fix: flakes now obey --eval-system

This required changing an excessive number of places since flakes code
is a delicious bowl of copy pasta.

I didn't change all of the usage sites since some of them run things on
the local machine and you actually want it to be the real system there.

Nevertheless, we probably should have the daemon do something much
different when it receives a `system` setting:
https://git.lix.systems/lix-project/lix/issues/694

Fixes: https://git.lix.systems/lix-project/lix/issues/692
Fixes: https://git.lix.systems/lix-project/lix/issues/673
Fixes: https://github.com/NixOS/nix/issues/11359
Change-Id: I55e696c09794d2520b60238a84829c98fcad7ccc

doc/manual/rl-next/flakes-eval-system.md

@@ -0,0 +1,18 @@
+---
+synopsis: 'Flakes follow `--eval-system` where it makes sense'
+issues: [fj#673, fj#692, gh#11359]
+cls: [2657]
+category: Fixes
+credits: [jade]
+---
+
+Most flake commands now follow `--eval-system` when choosing attributes to build/evaluate/etc.
+
+The exceptions are commands that actually run something on the local machine:
+- nix develop
+- nix run
+- nix upgrade-nix
+- nix fmt
+- nix bundle
+
+This is not a principled approach to cross compilation or anything, flakes still impede rather than support cross compilation, but this unbreaks many remote build use cases.

lix/libcmd/installables.cc

@@ -185,8 +185,8 @@ MixReadOnlyOption::MixReadOnlyOption()
 Strings SourceExprCommand::getDefaultFlakeAttrPaths()
 {
     return {
-        "packages." + settings.thisSystem.get() + ".default",
-        "defaultPackage." + settings.thisSystem.get()
+        "packages." + evalSettings.getCurrentSystem() + ".default",
+        "defaultPackage." + evalSettings.getCurrentSystem()
     };
 }
 
@@ -195,10 +195,10 @@ Strings SourceExprCommand::getDefaultFlakeAttrPathPrefixes()
     return {
         // As a convenience, look for the attribute in
         // 'outputs.packages'.
-        "packages." + settings.thisSystem.get() + ".",
+        "packages." + evalSettings.getCurrentSystem() + ".",
         // As a temporary hack until Nixpkgs is properly converted
         // to provide a clean 'packages' set, look in 'legacyPackages'.
-        "legacyPackages." + settings.thisSystem.get() + "."
+        "legacyPackages." + evalSettings.getCurrentSystem() + "."
     };
 }
 

lix/nix/bundle.cc

@@ -1,5 +1,6 @@
 #include "lix/libcmd/installable-flake.hh"
 #include "lix/libcmd/command.hh"
+#include "lix/libexpr/eval-settings.hh"
 #include "lix/libmain/common-args.hh"
 #include "lix/libmain/shared.hh"
 #include "lix/libstore/store-api.hh"
@@ -54,9 +55,10 @@ struct CmdBundle : InstallableCommand
     // FIXME: cut&paste from CmdRun.
     Strings getDefaultFlakeAttrPaths() override
     {
+        // eval-system, since the app could be remote built and then bundled locally
         Strings res{
-            "apps." + settings.thisSystem.get() + ".default",
-            "defaultApp." + settings.thisSystem.get()
+            "apps." + evalSettings.getCurrentSystem() + ".default",
+            "defaultApp." + evalSettings.getCurrentSystem()
         };
         for (auto & s : SourceExprCommand::getDefaultFlakeAttrPaths())
             res.push_back(s);
@@ -65,7 +67,8 @@ struct CmdBundle : InstallableCommand
 
     Strings getDefaultFlakeAttrPathPrefixes() override
     {
-        Strings res{"apps." + settings.thisSystem.get() + "."};
+        // eval-system, since the app could be remote built and then bundled locally
+        Strings res{"apps." + evalSettings.getCurrentSystem() + "."};
         for (auto & s : SourceExprCommand::getDefaultFlakeAttrPathPrefixes())
             res.push_back(s);
         return res;
@@ -82,6 +85,7 @@ struct CmdBundle : InstallableCommand
 
         auto [bundlerFlakeRef, bundlerName, extendedOutputsSpec] = parseFlakeRefWithFragmentAndExtendedOutputsSpec(bundler, absPath("."));
         const flake::LockFlags lockFlags{ .writeLockFile = false };
+        // Current system, since it needs to run locally
         InstallableFlake bundler{this,
             evaluator, std::move(bundlerFlakeRef), bundlerName, std::move(extendedOutputsSpec),
             {"bundlers." + settings.thisSystem.get() + ".default",

lix/nix/develop.cc

@@ -427,6 +427,7 @@ struct Common : InstallableCommand, MixProfile
 
     Strings getDefaultFlakeAttrPaths() override
     {
+        // These are current system since they have to run locally from inside the shell
         Strings paths{
             "devShells." + settings.thisSystem.get() + ".default",
             "devShell." + settings.thisSystem.get(),
@@ -621,6 +622,7 @@ struct CmdDevelop : Common, MixEnvironment
                 "bashInteractive",
                 ExtendedOutputsSpec::Default(),
                 Strings{},
+                // `system` not `eval-system` since we are actually executing it on the local machine
                 Strings{"legacyPackages." + settings.thisSystem.get() + "."},
                 nixpkgsLockFlags);
 

lix/nix/flake.cc

@@ -365,7 +365,7 @@ struct CmdFlakeCheck : FlakeCommand
 
         lockFlags.applyNixConfig = true;
         auto flake = lockFlake(*state);
-        auto localSystem = std::string(settings.thisSystem.get());
+        auto localSystem = std::string(evalSettings.getCurrentSystem());
 
         bool hasErrors = false;
         auto reportError = [&](const Error & e) {
@@ -613,7 +613,7 @@ struct CmdFlakeCheck : FlakeCommand
                                         auto drvPath = checkDerivation(
                                             fmt("%s.%s.%s", name, attr_name, evaluator->symbols[attr2.name]),
                                             *attr2.value, attr2.pos);
-                                        if (drvPath && attr_name == settings.thisSystem.get()) {
+                                        if (drvPath && attr_name == evalSettings.getCurrentSystem()) {
                                             drvPaths.push_back(DerivedPath::Built {
                                                 .drvPath = makeConstantStorePathRef(*drvPath),
                                                 .outputs = OutputsSpec::All { },
@@ -1129,7 +1129,7 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
         auto evaluator = getEvaluator();
         auto state = evaluator->begin(aio());
         auto flake = std::make_shared<LockedFlake>(lockFlake(*state));
-        auto localSystem = std::string(settings.thisSystem.get());
+        auto localSystem = std::string(evalSettings.getCurrentSystem());
 
         std::function<bool(
             eval_cache::AttrCursor & visitor,

lix/nix/fmt.cc

@@ -22,6 +22,7 @@ struct CmdFmt : SourceExprCommand {
     Category category() override { return catSecondary; }
 
     Strings getDefaultFlakeAttrPaths() override {
+        // We are running it locally so it should be the actual system
         return Strings{"formatter." + settings.thisSystem.get()};
     }
 

lix/nix/run.cc

@@ -188,6 +188,7 @@ struct CmdRun : InstallableCommand
 
     Strings getDefaultFlakeAttrPaths() override
     {
+        // We are running it locally so it should be the actual system
         Strings res{
             "apps." + settings.thisSystem.get() + ".default",
             "defaultApp." + settings.thisSystem.get(),
@@ -199,6 +200,7 @@ struct CmdRun : InstallableCommand
 
     Strings getDefaultFlakeAttrPathPrefixes() override
     {
+        // We are running it locally so it should be the actual system
         Strings res{"apps." + settings.thisSystem.get() + "."};
         for (auto & s : SourceExprCommand::getDefaultFlakeAttrPathPrefixes())
             res.push_back(s);

lix/nix/search.cc

@@ -57,8 +57,8 @@ struct CmdSearch : InstallableCommand, MixJSON
     Strings getDefaultFlakeAttrPaths() override
     {
         return {
-            "packages." + settings.thisSystem.get(),
-            "legacyPackages." + settings.thisSystem.get()
+            "packages." + evalSettings.getCurrentSystem(),
+            "legacyPackages." + evalSettings.getCurrentSystem()
         };
     }
 

tests/functional/flakes/check.sh

@@ -107,6 +107,11 @@ cat > $flakeDir/flake.nix <<EOF
 EOF
 
 nix flake check $flakeDir
+# --eval-system should be considered for which the local system is for flake
+# purposes, and thus it should fail checking that attr
+(! nix flake check --eval-system system-1 $flakeDir)
+# likewise with --system
+(! nix flake check --system system-1 $flakeDir)
 
 checkRes=$(nix flake check --all-systems --keep-going $flakeDir 2>&1 && fail "nix flake check --all-systems should have failed" || true)
 echo "$checkRes" | grepQuiet "packages.system-1.default"

tests/functional/flakes/flakes.sh

@@ -126,6 +126,11 @@ nix build -o $TEST_ROOT/result git+file://$flake1Dir
 nix build -o $flake1Dir/result git+file://$flake1Dir
 nix path-info $flake1Dir/result
 
+# Ensure that eval-system affects the chosen attribute
+cp -r "$flake1Dir" "$flake1Dir.kittified"
+sed -i "s#$system#kitty-kitty#" "$flake1Dir.kittified/flake.nix"
+nix build --eval-system kitty-kitty "$flake1Dir.kittified"
+
 # 'getFlake' on an unlocked flakeref should fail in pure mode, but
 # succeed in impure mode.
 (! nix build -o $TEST_ROOT/result --expr "(builtins.getFlake \"$flake1Dir\").packages.$system.default")

tests/functional/flakes/show.sh

@@ -7,6 +7,8 @@ writeSimpleFlake "$flakeDir"
 cd "$flakeDir"
 
 
+# FIXME(jade): the following is rather absurd. we have jq!
+
 # By default: Only show the packages content for the current system and no
 # legacyPackages at all
 nix flake show --json > show-output.json
@@ -19,6 +21,11 @@ assert show_output.legacyPackages.${builtins.currentSystem} == {};
 true
 '
 
+# Follow --eval-system for determining the system for flakes
+nix flake show --eval-system someOtherSystem --json > show-output.json
+drvTitle=$(jq -r '.packages.someOtherSystem.default.name' show-output.json)
+[[ $drvTitle == 'simple' ]]
+
 # With `--all-systems`, show the packages for all systems
 nix flake show --json --all-systems > show-output.json
 nix eval --impure --expr '