nixos-rebuild: Allow remote building when using flakes

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -214,6 +214,49 @@ nixBuild() {
   fi
 }
 
+nixFlakeBuild() {
+    if [ -z "$buildHost" ]; then
+        nix build "$@" --out-link "${tmpDir}/result"
+        readlink -f "${tmpDir}/result"
+    else
+        local attr="$1"
+        shift 1
+        local evalArgs=()
+        local buildArgs=()
+        while [ "$#" -gt 0 ]; do
+            local i="$1"; shift 1
+            case "$i" in
+              --recreate-lock-file|--no-update-lock-file|--no-write-lock-file|--no-registries|--commit-lock-file)
+                evalArgs+=("$i")
+                ;;
+              --update-input)
+                local j="$1"; shift 1
+                evalArgs+=("$i" "$j")
+                ;;
+              --override-input)
+                local j="$1"; shift 1
+                local k="$1"; shift 1
+                evalArgs+=("$i" "$j" "$k")
+                ;;
+              *)
+                buildArgs+=("$i")
+                ;;
+            esac
+        done
+
+        local drv="$(nix "${flakeFlags[@]}" eval --raw "${attr}.drvPath" "${evalArgs[@]}" "${extraBuildArgs[@]}")"
+        if [ -a "$drv" ]; then
+            NIX_SSHOPTS=$SSHOPTS nix "${flakeFlags[@]}" copy --derivation --to "ssh://$buildHost" "$drv"
+            # The 'nix-command flakes' part in "${flakeFlags[@]}" is seen as two separate args over SSH
+            buildHostCmd nix --experimental-features "'nix-command flakes'" build "${buildArgs[@]}" --out-link "${tmpDir}/result" "$drv"
+            buildHostCmd readlink -f "${tmpDir}/result"
+        else
+            echo "nix eval failed"
+            exit 1
+        fi
+    fi
+}
+
 
 if [ -z "$action" ]; then showSyntax; fi
 
@@ -315,8 +358,14 @@ fi
 
 tmpDir=$(mktemp -t -d nixos-rebuild.XXXXXX)
 SSHOPTS="$NIX_SSHOPTS -o ControlMaster=auto -o ControlPath=$tmpDir/ssh-%n -o ControlPersist=60"
+if [ -n "$buildHost" -a -n "$flake" ]; then
+    buildHostCmd mkdir -p "$tmpDir"
+fi
 
 cleanup() {
+    if [ -n "$buildHost" -a -n "$flake" ]; then
+        buildHostCmd rm -rf "$tmpDir"
+    fi
     for ctrl in "$tmpDir"/ssh-*; do
         ssh -o ControlPath="$ctrl" -O exit dummyhost 2>/dev/null || true
     done
@@ -418,10 +467,7 @@ if [ -z "$rollback" ]; then
         if [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' --no-out-link -A system "${extraBuildFlags[@]}")"
         else
-            outLink=$tmpDir/result
+            pathToConfig="$(nixFlakeBuild "$flake#$flakeAttr.config.system.build.toplevel" "${extraBuildFlags[@]}" "${lockFlags[@]}")"
-            nix "${flakeFlags[@]}" build "$flake#$flakeAttr.config.system.build.toplevel" \
-              "${extraBuildFlags[@]}" "${lockFlags[@]}" --out-link $outLink
-            pathToConfig="$(readlink -f $outLink)"
         fi
         copyToTarget "$pathToConfig"
         targetHostCmd nix-env -p "$profile" --set "$pathToConfig"
@@ -429,24 +475,19 @@ if [ -z "$rollback" ]; then
         if [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' -A system -k "${extraBuildFlags[@]}")"
         else
-            nix "${flakeFlags[@]}" build "$flake#$flakeAttr.config.system.build.toplevel" "${extraBuildFlags[@]}" "${lockFlags[@]}"
+            pathToConfig="$(nixFlakeBuild "$flake#$flakeAttr.config.system.build.toplevel" "${extraBuildFlags[@]}" "${lockFlags[@]}")"
-            pathToConfig="$(readlink -f ./result)"
         fi
     elif [ "$action" = build-vm ]; then
         if [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' -A vm -k "${extraBuildFlags[@]}")"
         else
-            nix "${flakeFlags[@]}" build "$flake#$flakeAttr.config.system.build.vm" \
+            pathToConfig="$(nixFlakeBuild "$flake#$flakeAttr.config.system.build.vm" "${extraBuildFlags[@]}" "${lockFlags[@]}")"
-              "${extraBuildFlags[@]}" "${lockFlags[@]}"
-            pathToConfig="$(readlink -f ./result)"
         fi
     elif [ "$action" = build-vm-with-bootloader ]; then
         if [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' -A vmWithBootLoader -k "${extraBuildFlags[@]}")"
         else
-            nix "${flakeFlags[@]}" build "$flake#$flakeAttr.config.system.build.vmWithBootLoader" \
+            pathToConfig="$(nixFlakeBuild "$flake#$flakeAttr.config.system.build.vmWithBootLoader" "${extraBuildFlags[@]}" "${lockFlags[@]}")"
-              "${extraBuildFlags[@]}" "${lockFlags[@]}"
-            pathToConfig="$(readlink -f ./result)"
         fi
     else
         showSyntax