tc cache: abort writing if a dependent target is not available

After successful build of an export target target a cache entry is
written. As those export targets have a dependency structure, care
is taken to write them in correct order.

Writing a cache entry for an export target requires knowledge of
the analysis result of that target. Originally, all targets used
to be analysed locally, so that information was available for
every target that was not read from the local target cache already.
However, with the introduction of serve endpoints, it can happen
that a target was analyzed locally, but it depends on targets
obtained from serve. In this case, we have to refrain from writing
a target-level cache entry to keep the the consistency invariants
of the target-level cache.

1 files changed, 9 insertions, 2 deletions

diff --git a/src/buildtool/main/build_utils.cpp b/src/buildtool/main/build_utils.cpp
index 5b8d142d..b29ff0c5 100644
--- a/src/buildtool/main/build_utils.cpp
+++ b/src/buildtool/main/build_utils.cpp
@@ -83,8 +83,15 @@ auto CreateTargetCacheWriterMap(
             TargetCacheKey tc_key{key};
             // check if entry actually needs storing
             if (not cache_targets.contains(tc_key)) {
-                // entry already in target-cache, so nothing to be done
-                (*setter)(nullptr);
+                if (tc.Read(tc_key)) {
+                    // entry already in target-cache, so nothing to be done
+                    (*setter)(nullptr);
+                    return;
+                }
+                (*logger)(fmt::format("Export target {} not analysed locally; "
+                                      "not caching anything depending on it",
+                                      key.ToString()),
+                          true);
                 return;
             }
             auto const& target = cache_targets.at(tc_key);