Git fetcher: Improve submodule handling

Instead of making a complete copy of the repo, fetching the
submodules, and writing the result to the store (which is all
superexpensive), we now fetch the submodules recursively using the Git
fetcher, and return a union accessor that "mounts" the accessors for
the submodules on top of the root accessor.

src/libfetchers/git-utils.cc

@@ -1,11 +1,13 @@
 #include "git-utils.hh"
 #include "input-accessor.hh"
 #include "cache.hh"
+#include "finally.hh"
 
 #include <boost/core/span.hpp>
 
 #include <git2/blob.h>
 #include <git2/commit.h>
+#include <git2/config.h>
 #include <git2/describe.h>
 #include <git2/errors.h>
 #include <git2/global.h>
@@ -14,6 +16,7 @@
 #include <git2/remote.h>
 #include <git2/repository.h>
 #include <git2/status.h>
+#include <git2/submodule.h>
 #include <git2/tree.h>
 
 #include <unordered_set>
@@ -63,6 +66,8 @@ typedef std::unique_ptr<git_reference, Deleter<git_reference_free>> Reference;
 typedef std::unique_ptr<git_describe_result, Deleter<git_describe_result_free>> DescribeResult;
 typedef std::unique_ptr<git_status_list, Deleter<git_status_list_free>> StatusList;
 typedef std::unique_ptr<git_remote, Deleter<git_remote_free>> Remote;
+typedef std::unique_ptr<git_config, Deleter<git_config_free>> GitConfig;
+typedef std::unique_ptr<git_config_iterator, Deleter<git_config_iterator_free>> ConfigIterator;
 
 // A helper to ensure that we don't leak objects returned by libgit2.
 template<typename T>
@@ -256,6 +261,17 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
         return std::nullopt;
     }
 
+    std::vector<Submodule> getSubmodules(const Hash & rev) override;
+
+    std::string resolveSubmoduleUrl(const std::string & url) override
+    {
+        git_buf buf = GIT_BUF_INIT;
+        if (git_submodule_resolve_url(&buf, *this, url.c_str()))
+            throw Error("resolving Git submodule URL '%s'", url);
+        Finally cleanup = [&]() { git_buf_dispose(&buf); };
+        return buf.ptr;
+    }
+
     bool hasObject(const Hash & oid_) override
     {
         auto oid = hashToOID(oid_);
@@ -400,6 +416,16 @@ struct GitInputAccessor : InputAccessor
         return readBlob(path, true);
     }
 
+    Hash getSubmoduleRev(const CanonPath & path)
+    {
+        auto entry = need(path);
+
+        if (git_tree_entry_type(entry) != GIT_OBJECT_COMMIT)
+            throw Error("'%s' is not a submodule", showPath(path));
+
+        return toHash(*git_tree_entry_id(entry));
+    }
+
     std::map<CanonPath, TreeEntry> lookupCache;
 
     /* Recursively look up 'path' relative to the root. */
@@ -495,4 +521,56 @@ ref<InputAccessor> GitRepoImpl::getAccessor(const Hash & rev)
     return make_ref<GitInputAccessor>(ref<GitRepoImpl>(shared_from_this()), rev);
 }
 
+std::vector<GitRepoImpl::Submodule> GitRepoImpl::getSubmodules(const Hash & rev)
+{
+    /* Read the .gitmodules files from this revision. */
+    CanonPath modulesFile(".gitmodules");
+
+    auto accessor = getAccessor(rev);
+    if (!accessor->pathExists(modulesFile)) return {};
+
+    /* Parse it. */
+    auto configS = accessor->readFile(modulesFile);
+
+    auto [fdTemp, pathTemp] = createTempFile("nix-git-submodules");
+    writeFull(fdTemp.get(), configS);
+
+    GitConfig config;
+    if (git_config_open_ondisk(Setter(config), pathTemp.c_str()))
+        throw Error("parsing .gitmodules file: %s", git_error_last()->message);
+
+    ConfigIterator it;
+    if (git_config_iterator_glob_new(Setter(it), config.get(), "^submodule\\..*\\.(path|url|branch)$"))
+        throw Error("iterating over .gitmodules: %s", git_error_last()->message);
+
+    std::map<std::string, std::string> entries;
+
+    while (true) {
+        git_config_entry * entry = nullptr;
+        if (auto err = git_config_next(&entry, it.get())) {
+            if (err == GIT_ITEROVER) break;
+            throw Error("iterating over .gitmodules: %s", git_error_last()->message);
+        }
+        entries.emplace(entry->name + 10, entry->value);
+    }
+
+    std::vector<Submodule> result;
+
+    for (auto & [key, value] : entries) {
+        if (!hasSuffix(key, ".path")) continue;
+        std::string key2(key, 0, key.size() - 5);
+        auto path = CanonPath(value);
+        auto rev = accessor.dynamic_pointer_cast<GitInputAccessor>()->getSubmoduleRev(path);
+        result.push_back(Submodule {
+            .path = path,
+            .url = entries[key2 + ".url"],
+            .branch = entries[key2 + ".branch"],
+            .rev = rev,
+        });
+    }
+
+    return result;
+}
+
+
 }