xorgserver: set log-dir to /var/log instead of the Nix store

Xorg creates the log-dir in its output path because X crashes if it can't write
to its logfile. On a regular distro, this dir would be installed into the root
to prevent that from happening but with Nix, it sits in the read-only Nix store.
Ironically, when Xorg tries to write here, it fails and crashes.

To make Xorg log to /var/log, we have to stop the build script from trying to
create the log-dir as the sandbox doesn't (and shouldn't) have access to /var.

This creates a runtime dependency on /var when running as root but that should
exist on any Linux system (on NixOS, journald always creates /var/log).

Previously, the startx displayManager required some workarounds for logfiles
which are obsolete now.

patchPhase -> postPatch because overriding the patchPhase prevents patches from
being applied

pkgs/servers/x11/xorg/dont-create-logdir-during-build.patch

@@ -0,0 +1,32 @@
+---
+ hw/xfree86/Makefile.am | 1 -
+ hw/xfree86/Makefile.in | 1 -
+ 2 files changed, 2 deletions(-)
+
+diff --git a/hw/xfree86/Makefile.am b/hw/xfree86/Makefile.am
+index 9aeaea1..dcca3b8 100644
+--- a/hw/xfree86/Makefile.am
++++ b/hw/xfree86/Makefile.am
+@@ -100,7 +100,6 @@ EXTRA_DIST = xorgconf.cpp
+ 
+ # Without logdir, X will post an error on the terminal and will not start
+ install-data-local:
+-	$(AM_V_GEN)$(MKDIR_P) $(DESTDIR)$(logdir)
+ if CYGWIN
+ 	$(INSTALL_DATA) libXorg.exe.a $(DESTDIR)$(libdir)/libXorg.exe.a
+ endif
+diff --git a/hw/xfree86/Makefile.in b/hw/xfree86/Makefile.in
+index c4fceee..74da8f1 100644
+--- a/hw/xfree86/Makefile.in
++++ b/hw/xfree86/Makefile.in
+@@ -1161,7 +1161,6 @@ uninstall-am: uninstall-binPROGRAMS uninstall-local \
+ 
+ # Without logdir, X will post an error on the terminal and will not start
+ install-data-local:
+-	$(AM_V_GEN)$(MKDIR_P) $(DESTDIR)$(logdir)
+ @CYGWIN_TRUE@	$(INSTALL_DATA) libXorg.exe.a $(DESTDIR)$(libdir)/libXorg.exe.a
+ 
+ install-exec-hook:
+-- 
+2.25.4
+

pkgs/servers/x11/xorg/overrides.nix

@@ -571,7 +571,7 @@ self: super:
       attrs =
         if (abiCompat == null || lib.hasPrefix abiCompat version) then
           attrs_passed // {
-            buildInputs = attrs_passed.buildInputs ++ [ libdrm.dev ]; patchPhase = ''
+            buildInputs = attrs_passed.buildInputs ++ [ libdrm.dev ]; postPatch = ''
             for i in dri3/*.c
             do
               sed -i -e "s|#include <drm_fourcc.h>|#include <libdrm/drm_fourcc.h>|" $i
@@ -626,6 +626,12 @@ self: super:
       if (!isDarwin)
       then {
         outputs = [ "out" "dev" ];
+        patches = [
+          # The build process tries to create the specified logdir when building.
+          #
+          # We set it to /var/log which can't be touched from inside the sandbox causing the build to hard-fail
+          ./dont-create-logdir-during-build.patch
+        ];
         buildInputs = commonBuildInputs ++ [ libdrm mesa ];
         propagatedBuildInputs = attrs.propagatedBuildInputs or [] ++ [ libpciaccess epoxy ] ++ commonPropagatedBuildInputs ++ lib.optionals stdenv.isLinux [
           udev
@@ -642,6 +648,7 @@ self: super:
           "--with-xkb-bin-directory=${self.xkbcomp}/bin"
           "--with-xkb-path=${self.xkeyboardconfig}/share/X11/xkb"
           "--with-xkb-output=$out/share/X11/xkb/compiled"
+          "--with-log-dir=/var/log"
           "--enable-glamor"
         ] ++ lib.optionals stdenv.hostPlatform.isMusl [
           "--disable-tls"