serve target: Improve logic for local build failure triggers

If the serve endpoint reports an internal error, local builds
should not continue and the error message should be provided.
Similarly, if the serve endpoint promises the target cache value to
be in remote CAS, but the client cannot find or process it as
needed, then a local build again should not continue and the reason
be provided as an error message.

1 files changed, 37 insertions, 24 deletions

diff --git a/src/buildtool/build_engine/target_map/export.cpp b/src/buildtool/build_engine/target_map/export.cpp
index e0bc36c3..e92d4c9c 100644
--- a/src/buildtool/build_engine/target_map/export.cpp
+++ b/src/buildtool/build_engine/target_map/export.cpp
@@ -160,31 +160,44 @@ void ExportRule(
                             key.target.ToString());
             }
             else {
-                if (res->index() == 0) {
-                    // target found but failed to analyse/build: this should be
-                    // a fatal error for the local build too
-                    (*logger)(
-                        fmt::format("Failure to remotely analyse or build "
-                                    "target {}\nDetailed log available on the "
-                                    "remote-execution endpoint as blob {}",
-                                    key.target.ToString(),
-                                    std::get<0>(*res)),
-                        /*fatal=*/true);
-                    return;
-                }
-                if (res->index() == 1) {
-                    // some other failure occurred while querying the serve
-                    // endpoint; log to debug and continue locally
-                    Logger::Log(LogLevel::Debug,
-                                "While querying serve endpoint for export "
-                                "target {}:\n{}",
+                switch (res->index()) {
+                    case 0: {
+                        // target found but failed to analyse/build: this should
+                        // be a fatal error for the local build too
+                        (*logger)(
+                            fmt::format(
+                                "Failure to remotely analyse or build target "
+                                "{}\nDetailed log available on the "
+                                "remote-execution endpoint as blob {}",
                                 key.target.ToString(),
-                                std::get<1>(*res));
-                }
-                else {
-                    // index == 2
-                    target_cache_value = std::get<2>(*res);
-                    from_just_serve = true;
+                                std::get<0>(*res)),
+                            /*fatal=*/true);
+                        return;
+                    }
+                    case 1: {
+                        // internal failure on the serve endpoint, or failures
+                        // on the client side: local build should not continue
+                        (*logger)(fmt::format("While querying serve endpoint "
+                                              "for export target {}:\n{}",
+                                              key.target.ToString(),
+                                              std::get<1>(*res)),
+                                  /*fatal=*/true);
+                        return;
+                    }
+                    case 2: {
+                        // some other failure occurred on the serve endpoint;
+                        // log to debug and continue locally
+                        Logger::Log(LogLevel::Debug,
+                                    "While querying serve endpoint for export "
+                                    "target {}:\n{}",
+                                    key.target.ToString(),
+                                    std::get<2>(*res));
+                    } break;
+                    default: {
+                        // index == 3
+                        target_cache_value = std::get<3>(*res);
+                        from_just_serve = true;
+                    }
                 }
             }
             exports_progress->TaskTracker().Stop(task);