2 files changed, 74 insertions, 4 deletions

diff --git a/src/buildtool/file_system/git_repo.cpp b/src/buildtool/file_system/git_repo.cpp
index a989c608..1b9eaf1f 100644
--- a/src/buildtool/file_system/git_repo.cpp
+++ b/src/buildtool/file_system/git_repo.cpp
@@ -1127,7 +1127,7 @@ auto GitRepo::CheckTreeExists(std::string const& tree_id,
         // preferably with a "fake" repository!
         if (not IsRepoFake()) {
             Logger::Log(LogLevel::Debug,
-                        "Subtree lookup called on a real repository");
+                        "Tree lookup called on a real repository");
         }
         // get git oid
         git_oid tree_oid;
@@ -1161,14 +1161,73 @@ auto GitRepo::CheckTreeExists(std::string const& tree_id,
         }
         return true;  // tree found
     } catch (std::exception const& ex) {
-        Logger::Log(LogLevel::Error,
-                    "check subtree exists failed with:\n{}",
-                    ex.what());
+        Logger::Log(
+            LogLevel::Error, "check tree exists failed with:\n{}", ex.what());
         return std::nullopt;
     }
 #endif  // BOOTSTRAP_BUILD_TOOL
 }
 
+auto GitRepo::TryReadBlob(std::string const& blob_id,
+                          anon_logger_ptr const& logger) noexcept
+    -> std::pair<bool, std::optional<std::string>> {
+#ifdef BOOTSTRAP_BUILD_TOOL
+    return std::pair(false, std::nullopt);
+#else
+    try {
+        // preferably with a "fake" repository!
+        if (not IsRepoFake()) {
+            Logger::Log(LogLevel::Debug,
+                        "Blob lookup called on a real repository");
+        }
+        // get git oid
+        git_oid blob_oid;
+        if (git_oid_fromstr(&blob_oid, blob_id.c_str()) != 0) {
+            (*logger)(fmt::format("blob ID parsing in git repository {} failed "
+                                  "with:\n{}",
+                                  GetGitCAS()->git_path_.string(),
+                                  GitLastError()),
+                      true /*fatal*/);
+            return std::pair(false, std::nullopt);
+        }
+        // get blob object
+        git_blob* blob_ptr = nullptr;
+        int lookup_res{};
+        {
+            // share the odb lock
+            std::shared_lock lock{GetGitCAS()->mutex_};
+            lookup_res = git_blob_lookup(&blob_ptr, repo_->Ptr(), &blob_oid);
+        }
+        git_blob_free(blob_ptr);
+        if (lookup_res != 0) {
+            if (lookup_res == GIT_ENOTFOUND) {
+                return std::pair(true, std::nullopt);  // blob not found
+            }
+            (*logger)(fmt::format("blob lookup in git repository {} failed "
+                                  "with:\n{}",
+                                  GetGitCAS()->git_path_.string(),
+                                  GitLastError()),
+                      true /*fatal*/);
+            return std::pair(false, std::nullopt);
+        }
+        // get data of found blob
+        if (auto data = GetGitCAS()->ReadObject(blob_id, /*is_hex_id=*/true)) {
+            return std::pair(true, std::move(*data));
+        }
+        (*logger)(fmt::format(
+                      "failed to read target for blob {} in git repository {}",
+                      blob_id,
+                      GetGitCAS()->git_path_.string()),
+                  true /*fatal*/);
+        return std::pair(false, std::nullopt);
+    } catch (std::exception const& ex) {
+        Logger::Log(
+            LogLevel::Error, "check blob exists failed with:\n{}", ex.what());
+        return std::pair(false, std::nullopt);
+    }
+#endif  // BOOTSTRAP_BUILD_TOOL
+}
+
 auto GitRepo::GetObjectByPathFromTree(std::string const& tree_id,
                                       std::string const& rel_path) noexcept
     -> std::optional<TreeEntryInfo> {
diff --git a/src/buildtool/file_system/git_repo.hpp b/src/buildtool/file_system/git_repo.hpp
index b2e12934..7314289a 100644
--- a/src/buildtool/file_system/git_repo.hpp
+++ b/src/buildtool/file_system/git_repo.hpp
@@ -219,6 +219,17 @@ class GitRepo {
                                        anon_logger_ptr const& logger) noexcept
         -> std::optional<bool>;
 
+    /// \brief Check if given blob ID is present in the directory structure of
+    /// the local repository and try to return it.
+    /// Calling it from a fake repository allows thread-safe use.
+    /// Returns a pair of a success flag, stating that no errors occurred while
+    /// performing the libgit2 calls, and an optional string containing the
+    /// content of the blob, if the blob is found. It guarantees the logger is
+    /// called exactly once with fatal if failure.
+    [[nodiscard]] auto TryReadBlob(std::string const& blob_id,
+                                   anon_logger_ptr const& logger) noexcept
+        -> std::pair<bool, std::optional<std::string>>;
+
     /// \brief Get the object info related to a given path inside a Git tree.
     /// Unlike GetSubtreeFromTree, we here ignore errors and only return a value
     /// when all is successful.