Add a setup hook to fill NIX_DEBUG_INFO_DIRS

This setup hook is propagated by gdb. Thus, a typical use is:

  $ nix-shell -p gdb nix nix.debug sqlite.debug ...

and gdb will be able find the debug symbols of nix etc. automatically.

pkgs/build-support/setup-hooks/setup-debug-info-dirs.sh

@@ -0,0 +1,5 @@
+setupDebugInfoDirs () {
+    addToSearchPath NIX_DEBUG_INFO_DIRS $1/lib/debug
+}
+
+envHooks+=(setupDebugInfoDirs)

pkgs/development/tools/misc/gdb/default.nix

@@ -11,6 +11,8 @@
 
 # Additional dependencies for GNU/Hurd.
 , mig ? null, hurd ? null
+
+, setupDebugInfoDirs
 }:
 
 let
@@ -34,7 +36,7 @@ stdenv.mkDerivation rec {
 
   patches = [ ./debug-info-from-env.patch ];
 
-  nativeBuildInputs = [ pkgconfig texinfo perl ]
+  nativeBuildInputs = [ pkgconfig texinfo perl setupDebugInfoDirs ]
     # TODO(@Ericson2314) not sure if should be host or target
     ++ stdenv.lib.optional targetPlatform.isHurd mig;
 
@@ -43,6 +45,8 @@ stdenv.mkDerivation rec {
     ++ stdenv.lib.optional targetPlatform.isHurd hurd
     ++ stdenv.lib.optional doCheck dejagnu;
 
+  propagatedNativeBuildInputs = [ setupDebugInfoDirs ];
+
   enableParallelBuilding = true;
 
   # darwin build fails with format hardening since v7.12

pkgs/top-level/all-packages.nix

@@ -347,6 +347,8 @@ with pkgs;
 
   separateDebugInfo = makeSetupHook { } ../build-support/setup-hooks/separate-debug-info.sh;
 
+  setupDebugInfoDirs = makeSetupHook { } ../build-support/setup-hooks/setup-debug-info-dirs.sh;
+
   useOldCXXAbi = makeSetupHook { } ../build-support/setup-hooks/use-old-cxx-abi.sh;
 
   iconConvTools = callPackage ../build-support/icon-conv-tools {};