Fix serialization of the target cache key

... which was accidentially a list of (a single) object,
instead of only a single JSON object.

(cherry-picked from 63a874517618a57dd5ca223d19a795c28a39c123)

2 files changed, 19 insertions, 11 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8e762967..22ea953c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,13 +4,21 @@ Bug fixes on top of release `1.2.2`.
 
 ### Fixes
 
-- The cache key used for an export target is now based on the export
-  target itself rather than that of the exported target. The latter
-  could lead to spourious cache hits, but only in the case where
-  an explicit file reference was exported and an regular target
-  with the same name exists. Where the new cache keys overlap with
-  the old ones, they refer to the same configured targets; so no
-  special action is to be taken when updating.
+- The cache key used for an export target is now based on the
+  export target itself rather than that of the exported target. The
+  latter could lead to spurious cache hits, but only in the case
+  where the exported target was an explicit file reference, and a
+  regular target with the same name existed as well. Where the new
+  cache keys would overlap with the old ones, they would refer to
+  the same configured targets. However, we used the fact that we
+  changed the target cache key to also clean up the serialization
+  format to only contain the JSON object describing repository,
+  target, and effective configuration, instead of a singleton list
+  containing this object. Therefore, old and new cache keys do not
+  overlap at all. In particular, no special care has to be taken
+  on upgrading or downgrading. However, old target-level cache
+  entries will not be used leading potentially to rebuilding of
+  some targets.
 - Fixed a race condition in an internal cache of `just execute` used for keeping
   track of running operations.
 
diff --git a/src/buildtool/storage/target_cache_key.cpp b/src/buildtool/storage/target_cache_key.cpp
index 11ea5c84..7d300338 100644
--- a/src/buildtool/storage/target_cache_key.cpp
+++ b/src/buildtool/storage/target_cache_key.cpp
@@ -31,10 +31,10 @@ auto TargetCacheKey::Create(std::string const& repo_key,
     try {
         // target's repository is content-fixed, we can compute a cache key
         auto target_desc = nlohmann::json{
-            {{"repo_key", repo_key},
-             {"target_name",
-              nlohmann::json{target_name.module, target_name.name}.dump()},
-             {"effective_config", effective_config.ToString()}}};
+            {"repo_key", repo_key},
+            {"target_name",
+             nlohmann::json{target_name.module, target_name.name}.dump()},
+            {"effective_config", effective_config.ToString()}};
         if (auto target_key = Storage::Instance().CAS().StoreBlob(
                 target_desc.dump(2), /*is_executable=*/false)) {
             return TargetCacheKey{