Make nix flake metadata|update|lock lazy

These don't need to evaluate anything (except for the flake metadata
in flake.nix) so we can make these commands operate on lazy trees
without risk of any semantic change in the evaluator.

However, `nix flake metadata` now no longer prints the store path,
which is a breaking change (but unavoidable if we want lazy trees).

src/libflake/flake/flake.cc

@@ -398,7 +398,8 @@ static Flake getFlake(
     const FlakeRef & originalRef,
     bool useRegistries,
     FlakeCache & flakeCache,
-    const InputAttrPath & lockRootAttrPath)
+    const InputAttrPath & lockRootAttrPath,
+    bool forceLazy)
 {
     // Fetch a lazy tree first.
     auto [accessor, resolvedRef, lockedRef] = fetchOrSubstituteTree(
@@ -420,17 +421,22 @@ static Flake getFlake(
         lockedRef = lockedRef2;
     }
 
-    // Copy the tree to the store.
-    auto storePath = copyInputToStore(state, lockedRef.input, accessor);
-
     // Re-parse flake.nix from the store.
-    return readFlake(state, originalRef, resolvedRef, lockedRef, state.rootPath(state.store->toRealPath(storePath)), lockRootAttrPath);
+    return readFlake(
+        state, originalRef, resolvedRef, lockedRef,
+        forceLazy && lockedRef.input.isLocked()
+        ? SourcePath(accessor)
+        : // Copy the tree to the store.
+          state.rootPath(
+            state.store->toRealPath(
+                copyInputToStore(state, lockedRef.input, accessor))),
+        lockRootAttrPath);
 }
 
-Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool useRegistries)
+Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool useRegistries, bool forceLazy)
 {
     FlakeCache flakeCache;
-    return getFlake(state, originalRef, useRegistries, flakeCache, {});
+    return getFlake(state, originalRef, useRegistries, flakeCache, {}, forceLazy);
 }
 
 static LockFile readLockFile(
@@ -456,7 +462,7 @@ LockedFlake lockFlake(
 
     auto useRegistries = lockFlags.useRegistries.value_or(settings.useRegistries);
 
-    auto flake = getFlake(state, topRef, useRegistries, flakeCache, {});
+    auto flake = getFlake(state, topRef, useRegistries, flakeCache, {}, lockFlags.forceLazy);
 
     if (lockFlags.applyNixConfig) {
         flake.config.apply(settings);
@@ -632,7 +638,7 @@ LockedFlake lockFlake(
                         if (auto resolvedPath = resolveRelativePath()) {
                             return readFlake(state, *input.ref, *input.ref, *input.ref, *resolvedPath, inputAttrPath);
                         } else {
-                            return getFlake(state, *input.ref, useRegistries, flakeCache, inputAttrPath);
+                            return getFlake(state, *input.ref, useRegistries, flakeCache, inputAttrPath, lockFlags.forceLazy);
                         }
                     };
 
@@ -783,10 +789,14 @@ LockedFlake lockFlake(
                                     auto [accessor, resolvedRef, lockedRef] = fetchOrSubstituteTree(
                                         state, *input.ref, useRegistries, flakeCache);
 
-                                    // FIXME: allow input to be lazy.
-                                    auto storePath = copyInputToStore(state, lockedRef.input, accessor);
-
-                                    return {state.rootPath(state.store->toRealPath(storePath)), lockedRef};
+                                    return {
+                                        lockFlags.forceLazy && lockedRef.input.isLocked()
+                                        ? SourcePath(accessor)
+                                        : state.rootPath(
+                                            state.store->toRealPath(
+                                                copyInputToStore(state, lockedRef.input, accessor))),
+                                        lockedRef
+                                    };
                                 }
                             }();
 
@@ -896,7 +906,7 @@ LockedFlake lockFlake(
                            repo, so we should re-read it. FIXME: we could
                            also just clear the 'rev' field... */
                         auto prevLockedRef = flake.lockedRef;
-                        flake = getFlake(state, topRef, useRegistries);
+                        flake = getFlake(state, topRef, useRegistries, lockFlags.forceLazy);
 
                         if (lockFlags.commitLockFile &&
                             flake.lockedRef.input.getRev() &&

src/libflake/flake/flake.hh

@@ -123,7 +123,11 @@ struct Flake
     }
 };
 
-Flake getFlake(EvalState & state, const FlakeRef & flakeRef, bool useRegistries);
+Flake getFlake(
+    EvalState & state,
+    const FlakeRef & flakeRef,
+    bool useRegistries,
+    bool forceLazy = false);
 
 /**
  * Fingerprint of a locked flake; used as a cache key.
@@ -221,6 +225,11 @@ struct LockFlags
      * for those inputs will be ignored.
      */
     std::set<InputAttrPath> inputUpdates;
+
+    /**
+     * If set, do not copy the flake to the Nix store.
+     */
+    bool forceLazy = false;
 };
 
 LockedFlake lockFlake(

src/nix/flake.cc

@@ -133,6 +133,7 @@ struct CmdFlakeUpdate : FlakeCommand
         lockFlags.recreateLockFile = updateAll;
         lockFlags.writeLockFile = true;
         lockFlags.applyNixConfig = true;
+        lockFlags.forceLazy = true;
 
         lockFlake();
     }
@@ -165,6 +166,7 @@ struct CmdFlakeLock : FlakeCommand
         lockFlags.writeLockFile = true;
         lockFlags.failOnUnlocked = true;
         lockFlags.applyNixConfig = true;
+        lockFlags.forceLazy = true;
 
         lockFlake();
     }
@@ -211,12 +213,10 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
 
     void run(nix::ref<nix::Store> store) override
     {
+        lockFlags.forceLazy = true;
         auto lockedFlake = lockFlake();
         auto & flake = lockedFlake.flake;
 
-        // Currently, all flakes are in the Nix store via the rootFS accessor.
-        auto storePath = store->printStorePath(sourcePathToStorePath(store, flake.path).first);
-
         if (json) {
             nlohmann::json j;
             if (flake.description)
@@ -237,7 +237,6 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 j["revCount"] = *revCount;
             if (auto lastModified = flake.lockedRef.input.getLastModified())
                 j["lastModified"] = *lastModified;
-            j["path"] = storePath;
             j["locks"] = lockedFlake.lockFile.toJSON().first;
             if (auto fingerprint = lockedFlake.getFingerprint(store, fetchSettings))
                 j["fingerprint"] = fingerprint->to_string(HashFormat::Base16, false);
@@ -254,9 +253,6 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 logger->cout(
                     ANSI_BOLD "Description:" ANSI_NORMAL "   %s",
                     *flake.description);
-            logger->cout(
-                ANSI_BOLD "Path:" ANSI_NORMAL "          %s",
-                storePath);
             if (auto rev = flake.lockedRef.input.getRev())
                 logger->cout(
                     ANSI_BOLD "Revision:" ANSI_NORMAL "      %s",

tests/functional/flakes/flakes.sh

@@ -69,7 +69,6 @@ nix flake metadata "$flake1Dir" | grepQuiet 'URL:.*flake1.*'
 # Test 'nix flake metadata --json'.
 json=$(nix flake metadata flake1 --json | jq .)
 [[ $(echo "$json" | jq -r .description) = 'Bla bla' ]]
-[[ -d $(echo "$json" | jq -r .path) ]]
 [[ $(echo "$json" | jq -r .lastModified) = $(git -C "$flake1Dir" log -n1 --format=%ct) ]]
 hash1=$(echo "$json" | jq -r .revision)
 [[ -n $(echo "$json" | jq -r .fingerprint) ]]