Add 60s timeout to thumbnail generation to prevent runner OOM kills

Large global datasets (e.g. ECMWF IFS ENS) can hang when loading
chunks for thumbnail generation, causing the Actions runner to be
killed. A SIGALRM-based timeout skips the thumbnail gracefully instead.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: rsignell

scripts/build_dynamical_stac.py

@@ -40,6 +40,7 @@
 import logging
 import os
 import shutil
+import signal
 import subprocess
 import sys
 import tempfile
@@ -62,6 +63,21 @@
 
 log = logging.getLogger(__name__)
 
+# ---------------------------------------------------------------------------
+# Thumbnail timeout
+# ---------------------------------------------------------------------------
+
+THUMBNAIL_TIMEOUT_SECONDS = 60
+
+
+class _ThumbnailTimeout(Exception):
+    pass
+
+
+def _thumbnail_timeout_handler(signum, frame):
+    raise _ThumbnailTimeout()
+
+
 # ---------------------------------------------------------------------------
 # Constants
 # ---------------------------------------------------------------------------
@@ -234,6 +250,8 @@ def generate_thumbnail(
         log.warning("  No temperature_2m in %s -- skipping thumbnail", item_id)
         return None
 
+    signal.signal(signal.SIGALRM, _thumbnail_timeout_handler)
+    signal.alarm(THUMBNAIL_TIMEOUT_SECONDS)
     try:
         import cartopy.crs as ccrs
         import cartopy.feature as cfeature
@@ -348,13 +366,19 @@ def generate_thumbnail(
         log.info("  Thumbnail saved: %s", out_path)
         return out_path
 
+    except _ThumbnailTimeout:
+        log.warning("  Thumbnail timed out after %ds for %s -- skipping",
+                    THUMBNAIL_TIMEOUT_SECONDS, item_id)
+        return None
     except Exception as exc:
         log.warning("  Thumbnail generation failed for %s: %s", item_id, exc)
+        return None
+    finally:
+        signal.alarm(0)  # cancel any remaining alarm
         try:
             plt.close("all")
         except Exception:
             pass
-        return None
 
 
 # ---------------------------------------------------------------------------