Fetch Git submodules lazily

src/libfetchers/git.cc

@@ -10,6 +10,7 @@
 #include "processes.hh"
 #include "git.hh"
 #include "mounted-source-accessor.hh"
+#include "lazy-source-accessor.hh"
 #include "git-utils.hh"
 #include "logging.hh"
 #include "finally.hh"
@@ -645,22 +646,27 @@ struct GitInputScheme : InputScheme
             std::map<CanonPath, nix::ref<SourceAccessor>> mounts;
 
             for (auto & [submodule, submoduleRev] : repo->getSubmodules(rev, exportIgnore)) {
-                auto resolved = repo->resolveSubmoduleUrl(submodule.url);
-                debug("Git submodule %s: %s %s %s -> %s",
-                    submodule.path, submodule.url, submodule.branch, submoduleRev.gitRev(), resolved);
-                fetchers::Attrs attrs;
-                attrs.insert_or_assign("type", "git");
-                attrs.insert_or_assign("url", resolved);
-                if (submodule.branch != "")
-                    attrs.insert_or_assign("ref", submodule.branch);
-                attrs.insert_or_assign("rev", submoduleRev.gitRev());
-                attrs.insert_or_assign("exportIgnore", Explicit<bool>{ exportIgnore });
-                attrs.insert_or_assign("submodules", Explicit<bool>{ true });
-                attrs.insert_or_assign("allRefs", Explicit<bool>{ true });
-                auto submoduleInput = fetchers::Input::fromAttrs(*input.settings, std::move(attrs));
-                auto [submoduleAccessor, submoduleInput2] =
-                    submoduleInput.getAccessor(store);
-                submoduleAccessor->setPathDisplay("«" + submoduleInput.to_string() + "»");
+                auto submoduleAccessor = make_ref<LazySourceAccessor>([repoDir, submodule, submoduleRev, exportIgnore, store, settings(input.settings)]()
+                {
+                    auto repo = GitRepo::openRepo(repoDir);
+                    auto resolved = repo->resolveSubmoduleUrl(submodule.url);
+                    debug("Git submodule %s: %s %s %s -> %s",
+                        submodule.path, submodule.url, submodule.branch, submoduleRev.gitRev(), resolved);
+                    fetchers::Attrs attrs;
+                    attrs.insert_or_assign("type", "git");
+                    attrs.insert_or_assign("url", resolved);
+                    if (submodule.branch != "")
+                        attrs.insert_or_assign("ref", submodule.branch);
+                    attrs.insert_or_assign("rev", submoduleRev.gitRev());
+                    attrs.insert_or_assign("exportIgnore", Explicit<bool>{ exportIgnore });
+                    attrs.insert_or_assign("submodules", Explicit<bool>{ true });
+                    attrs.insert_or_assign("allRefs", Explicit<bool>{ true });
+                    auto submoduleInput = fetchers::Input::fromAttrs(*settings, std::move(attrs));
+                    auto [submoduleAccessor, submoduleInput2] =
+                        submoduleInput.getAccessor(store);
+                    submoduleAccessor->setPathDisplay("«" + submoduleInput.to_string() + "»");
+                    return submoduleAccessor;
+                });
                 mounts.insert_or_assign(submodule.path, submoduleAccessor);
             }
 

src/libutil/lazy-source-accessor.hh

@@ -0,0 +1,69 @@
+#pragma once
+
+#include "source-accessor.hh"
+#include "sync.hh"
+
+#include <variant>
+
+namespace nix {
+
+/**
+ * A wrapper `SourceAccessor` that lazily constructs an underlying
+ * `SourceAccessor`.
+ */
+struct LazySourceAccessor : SourceAccessor
+{
+    using Fun = std::function<ref<SourceAccessor>()>;
+
+    Sync<std::variant<ref<SourceAccessor>, Fun>> next_;
+
+    LazySourceAccessor(Fun next)
+        : next_{{std::move(next)}}
+    {
+    }
+
+    ref<SourceAccessor> getNext()
+    {
+        auto next(next_.lock());
+        if (auto accessor = std::get_if<ref<SourceAccessor>>(&*next))
+            return *accessor;
+        else {
+            auto fun = std::get<Fun>(*next);
+            auto acc = fun();
+            *next = acc;
+            return acc;
+        }
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        return getNext()->readFile(path);
+    }
+
+    void readFile(const CanonPath & path, Sink & sink, std::function<void(uint64_t)> sizeCallback) override
+    {
+        return getNext()->readFile(path, sink, sizeCallback);
+    }
+
+    bool pathExists(const CanonPath & path) override
+    {
+        return getNext()->pathExists(path);
+    }
+
+    std::optional<Stat> maybeLstat(const CanonPath & path) override
+    {
+        return getNext()->maybeLstat(path);
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        return getNext()->readDirectory(path);
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        return getNext()->readLink(path);
+    }
+};
+
+}

src/libutil/meson.build

@@ -202,6 +202,7 @@ headers = [config_h] + files(
   'hilite.hh',
   'json-impls.hh',
   'json-utils.hh',
+  'lazy-source-accessor.hh',
   'logging.hh',
   'lru-cache.hh',
   'memory-source-accessor.hh',