:lbt_set_forward, libblastrampoline),
+        Int32,
+        (Cstring, Ptr{Cvoid}, Int32, Int32, Int32),
+        string(symbol_name),
+        addr,
+        Int32(interface),
+        Int32(f2c),
+        verbose ? Int32(1) : Int32(0),
+    )
+end
+function lbt_set_forward(symbol_name, addr, interface::Symbol, f2c::Symbol = :plain; kwargs...)
+    return lbt_set_forward(symbol_name, addr, LBT_INV_INTERFACE_MAP[interface], LBT_INV_F2C_MAP[f2c]; kwargs...)
+end
+
+function lbt_get_forward(symbol_name, interface, f2c = LBT_F2C_PLAIN)
+    return ccall(
+        (:lbt_get_forward, libblastrampoline),
+        Ptr{Cvoid},
+        (Cstring, Int32, Int32),
+        string(symbol_name),
+        Int32(interface),
+        Int32(f2c),
+    )
+end
+function lbt_get_forward(symbol_name, interface::Symbol, f2c::Symbol = :plain)
+    return lbt_get_forward(symbol_name, LBT_INV_INTERFACE_MAP[interface], LBT_INV_F2C_MAP[f2c])
 end
-=#