GitCAS: implement method for creation of an empty GitCAS

...and use it in GitRepo to set custom backends.

(cherry-picked 99860b78304817c4d5b27ec4c661b733e30a430a)

4 files changed, 37 insertions, 16 deletions

diff --git a/src/buildtool/file_system/TARGETS b/src/buildtool/file_system/TARGETS
index 790111f1..ff145084 100644
--- a/src/buildtool/file_system/TARGETS
+++ b/src/buildtool/file_system/TARGETS
@@ -68,7 +68,8 @@
   , "name": ["git_cas"]
   , "hdrs": ["git_cas.hpp"]
   , "srcs": ["git_cas.cpp"]
-  , "deps": ["git_context", "git_utils", ["@", "gsl", "", "gsl"]]
+  , "deps":
+    ["git_context", "git_utils", "object_type", ["@", "gsl", "", "gsl"]]
   , "stage": ["src", "buildtool", "file_system"]
   , "private-deps":
     [ ["", "libgit2"]
diff --git a/src/buildtool/file_system/git_cas.cpp b/src/buildtool/file_system/git_cas.cpp
index 13177cea..f40c83cf 100644
--- a/src/buildtool/file_system/git_cas.cpp
+++ b/src/buildtool/file_system/git_cas.cpp
@@ -109,6 +109,24 @@ auto GitCAS::Open(std::filesystem::path const& repo_path) noexcept
 #endif
 }
 
+auto GitCAS::CreateEmpty() noexcept -> GitCASPtr {
+#ifdef BOOTSTRAP_BUILD_TOOL
+    return nullptr;
+#else
+    auto result = std::make_shared<GitCAS>();
+
+    git_odb* odb_ptr{nullptr};
+    if (git_odb_new(&odb_ptr) != 0 or odb_ptr == nullptr) {
+        Logger::Log(LogLevel::Error,
+                    "creating an empty database failed with:\n{}",
+                    GitLastError());
+        return nullptr;
+    }
+    result->odb_.reset(odb_ptr);  // retain odb pointer
+    return std::const_pointer_cast<GitCAS const>(result);
+#endif
+}
+
 auto GitCAS::ReadObject(std::string const& id, bool is_hex_id) const noexcept
     -> std::optional<std::string> {
 #ifdef BOOTSTRAP_BUILD_TOOL
diff --git a/src/buildtool/file_system/git_cas.hpp b/src/buildtool/file_system/git_cas.hpp
index 78cb6d0c..f358e65b 100644
--- a/src/buildtool/file_system/git_cas.hpp
+++ b/src/buildtool/file_system/git_cas.hpp
@@ -24,6 +24,7 @@
 #include <utility>
 #include <vector>
 
+#include "gsl/gsl"
 #include "src/buildtool/file_system/git_context.hpp"
 #include "src/buildtool/file_system/git_utils.hpp"
 
@@ -36,6 +37,8 @@ class GitCAS {
     [[nodiscard]] static auto Open(
         std::filesystem::path const& repo_path) noexcept -> GitCASPtr;
 
+    [[nodiscard]] static auto CreateEmpty() noexcept -> GitCASPtr;
+
     GitCAS() noexcept;
     ~GitCAS() noexcept = default;
 
@@ -45,6 +48,10 @@ class GitCAS {
     auto operator=(GitCAS const&) = delete;
     auto operator=(GitCAS&& other) = delete;
 
+    [[nodiscard]] auto GetODB() const noexcept -> gsl::not_null<git_odb*> {
+        return odb_.get();
+    }
+
     /// \brief Read object from CAS.
     /// \param id         The object id.
     /// \param is_hex_id  Specify whether `id` is hex string or raw.
diff --git a/src/buildtool/file_system/git_repo.cpp b/src/buildtool/file_system/git_repo.cpp
index def30f6a..4a2fb626 100644
--- a/src/buildtool/file_system/git_repo.cpp
+++ b/src/buildtool/file_system/git_repo.cpp
@@ -1972,7 +1972,6 @@ auto GitRepo::ReadTreeData(
 #ifndef BOOTSTRAP_BUILD_TOOL
     try {
         InMemoryODBBackend b{.parent = kInMemoryODBParent};
-        auto cas = std::make_shared<GitCAS>();
         if (auto raw_id =
                 is_hex_id ? FromHexString(id) : std::make_optional(id)) {
             try {
@@ -1980,17 +1979,16 @@ auto GitRepo::ReadTreeData(
             } catch (...) {
                 return std::nullopt;
             }
-            // create a GitCAS from a special-purpose in-memory object database.
-            git_odb* odb_ptr{nullptr};
-            if (git_odb_new(&odb_ptr) == 0 and
+            // create a GitCAS from a special-purpose in-memory object
+            // database.
+            auto cas = GitCAS::CreateEmpty();
+            if (cas != nullptr and
                 git_odb_add_backend(
-                    odb_ptr,
+                    cas->GetODB(),
                     reinterpret_cast<git_odb_backend*>(&b),  // NOLINT
                     0) == 0) {
-                cas->odb_.reset(odb_ptr);  // take ownership of odb
                 // wrap odb in "fake" repo
-                auto repo =
-                    GitRepo(std::static_pointer_cast<GitCAS const>(cas));
+                auto repo = GitRepo(cas);
                 return repo.ReadTree(
                     *raw_id, check_symlinks, /*is_hex_id=*/false);
             }
@@ -2008,17 +2006,14 @@ auto GitRepo::CreateShallowTree(tree_entries_t const& entries) noexcept
 #ifndef BOOTSTRAP_BUILD_TOOL
     try {
         InMemoryODBBackend b{.parent = kInMemoryODBParent, .entries = &entries};
-        auto cas = std::make_shared<GitCAS>();
-        // create a GitCAS from a special-purpose in-memory object database.
-        git_odb* odb_ptr{nullptr};
-        if (git_odb_new(&odb_ptr) == 0 and
+        auto cas = GitCAS::CreateEmpty();
+        if (cas != nullptr and
             git_odb_add_backend(
-                odb_ptr,
+                cas->GetODB(),
                 reinterpret_cast<git_odb_backend*>(&b),  // NOLINT
                 0) == 0) {
-            cas->odb_.reset(odb_ptr);  // take ownership of odb
             // wrap odb in "fake" repo
-            auto repo = GitRepo(std::static_pointer_cast<GitCAS const>(cas));
+            auto repo = GitRepo(cas);
             if (auto raw_id = repo.CreateTree(entries)) {
                 // read result from in-memory trees
                 if (auto it = b.trees.find(*raw_id); it != b.trees.end()) {