serve source tree: Increase server-side granularity in response statuses

For archives and Git repositories we should ensure that not finding
the witnessing entity (archive content blob or Git commit,
respectively) results in a distinct status in the response to a
request that sets up roots on the serve endpoint. This will allow
just-mr to better handle its interaction with the serve endpoint.

1 files changed, 11 insertions, 2 deletions

diff --git a/src/buildtool/file_system/git_repo.hpp b/src/buildtool/file_system/git_repo.hpp
index 05bcfaf3..09e65402 100644
--- a/src/buildtool/file_system/git_repo.hpp
+++ b/src/buildtool/file_system/git_repo.hpp
@@ -15,7 +15,13 @@
 #ifndef INCLUDED_SRC_BUILDTOOL_FILE_SYSTEM_GIT_REPO_HPP
 #define INCLUDED_SRC_BUILDTOOL_FILE_SYSTEM_GIT_REPO_HPP
 
+#include <filesystem>
 #include <functional>
+#include <optional>
+#include <string>
+#include <unordered_map>
+#include <variant>
+#include <vector>
 
 #include "src/buildtool/common/bazel_types.hpp"
 #include "src/buildtool/file_system/git_cas.hpp"
@@ -178,12 +184,15 @@ class GitRepo {
 
     /// \brief Get the tree id of a subtree given the root commit
     /// Calling it from a fake repository allows thread-safe use.
-    /// Returns the subtree hash, as a string, or nullopt if failure.
+    /// Returns an error + data union, where at index 0 is a flag stating the
+    /// nature of the error on failure (true is fatal, false is non-fatal, i.e.,
+    /// commit not found), and at index 1 is the subtree hash on success.
     /// It guarantees the logger is called exactly once with fatal if failure.
     [[nodiscard]] auto GetSubtreeFromCommit(
         std::string const& commit,
         std::string const& subdir,
-        anon_logger_ptr const& logger) noexcept -> std::optional<std::string>;
+        anon_logger_ptr const& logger) noexcept
+        -> std::variant<bool, std::string>;
 
     /// \brief Get the tree id of a subtree given the root tree hash
     /// Calling it from a fake repository allows thread-safe use.