build_mix: no stray print inside BUILD_JSON block (E-019); manifest gains n_shards

build/build_mix.py

@@ -598,6 +598,7 @@ def main():
                   for k, v in st.d["staged"].items()},
         "staged_tokens": st.staged_tokens_total(),
         "shards": st.d["merged_shards"],
+        "n_shards": len(st.d["merged_shards"]),
         "val_shards": st.d.get("val_shards", []),
         "total_tokens": sum(s["tokens"] for s in st.d["merged_shards"]),
         "val_tokens": sum(s["tokens"] for s in st.d.get("val_shards", [])),
@@ -609,11 +610,12 @@ def main():
     }
     with open(os.path.join(args.root, "manifest.json"), "w") as f:
         json.dump(manifest, f, indent=1)
+    # Nothing may be printed between the JSON and its END marker: the caller parses this span with
+    # json.loads, and a stray status line turned a passing rehearsal red (E-019).
     print("BUILD_JSON_BEGIN")
     print(json.dumps({k: manifest[k] for k in
-                      ("staged_tokens", "total_tokens", "distinct_sources_per_shard_min",
-                       "seconds")}, indent=1))
-    print("shards=", len(manifest["shards"]), flush=True)
+                      ("staged_tokens", "total_tokens", "val_tokens", "n_shards",
+                       "distinct_sources_per_shard_min", "seconds")}, indent=1))
     print("BUILD_JSON_END")