CGI: Take a list of feature IDs to query to avoid pounding the backend.

application() returns a iterator, so we serve responses immediately as
they as received from the server (no buffering).

1 files changed, 43 insertions, 16 deletions

diff --git a/webmap-cgi b/webmap-cgi
index 616b2fd..c32a389 100755
--- a/webmap-cgi
+++ b/webmap-cgi
@@ -63,11 +63,15 @@ def get_query(layername : str) -> bytes:
     query +=                                'pretty_bool=>\'f\'),'
     query +=                   '\'UTF8\') AS "GeoJSON" '
     query +=  'FROM ('
-    query +=    'SELECT l.* '
+    query +=    'SELECT l.*, %s AS layer_group, %s AS layer '
     query +=      'FROM ' + common.escape_identifier(SCHEMA_NAME)
     query +=            '.' + common.escape_identifier(layername) + ' l '
     query +=      'WHERE l.ogc_fid = %s'
     query +=  ') m'
+    # The query never returns more than one row since we filter on a single FID.
+    # Don't try to be clever and batch queries in an IN set or ANY as we
+    # want to preserve the order in the response (so the feature(s)
+    # exactly under the cursor are returned first).
     return query.encode('utf-8')
 
 
@@ -99,26 +103,48 @@ def application(env, start_response) -> Iterator[bytes]:
     first = True
     try:
         body = json_load(env['wsgi.input'])
-        if not isinstance(body, dict):
+        if not isinstance(body, list):
             raise ValueError
 
         start_response(STATUS_OK, [CONTENT_TYPE_JSON])
+        if not body:
+            yield b'[]'
+            return
+
+        if len(body) > MAX_FEATURE_COUNT:
+            logging.warning('Query has too many feature requests (%d), truncating to %d',
+                            len(body), MAX_FEATURE_COUNT)
+            body = body[:MAX_FEATURE_COUNT]
+
         # pylint: disable-next=no-member
         with PG_CONN.cursor(binary=True, scrollable=False, withhold=False) as cur:
-            if not isinstance(body, dict):
-                raise ValueError
-            mvt = body.get('mvt', None)
-            layername = body.get('layer', None)
-            if not isinstance(mvt, str) or not isinstance(layername, str):
-                raise ValueError
-            query = QUERY_MAP[TABLE_MAP[(mvt, layername)]]
-            fid = body.get('fid', None)
-            if not isinstance(fid, int):
-                raise ValueError
-            cur.execute(query, params=(fid,))
-            resp = cur.fetchone()
-            if resp is not None:
+            for item in body:
+                if not isinstance(item, dict):
+                    raise ValueError
+                layer_group = item.get('layer_group', None)
+                layer = item.get('layer', None)
+                if not isinstance(layer_group, str) or not isinstance(layer, str):
+                    raise ValueError
+                query = QUERY_MAP[TABLE_MAP[(layer_group, layer)]]
+                fid = item.get('fid', None)
+                if not isinstance(fid, int):
+                    raise ValueError
+                cur.execute(query, params=(layer_group, layer, fid))
+                resp = cur.fetchone()
+                if resp is None:
+                    continue # no match for this tuple
+                if first:
+                    yield b'['
+                    first = False
+                else:
+                    yield b','
                 yield resp[0]
+                # the query never returns more than one row since we filter on a single FID
+        if first:
+            yield b'[]' # no match, empty response
+            first = False
+        else:
+            yield b']'
 
     except (JSONDecodeError, LookupError, UnicodeDecodeError, ValueError) as exc:
         logging.exception('Invalid request body')
@@ -170,8 +196,9 @@ PG_CONN.execute( # pylint: disable=no-member
     'SET statement_timeout TO 15000', # 15s
     prepare=False)
 
-del sys.modules['common']
+# drop functions and modules we don't need anymore
 del common
+del sys.modules['common']
 del get_query
 del get_table_map
 del os_path