Use fedora27 bindist on Linux-x86_64 by default

Debian 9 still uses ncurses 5, which complicates installation.
Specifically, ncurses 5 provided by nixpkgs appears not to provide
shared object version information, whereas Debian does. While
`patchelfUnstable` can [1] update library version requirements, it
cannot remove them entirely.

Rather than find some terrible workaround, it's much easier to simply
use a bindist that expects ncurses 6 with symbol versioning. Fedora
27 provides exactly this.

[1]  NixOS/patchelf#85

artifact.nix

@@ -1,16 +1,21 @@
-{ stdenv, lib
+{ stdenv, lib, patchelfUnstable
 , perl, gcc, llvm_39
-, ncurses5, gmp, glibc, libiconv
-}: { bindistTarball }:
+, ncurses6, ncurses5, gmp, glibc, libiconv
+}: { bindistTarball, ncursesVersion }:
 
 # Prebuilt only does native
 assert stdenv.targetPlatform == stdenv.hostPlatform;
 
 let
   libPath = stdenv.lib.makeLibraryPath ([
-    ncurses5 gmp
+    selectedNcurses gmp
   ] ++ stdenv.lib.optional (stdenv.hostPlatform.isDarwin) libiconv);
 
+  selectedNcurses = {
+    "5" = ncurses5;
+    "6" = ncurses6;
+  }."${ncursesVersion}";
+
   libEnvVar = stdenv.lib.optionalString stdenv.hostPlatform.isDarwin "DY"
     + "LD_LIBRARY_PATH";
 
@@ -93,10 +98,11 @@ stdenv.mkDerivation rec {
           -exec sed -i "s@extra-lib-dirs: @extra-lib-dirs: ${libiconv}/lib@" {} \;
     '' +
     # Rename needed libraries and binaries, fix interpreter
+    # N.B. Use patchelfUnstable due to https://github.com/NixOS/patchelf/pull/85
     stdenv.lib.optionalString stdenv.isLinux ''
-      find . -type f -perm -0100 -exec patchelf \
-          --replace-needed libncurses${stdenv.lib.optionalString stdenv.is64bit "w"}.so.5 libncurses.so \
-          --replace-needed libtinfo.so libtinfo.so.5 \
+      find . -type f -perm -0100 -exec ${patchelfUnstable}/bin/patchelf \
+          --replace-needed libncurses${stdenv.lib.optionalString stdenv.is64bit "w"}.so.${ncursesVersion} libncurses.so \
+          --replace-needed libtinfo.so.${ncursesVersion} libncurses.so.${ncursesVersion} \
           --interpreter ${glibcDynLinker} {} \;
 
       sed -i "s|/usr/bin/perl|perl\x00        |" ghc*/ghc/stage2/build/tmp/ghc-stage2

default.nix

@@ -4,9 +4,10 @@ let
   ghc = self: ref: self.callPackage ./artifact.nix {} ref;
   ol = self: super:
     {
-      ghcHEAD = ghc self {
-        bindistTarball = self.callPackage ./gitlab-artifact.nix {} { inherit fork branch; };
-      };
+      ghcHEAD =
+        ghc self (self.callPackage ./gitlab-artifact.nix {} {
+          inherit fork branch;
+        });
     };
 in
   import <nixpkgs> { overlays = [ol]; }

gitlab-artifact.nix

@@ -3,20 +3,33 @@
 
 let
   mkUrl = job: "https://gitlab.haskell.org/${fork}/ghc/-/jobs/artifacts/${branch}/raw/ghc.tar.xz?job=${job}";
-  url = {
+
+  # job: the GitLab CI job we should pull the bindist from
+  # ncursesVersion: the ncurses version which the bindist expects
+  configs = {
     "i386-linux"   = {
-      url = mkUrl "validate-i386-linux-deb9";
+      job = mkUrl "validate-i386-linux-fedora27";
+      ncursesVersion = "6";
     };
     "x86_64-linux" = {
-      url = mkUrl "validate-x86_64-linux-deb8";
+      job = mkUrl "validate-x86_64-linux-fedora27";
+      ncursesVersion = "6";
     };
     "aarch64-linux" = {
-      url = mkUrl "validate-aarch64-linux-deb9";
+      job = mkUrl "validate-aarch64-linux-deb9";
+      ncursesVersion = "5";
     };
     "x86_64-darwin" = {
-      url = mkUrl "validate-x86_64-darwin";
+      job = mkUrl "validate-x86_64-darwin";
+      ncursesVersion = "6";
     };
-  }.${stdenv.hostPlatform.system}
+  };
+
+  config = configs.${stdenv.hostPlatform.system}
     or (throw "cannot bootstrap GHC on this platform");
-in builtins.fetchurl url
+
+in {
+  bindistTarball = builtins.fetchurl (mkUrl config.job);
+  inherit (config) ncursesVersion;
+}