nix flake show: don't evaluate derivations for foreign systems by default

src/nix/flake.cc

@@ -966,6 +966,7 @@ struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
 struct CmdFlakeShow : FlakeCommand, MixJSON
 {
     bool showLegacy = false;
+    bool showAllSystems = false;
 
     CmdFlakeShow()
     {
@@ -974,6 +975,11 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
             .description = "Show the contents of the `legacyPackages` output.",
             .handler = {&showLegacy, true}
         });
+        addFlag({
+            .longName = "all-systems",
+            .description = "Show the contents of outputs for all systems.",
+            .handler = {&showAllSystems, true}
+        });
     }
 
     std::string description() override
@@ -994,6 +1000,7 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
 
         auto state = getEvalState();
         auto flake = std::make_shared<LockedFlake>(lockFlake());
+        auto localSystem = std::string(settings.thisSystem.get());
 
         std::function<nlohmann::json(
             eval_cache::AttrCursor & visitor,
@@ -1084,10 +1091,18 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
                     || (attrPath.size() == 3 && (attrPathS[0] == "checks" || attrPathS[0] == "packages" || attrPathS[0] == "devShells"))
                     )
                 {
-                    if (visitor.isDerivation())
-                        showDerivation();
-                    else
-                        throw Error("expected a derivation");
+                    if (!showAllSystems && std::string(attrPathS[1]) != localSystem) {
+                        if (!json)
+                            logger->cout(fmt("%s " ANSI_WARNING "omitted" ANSI_NORMAL " (use '--all-systems' to show)", headerPrefix));
+                        else {
+                            logger->warn(fmt("%s omitted (use '--all-systems' to show)", concatStringsSep(".", attrPathS)));
+                        }
+                    } else {
+                        if (visitor.isDerivation())
+                            showDerivation();
+                        else
+                            throw Error("expected a derivation");
+                    }
                 }
 
                 else if (attrPath.size() > 0 && attrPathS[0] == "hydraJobs") {
@@ -1106,6 +1121,12 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
                         else {
                             logger->warn(fmt("%s omitted (use '--legacy' to show)", concatStringsSep(".", attrPathS)));
                         }
+                    } else if (!showAllSystems && std::string(attrPathS[1]) != localSystem) {
+                        if (!json)
+                            logger->cout(fmt("%s " ANSI_WARNING "omitted" ANSI_NORMAL " (use '--all-systems' to show)", headerPrefix));
+                        else {
+                            logger->warn(fmt("%s omitted (use '--all-systems' to show)", concatStringsSep(".", attrPathS)));
+                        }
                     } else {
                         if (visitor.isDerivation())
                             showDerivation();

tests/flakes/common.sh

@@ -20,9 +20,13 @@ writeSimpleFlake() {
       foo = import ./simple.nix;
       default = foo;
     };
+    packages.someOtherSystem = rec {
+      foo = import ./simple.nix;
+      default = foo;
+    };
 
     # To test "nix flake init".
-    legacyPackages.x86_64-linux.hello = import ./simple.nix;
+    legacyPackages.$system.hello = import ./simple.nix;
   };
 }
 EOF

tests/flakes/init.sh

@@ -41,8 +41,8 @@ cat > $templatesDir/trivial/flake.nix <<EOF
   description = "A flake for building Hello World";
 
   outputs = { self, nixpkgs }: {
-    packages.x86_64-linux = rec {
-      hello = nixpkgs.legacyPackages.x86_64-linux.hello;
+    packages.$system = rec {
+      hello = nixpkgs.legacyPackages.$system.hello;
       default = hello;
     };
   };

tests/flakes/show.sh

@@ -0,0 +1,39 @@
+source ./common.sh
+
+flakeDir=$TEST_ROOT/flake
+mkdir -p "$flakeDir"
+
+writeSimpleFlake "$flakeDir"
+cd "$flakeDir"
+
+
+# By default: Only show the packages content for the current system and no
+# legacyPackages at all
+nix flake show --json > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output.packages.someOtherSystem.default == {};
+assert show_output.packages.${builtins.currentSystem}.default.name == "simple";
+assert show_output.legacyPackages.${builtins.currentSystem} == {};
+true
+'
+
+# With `--all-systems`, show the packages for all systems
+nix flake show --json --all-systems > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output.packages.someOtherSystem.default.name == "simple";
+assert show_output.legacyPackages.${builtins.currentSystem} == {};
+true
+'
+
+# With `--legacy`, show the legacy packages
+nix flake show --json --legacy > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output.legacyPackages.${builtins.currentSystem}.hello.name == "simple";
+true
+'

tests/local.mk

@@ -113,6 +113,7 @@ nix_tests = \
   store-ping.sh \
   fetchClosure.sh \
   completions.sh \
+  flakes/show.sh \
   impure-derivations.sh \
   path-from-hash-part.sh \
   toString-path.sh