installer: simplify the per-build installation

The goal is to allow the installation and testing of arbitrary Nix
versions. Extend the base installer to accept a `--tarball-url-prefix
<url>` to change where the Nix tarball is getting downloaded from.

Once this is merged it should allow to:
1. Pick an evaluation at https://hydra.nixos.org/jobset/nix/master that
   looks healthy
2. Select the installedScript build and find the store path.

Now equipped with all of this, use an instance of nar-serve to fetch the
install script and release tarballs:

    curl -sfL https://nar-serve.numtide.com/nix/store/rkv4yh7pym941bhj0849zqdkg2546bdv-installer-script/install \
      | sh --tarball-url-prefix https://nar-serve.numtide.com/nix/store

Or with cachix, strip the /nix/store and derivation name and then:

    curl -sfL https://mycache.cachix.org/serve/rkv4yh7pym941bhj0849zqdkg2546bdv/install \
      | sh --tarball-url-prefix https://mycache.cachix.org/serve

Fixes #4047

2 files changed, 41 insertions, 8 deletions

diff --git a/scripts/install-nix-from-closure.sh b/scripts/install-nix-from-closure.sh
index 2de41a6a4..6352a8fac 100644
--- a/scripts/install-nix-from-closure.sh
+++ b/scripts/install-nix-from-closure.sh
@@ -98,6 +98,9 @@ while [ $# -gt 0 ]; do
                 echo ""
                 echo " --nix-extra-conf-file: Path to nix.conf to prepend when installing /etc/nix.conf"
                 echo ""
+                if [ -n "${INVOKED_FROM_INSTALL_IN:-}" ]; then
+                    echo " --tarball-url-prefix URL: Base URL to download the Nix tarball from."
+                fi
             ) >&2
 
             # darwin and Catalina+
diff --git a/scripts/install.in b/scripts/install.in
index 9a281d776..7c3b795cc 100644
--- a/scripts/install.in
+++ b/scripts/install.in
@@ -25,18 +25,47 @@ require_util() {
 }
 
 case "$(uname -s).$(uname -m)" in
-    Linux.x86_64) system=x86_64-linux; hash=@binaryTarball_x86_64-linux@;;
-    Linux.i?86) system=i686-linux; hash=@binaryTarball_i686-linux@;;
-    Linux.aarch64) system=aarch64-linux; hash=@binaryTarball_aarch64-linux@;;
-    Darwin.x86_64) system=x86_64-darwin; hash=@binaryTarball_x86_64-darwin@;;
-    # eventually maybe: system=arm64-darwin; hash=@binaryTarball_arm64-darwin@;;
-    Darwin.arm64) system=x86_64-darwin; hash=@binaryTarball_x86_64-darwin@;;
+    Linux.x86_64)
+        hash=@tarballHash_x86_64-linux@
+        path=@tarballPath_x86_64-linux@
+        system=x86_64-linux
+        ;;
+    Linux.i?86)
+        hash=@tarballHash_i686-linux@
+        path=@tarballPath_i686-linux@
+        system=i686-linux
+        ;;
+    Linux.aarch64)
+        hash=@tarballHash_aarch64-linux@
+        path=@tarballPath_aarch64-linux@
+        system=aarch64-linux
+        ;;
+    Darwin.x86_64)
+        hash=@tarballHash_x86_64-darwin@
+        path=@tarballPath_x86_64-darwin@
+        system=x86_64-darwin
+        ;;
+    Darwin.arm64)
+        hash=@binaryTarball_x86_64-darwin@
+        path=@tarballPath_x86_64-darwin@
+        # eventually maybe: arm64-darwin
+        system=x86_64-darwin
+        ;;
     *) oops "sorry, there is no binary distribution of Nix for your platform";;
 esac
 
-url="https://releases.nixos.org/nix/nix-@nixVersion@/nix-@nixVersion@-$system.tar.xz"
+# Use this command-line option to fetch the tarballs using nar-serve or Cachix
+if "${1:---tarball-url-prefix}"; then
+    if [ -z "${2:-}" ]; then
+        oops "missing argument for --tarball-url-prefix"
+    fi
+    url=${2}/${path}
+    shift 2
+else
+    url=https://releases.nixos.org/nix/nix-@nixVersion@/nix-@nixVersion@-$system.tar.xz
+fi
 
-tarball="$tmpDir/$(basename "$tmpDir/nix-@nixVersion@-$system.tar.xz")"
+tarball=$tmpDir/nix-@nixVersion@-$system.tar.xz
 
 require_util curl "download the binary tarball"
 require_util tar "unpack the binary tarball"
@@ -68,6 +97,7 @@ tar -xJf "$tarball" -C "$unpack" || oops "failed to unpack '$url'"
 script=$(echo "$unpack"/*/install)
 
 [ -e "$script" ] || oops "installation script is missing from the binary tarball!"
+export INVOKED_FROM_INSTALL_IN=1
 "$script" "$@"
 
 } # End of wrapping