Implement matrix sharding

Author: zanieb

ci-matrix.py

@@ -16,6 +16,7 @@
 CI_TARGETS_YAML = "ci-targets.yaml"
 CI_RUNNERS_YAML = "ci-runners.yaml"
 CI_EXTRA_SKIP_LABELS = ["documentation"]
+CI_MATRIX_SIZE_LIMIT = 256  # The maximum size of a matrix in GitHub Actions
 
 
 def meets_conditional_version(version: str, min_version: str) -> bool:
@@ -246,14 +247,19 @@ def main() -> None:
             if runner_config.get("free")
         }
 
-    matrix = {
-        "include": generate_matrix_entries(
-            config,
-            runners,
-            args.platform,
-            labels,
-        )
-    }
+    entries = generate_matrix_entries(
+        config,
+        runners,
+        args.platform,
+        labels,
+    )
+
+    matrix = {}
+    shards = len(entries) // CI_MATRIX_SIZE_LIMIT
+    for shard in shards:
+        matrix[str(shard)] = entries[
+            shard * CI_MATRIX_SIZE_LIMIT : (shard + 1) * CI_MATRIX_SIZE_LIMIT
+        ]
 
     print(json.dumps(matrix))