serve target: Differentiate between fatal and non-fatal orchestrated builds

...by increasing granularity in client-side reporting. This allows
to correctly continue with builds of local targets if the serve
endpoint does not have the requested target, as well as improve
the reporting for users on failure.

1 files changed, 16 insertions, 4 deletions

diff --git a/src/buildtool/serve_api/remote/target_client.hpp b/src/buildtool/serve_api/remote/target_client.hpp
index c9540cb0..948e9337 100644
--- a/src/buildtool/serve_api/remote/target_client.hpp
+++ b/src/buildtool/serve_api/remote/target_client.hpp
@@ -19,6 +19,7 @@
 #include <optional>
 #include <string>
 #include <utility>
+#include <variant>
 #include <vector>
 
 #include "gsl/gsl"
@@ -31,6 +32,16 @@
 #include "src/buildtool/storage/target_cache_entry.hpp"
 #include "src/buildtool/storage/target_cache_key.hpp"
 
+/// \brief Result union for the ServeTarget request.
+/// Index 0 will contain the hash of the blob containing the logged
+/// analysis/build failure received from the endpoint, as a string.
+/// Index 1 will contain any other failure message, as a string.
+/// Index 2 will contain the target cache value information on success.
+using serve_target_result_t =
+    std::variant<std::string,
+                 std::string,
+                 std::pair<TargetCacheEntry, Artifact::ObjectInfo>>;
+
 /// Implements client side for Target service defined in:
 /// src/buildtool/serve_api/serve_service/just_serve.proto
 class TargetClient {
@@ -39,12 +50,13 @@ class TargetClient {
 
     /// \brief Retrieve the pair of TargetCacheEntry and ObjectInfo associated
     /// to the given key.
-    /// \param[in] key The TargetCacheKey of an export target
-    /// \param[in] repo_key The RepositoryKey to upload as precondition
-    /// \returns Pair of cache entry and its object info on success or nullopt.
+    /// \param[in] key The TargetCacheKey of an export target.
+    /// \param[in] repo_key The RepositoryKey to upload as precondition.
+    /// \returns A correspondingly populated result union, or nullopt if remote
+    /// reported that the target was not found.
     [[nodiscard]] auto ServeTarget(const TargetCacheKey& key,
                                    const std::string& repo_key) noexcept
-        -> std::optional<std::pair<TargetCacheEntry, Artifact::ObjectInfo>>;
+        -> std::optional<serve_target_result_t>;
 
     /// \brief Retrieve the flexible config variables of an export target.
     /// \param[in] target_root_id Hash of target-level root tree.