allow to pass a nvidiaVersionFile

This allows to better control which driver is being loaded.

all.nix

@@ -1,4 +1,16 @@
-import ./default.nix {
+let
+  pkgs = import <nixpkgs> { config = { allowUnfree = true; }; };
+in
+{
+  pure = pkgs.recurseIntoAttrs (pkgs.callPackage ./nixGL.nix {
     nvidiaVersion = "440.82";
     nvidiaHash = "edd415acf2f75a659e0f3b4f27c1fab770cf21614e84a18152d94f0d004a758e";
+  });
+
+  versionFile = pkgs.recurseIntoAttrs (pkgs.callPackage ./nixGL.nix {
+    nvidiaVersionFile = pkgs.writeText "nvidia-version-440.82" ''
+      NVRM version: NVIDIA UNIX x86_64 Kernel Module  440.82  Wed Apr  1 20:04:33 UTC 2020
+      GCC version:  gcc version 9.3.0 (Arch Linux 9.3.0-1)
+    '';
+  });
 }

default.nix

@@ -4,6 +4,11 @@
   # Hash of the Nvidia driver .run file. null is fine, but fixing a value here
   # will be more reproducible and more efficient.
   nvidiaHash ? null,
+  # Alternatively, you can pass a path that points to a nvidia version file
+  # and let nixGL extract the version from it. That file must be a copy of
+  # /proc/driver/nvidia/version. Nix doesn't like zero-sized files (see
+  # https://github.com/NixOS/nix/issues/3539 ).
+  nvidiaVersionFile ? null,
   # Enable 32 bits driver
   # This is one by default, you can switch it to off if you want to reduce a
   # bit the size of nixGL closure.
@@ -15,5 +20,10 @@
   }
 }:
 pkgs.callPackage ./nixGL.nix {
-  inherit nvidiaVersion nvidiaHash enable32bits;
+  inherit
+    nvidiaVersion
+    nvidiaVersionFile
+    nvidiaHash
+    enable32bits
+    ;
 }

nixGL.nix

@@ -4,6 +4,11 @@
   # Hash of the Nvidia driver .run file. null is fine, but fixing a value here
   # will be more reproducible and more efficient.
   nvidiaHash ? null,
+  # Alternatively, you can pass a path that points to a nvidia version file
+  # and let nixGL extract the version from it. That file must be a copy of
+  # /proc/driver/nvidia/version. Nix doesn't like zero-sized files (see
+  # https://github.com/NixOS/nix/issues/3539 ).
+  nvidiaVersionFile ? null,
   # Enable 32 bits driver
   # This is one by default, you can switch it to off if you want to reduce a
   # bit the size of nixGL closure.
@@ -14,27 +19,35 @@
 }:
 
 let
-  # The nvidia version. Either fixed by the `nvidiaVersion` argument, or
+  _nvidiaVersionFile =
-  # auto-detected.
+    if nvidiaVersionFile != null then
-  _nvidiaVersion = if nvidiaVersion != null
+      nvidiaVersionFile
-  then nvidiaVersion
     else
-    # This is the auto-detection mecanism. This is ugly.
+      # HACK: Get the version from /proc. It turns out that /proc is mounted
-    # We read /proc/driver/nvidia/version which is set by the Nvidia driver kernel module.
+      # inside of the build sandbox and varies from machine to machine.
-    # This fails if the nvidia driver kernel module is not loaded.
+      #
-    # I'd like to just read the file using `${/proc/driver/nvidia/version}` and
+      # builtins.readFile is not able to read /proc files. See
-    # then let nix invalidate the derivation if the content of this file
+      # https://github.com/NixOS/nix/issues/3539.
-    # changes, but that's not possible, see
+      runCommand "impure-nvidia-version-file" {
-    # https://github.com/NixOS/nix/issues/3539
+        # To avoid sharing the build result over time or between machine,
-    # But /proc is readable at build time! So runCommand works fine.
+        # Add an impure parameter to force the rebuild on each access.
-    import (runCommand "auto-detect-nvidia" {
         time = builtins.currentTime;
       }
-    ''
+      "cp /proc/driver/nvidia/version $out";
-        # Written this way so if the version file does not exists, the script crashs
+
-        VERSION="$(${pcre}/bin/pcregrep -o1 'Module +([0-9]+\.[0-9]+)' /proc/driver/nvidia/version)"
+  # The nvidia version. Either fixed by the `nvidiaVersion` argument, or
-        echo "\"$VERSION\"" > $out
+  # auto-detected. Auto-detection is impure.
-    '');
+  _nvidiaVersion =
+    if nvidiaVersion != null then
+      nvidiaVersion
+    else
+      # Get if from the nvidiaVersionFile
+      let
+        data = builtins.readFile _nvidiaVersionFile;
+        versionMatch = builtins.match ".*Module +([0-9]+\\.[0-9]+).*" data;
+      in
+      assert builtins.length versionMatch == 1;
+      builtins.head versionMatch;
 
   addNvidiaVersion = drv: drv.overrideAttrs(oldAttrs: {
     name = oldAttrs.name + "-${_nvidiaVersion}";
@@ -57,7 +70,6 @@ let
      done
     '';
   };
-
 in
   rec {
     nvidia = (linuxPackages.nvidia_x11.override {