Register flake source trees as GC roots

This ensures that flakes don't get garbage-collected, which is
important to get nix-channel-like behaviour.

For example, running

  $ nix build hydra:

will create a GC root

  ~/.cache/nix/flake-closures/hydra -> /nix/store/xarfiqcwa4w8r4qpz1a769xxs8c3phgn-flake-closure

where the contents/references of the linked file in the store are the
flake source trees used by the 'hydra' flake:

  /nix/store/n6d5f5lkpfjbmkyby0nlg8y1wbkmbc7i-source
  /nix/store/vbkg4zy1qd29fnhflsv9k2j9jnbqd5m2-source
  /nix/store/z46xni7d47s5wk694359mq9ay353ar94-source

Note that this in itself is not enough to allow offline use; the
fetcher for the flakeref (e.g. fetchGit or downloadCached) must not
fail if it cannot fetch the latest version of the file, so long as it
knows a cached version.

Issue #2868.

src/libexpr/primops/flake.hh

@@ -132,6 +132,8 @@ struct ResolvedFlake
 
 ResolvedFlake resolveFlake(EvalState &, const FlakeRef &, HandleLockFile);
 
+void callFlake(EvalState & state, const ResolvedFlake & resFlake, Value & v);
+
 void updateLockFile(EvalState &, const FlakeRef & flakeRef, bool recreateLockFile);
 
 void gitCloneFlake(FlakeRef flakeRef, EvalState &, Registries, const Path & destDir);

src/nix/installables.cc

@@ -10,6 +10,7 @@
 #include "primops/flake.hh"
 
 #include <regex>
+#include <queue>
 
 namespace nix {
 
@@ -162,6 +163,44 @@ struct InstallableAttrPath : InstallableValue
     }
 };
 
+void makeFlakeClosureGCRoot(Store & store, const FlakeRef & origFlakeRef, const ResolvedFlake & resFlake)
+{
+    if (std::get_if<FlakeRef::IsPath>(&origFlakeRef.data)) return;
+
+    /* Get the store paths of all non-local flakes. */
+    PathSet closure;
+
+    std::queue<std::reference_wrapper<const ResolvedFlake>> queue;
+    queue.push(resFlake);
+
+    while (!queue.empty()) {
+        const ResolvedFlake & flake = queue.front();
+        queue.pop();
+        if (!std::get_if<FlakeRef::IsPath>(&flake.flake.resolvedRef.data))
+            closure.insert(flake.flake.storePath);
+        for (const auto & dep : flake.flakeDeps)
+            queue.push(dep.second);
+    }
+
+    if (closure.empty()) return;
+
+    /* Write the closure to a file in the store. */
+    auto closurePath = store.addTextToStore("flake-closure", concatStringsSep(" ", closure), closure);
+
+    Path cacheDir = getCacheDir() + "/nix/flake-closures";
+    createDirs(cacheDir);
+
+    auto s = origFlakeRef.to_string();
+    assert(s[0] != '.');
+    s = replaceStrings(s, "%", "%25");
+    s = replaceStrings(s, "/", "%2f");
+    s = replaceStrings(s, ":", "%3a");
+    Path symlink = cacheDir + "/" + s;
+    debug("writing GC root '%s' for flake closure of '%s'", symlink, origFlakeRef);
+    replaceSymlink(closurePath, symlink);
+    store.addIndirectRoot(symlink);
+}
+
 struct InstallableFlake : InstallableValue
 {
     FlakeRef flakeRef;
@@ -182,7 +221,11 @@ struct InstallableFlake : InstallableValue
     {
         auto vFlake = state.allocValue();
 
-        makeFlakeValue(state, flakeRef, cmd.getLockFileMode(), *vFlake);
+        auto resFlake = resolveFlake(state, flakeRef, cmd.getLockFileMode());
+
+        callFlake(state, resFlake, *vFlake);
+
+        makeFlakeClosureGCRoot(*state.store, flakeRef, resFlake);
 
         auto vProvides = (*vFlake->attrs->get(state.symbols.create("provides")))->value;
 

tests/flakes.sh

@@ -5,7 +5,10 @@ if [[ -z $(type -p git) ]]; then
     exit 99
 fi
 
+export _NIX_FORCE_HTTP=1
+
 clearStore
+rm -rf $TEST_HOME/.cache
 
 registry=$TEST_ROOT/registry.json
 
@@ -14,7 +17,7 @@ flake2Dir=$TEST_ROOT/flake2
 flake3Dir=$TEST_ROOT/flake3
 
 for repo in $flake1Dir $flake2Dir $flake3Dir; do
-    rm -rf $repo
+    rm -rf $repo $repo.tmp
     mkdir $repo
     git -C $repo init
     git -C $repo config user.email "foobar@example.com"
@@ -142,7 +145,7 @@ nix build -o $TEST_ROOT/result --flake-registry $registry flake2:bar
 
 # Or without a registry.
 # FIXME: shouldn't need '--flake-registry /no-registry'?
-nix build -o $TEST_ROOT/result --flake-registry /no-registry file://$flake2Dir:bar
+nix build -o $TEST_ROOT/result --flake-registry /no-registry file://$flake2Dir:bar --tarball-ttl 0
 
 # Test whether indirect dependencies work.
 nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:xyzzy
@@ -184,3 +187,15 @@ nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:sth 2>&1 |
 nix flake list --flake-registry file://$registry | grep -q flake3
 mv $registry $registry.tmp
 nix flake list --flake-registry file://$registry --tarball-ttl 0 | grep -q flake3
+mv $registry.tmp $registry
+
+# Test whether flakes are registered as GC roots for offline use.
+rm -rf $TEST_HOME/.cache
+nix build -o $TEST_ROOT/result --flake-registry file://$registry file://$flake2Dir:bar
+mv $flake1Dir $flake1Dir.tmp
+mv $flake2Dir $flake2Dir.tmp
+nix-store --gc
+nix build -o $TEST_ROOT/result --flake-registry file://$registry file://$flake2Dir:bar
+nix build -o $TEST_ROOT/result --flake-registry file://$registry file://$flake2Dir:bar --tarball-ttl 0
+mv $flake1Dir.tmp $flake1Dir
+mv $flake2Dir.tmp $flake2Dir