gnu: Add julia-jllwrappers.

* gnu/packages/julia-xyz.scm (julia-jllwrappers): New variable.
* doc/guix.texi (julia-build-system): Document how to use jllwrappers to use
  to guix binary packages.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -7752,10 +7752,31 @@ The Julia package name is read from the file @file{Project.toml}.  This
 value can be overridden by passing the argument @code{#:julia-package-name}
 (which must be correctly capitalized).
 
-For packages requiring shared library dependencies, you may need to write the
-@file{/deps/deps.jl} file manually.  It's usually a line of @code{const
-variable = /gnu/store/library.so} for each dependency, plus a void function
-@code{check_deps() = nothing}.
+Julia packages usually manage they binary dependencies via
+@code{JLLWrappers.jl}, a Julia package that creates a module (named
+after the wrapped library followed by @code{_jll.jl}.
+
+To add the binary path @code{_jll.jl} packages, you need to patch the
+files under @file{src/wrappers/}, replacing the call to the macro
+@code{JLLWrappers.@@generate_wrapper_header}, adding as a secound
+argument containing the store path the binary.
+
+As an example, in the MbedTLS Julia package, we add a build phase
+(@pxref{Build Phases}) to insert the absolute file name of the wrapped
+MbedTLS package:
+
+@lisp
+(add-after 'unpack 'override-binary-path
+  (lambda* (#:key inputs #:allow-other-keys)
+    (for-each (lambda (wrapper)
+                (substitute* wrapper
+                  (("generate_wrapper_header.*")
+                   (string-append
+                    "generate_wrapper_header(\"MbedTLS\", \""
+                    (assoc-ref inputs "mbedtls-apache") "\")\n"))))
+              ;; There's a Julia file for each platform, override them all.
+              (find-files "src/wrappers/" "\\.jl$"))))
+@end lisp
 
 Some older packages that aren't using @file{Package.toml} yet, will require
 this file to be created, too.  The function @code{julia-create-package-toml}

gnu/packages/julia-xyz.scm

@@ -147,6 +147,49 @@ scaled by a constant factor.  Consequently, they have a fixed number of
 digits (bits) after the decimal (radix) point.")
     (license license:expat)))
 
+(define-public julia-jllwrappers
+  (package
+    (name "julia-jllwrappers")
+    (version "1.2.0")
+    (source
+     (origin
+       (method git-fetch)
+       (uri (git-reference
+             (url "https://github.com/JuliaPackaging/JLLWrappers.jl")
+             (commit (string-append "v" version))))
+       (file-name (git-file-name name version))
+       (sha256
+        (base32 "1sj3mi2dcc13apqfpy401wic5n0pgbck1p98b2g3zw0mln9s83m4"))))
+    (arguments
+     ;; Wants to download stuff
+     '(#:tests? #f
+       #:phases
+       (modify-phases %standard-phases
+         (add-after 'unpack 'custom-override-path
+           (lambda* (#:key inputs #:allow-other-keys)
+             ;; Make @generate_wrapper_header take an optional argument that
+             ;; guix packagers can pass to override the default "override"
+             ;; binary path.  This won't be needed when something like
+             ;; https://github.com/JuliaPackaging/JLLWrappers.jl/pull/27
+             ;; will be merged.
+             (substitute* "src/wrapper_generators.jl"
+               (("generate_wrapper_header.*")
+                "generate_wrapper_header(src_name, override_path = nothing)\n")
+               (("pkg_dir = .*" all)
+                (string-append
+                 all "\n" "override = something(override_path,"
+                 "joinpath(dirname(pkg_dir), \"override\"))\n"))
+               (("@static if isdir.*") "@static if isdir($override)\n")
+               (("return joinpath.*") "return $override\n"))
+             #t)))))
+    (build-system julia-build-system)
+    (home-page "https://github.com/JuliaPackaging/JLLWrappers.jl")
+    (synopsis "Julia macros used by JLL packages")
+    (description "This package contains Julia macros that enable JLL packages
+to generate themselves.  It is not intended to be used by users, but rather is
+used in autogenerated packages via @code{BinaryBuilder.jl}.")
+    (license license:expat)))
+
 (define-public julia-json
   (package
     (name "julia-json")