system: Modify GDB skeleton to find debug files for any profile.

Instead of hard coding the debug directory to that of the user profile, use
Guile scripting in combination with a new search path specification on GDB to
specify the debug file directories.

* gnu/packages/gdb.scm (gdb-9.1): Add a search path specification for the
GDB_DEBUG_FILE_DIRECTORY environment variable.
* gnu/system/shadow.scm (default-skeletons)[gdbinit]: Derive the value of
DEBUG-FILE-DIRECTORY via the GDB_DEBUG_FILE_DIRECTORY environment variable.
* doc/guix.texi (Installing Debugging Files): Document it.

doc/guix.texi

@@ -29919,6 +29919,20 @@ GDB}):
 From there on, GDB will pick up debugging information from the
 @file{.debug} files under @file{~/.guix-profile/lib/debug}.
 
+Below is an alternative GDB script which is useful when working with
+other profiles.  It takes advantage of the optional Guile integration in
+GDB.  This snippet is included by default on Guix System in the
+@file{~/.gdbinit} file.
+
+@example
+guile
+(use-modules (gdb))
+(execute (string-append "set debug-file-directory "
+                        (or (getenv "GDB_DEBUG_FILE_DIRECTORY")
+                            "~/.guix-profile/lib/debug")))
+end
+@end example
+
 In addition, you will most likely want GDB to be able to show the source
 code being debugged.  To do that, you will have to unpack the source
 code of the package of interest (obtained with @code{guix build

gnu/packages/gdb.scm

@@ -5,6 +5,7 @@
 ;;; Copyright © 2020 Marius Bakke <mbakke@fastmail.com>
 ;;; Copyright © 2020 Vincent Legoll <vincent.legoll@gmail.com>
 ;;; Copyright © 2020 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -117,6 +118,12 @@
         ("dejagnu" ,dejagnu)
         ("pkg-config" ,pkg-config)
         ,@(if (hurd-target?) `(("mig" ,mig)) '())))
+    ;; TODO: Add support for the GDB_DEBUG_FILE_DIRECTORY environment variable
+    ;; in GDB itself instead of relying on some glue code in the Guix-provided
+    ;; .gdbinit file.
+    (native-search-paths (list (search-path-specification
+                                (variable "GDB_DEBUG_FILE_DIRECTORY")
+                                (files '("lib/debug")))))
     (home-page "https://www.gnu.org/software/gdb/")
     (synopsis "The GNU debugger")
     (description

gnu/system/shadow.scm

@@ -3,6 +3,7 @@
 ;;; Copyright © 2016 Alex Griffin <a@ajgrf.com>
 ;;; Copyright © 2020 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
 ;;; Copyright © 2020 Efraim Flashner <efraim@flashner.co.il>
+;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -162,7 +163,12 @@ XTerm*utf8: always
 XTerm*metaSendsEscape: true\n"))
         (gdbinit   (plain-file "gdbinit" "\
 # Tell GDB where to look for separate debugging files.
-set debug-file-directory ~/.guix-profile/lib/debug
+guile
+(use-modules (gdb))
+(execute (string-append \"set debug-file-directory \"
+                        (or (getenv \"GDB_DEBUG_FILE_DIRECTORY\")
+                            \"~/.guix-profile/lib/debug\")))
+end
 
 # Authorize extensions found in the store, such as the
 # pretty-printers of libstdc++.