fix: geojson invalid query on unavailable PostGIS

Closes https://github.com/PostgREST/postgrest/issues/4245.

It adds a query on schema cache construction, but doesn't add a new
attribute to the SchemaCache type. We only need the information to be
passed to the `initialMediaHandlers` function to build the builtin media
handlers.

src/PostgREST/SchemaCache.hs

@@ -152,6 +152,7 @@ querySchemaCache conf@AppConfig{..} = do
   reps    <- SQL.statement conf $ dataRepresentations prepared
   mHdlers <- SQL.statement conf $ mediaHandlers prepared
   tzones  <- SQL.statement mempty $ timezones prepared
+  hasPgis <- SQL.statement conf $ postgisFunc prepared
   _       <-
     let sleepCall = SQL.Statement "select pg_sleep($1 / 1000.0)" (param HE.int4) HD.noResult prepared in
     whenJust configInternalSCSleep (`SQL.statement` sleepCall) -- only used for testing
@@ -164,7 +165,7 @@ querySchemaCache conf@AppConfig{..} = do
     , dbRelationships = getOverrideRelationshipsMap rels cRels
     , dbRoutines = funcs
     , dbRepresentations = reps
-    , dbMediaHandlers = HM.union mHdlers initialMediaHandlers -- the custom handlers will override the initial ones
+    , dbMediaHandlers = HM.union mHdlers $ initialMediaHandlers hasPgis -- the custom handlers will override the initial ones
     , dbTimezones = tzones
     }
   where
@@ -1048,12 +1049,14 @@ allViewsKeyDependencies =
       having ncol = array_length(array_agg(row(col.attname, view_columns) order by pks_fks.ord), 1)
       |]
 
-initialMediaHandlers :: MediaHandlerMap
-initialMediaHandlers =
+initialMediaHandlers :: Bool -> MediaHandlerMap
+initialMediaHandlers hasPostgisFunc =
   HM.insert (RelAnyElement, MediaType.MTAny            ) (BuiltinOvAggJson,    MediaType.MTApplicationJSON) $
   HM.insert (RelAnyElement, MediaType.MTApplicationJSON) (BuiltinOvAggJson,    MediaType.MTApplicationJSON) $
   HM.insert (RelAnyElement, MediaType.MTTextCSV        ) (BuiltinOvAggCsv,     MediaType.MTTextCSV) $
-  HM.insert (RelAnyElement, MediaType.MTGeoJSON        ) (BuiltinOvAggGeoJson, MediaType.MTGeoJSON)
+  (if hasPostgisFunc
+    then HM.insert (RelAnyElement, MediaType.MTGeoJSON        ) (BuiltinOvAggGeoJson, MediaType.MTGeoJSON)
+    else mempty)
   HM.empty
 
 mediaHandlers :: Bool -> SQL.Statement AppConfig MediaHandlerMap
@@ -1139,6 +1142,35 @@ timezones = SQL.Statement sql HE.noParams decodeTimezones
     decodeTimezones :: HD.Result TimezoneNames
     decodeTimezones = S.fromList <$> HD.rowList (column HD.text)
 
+
+-- Find the postgis function that has the signature:
+-- st_asgeojson(record,...) returns text
+postgisFunc :: Bool -> SQL.Statement AppConfig Bool
+postgisFunc = SQL.Statement sql params decoder
+  where
+    params =
+      (map escapeIdent . toList . configDbSchemas >$< arrayParam HE.text) <>
+      (map escapeIdent . toList . configDbExtraSearchPath >$< arrayParam HE.text)
+    decoder = HD.singleRow (column HD.bool)
+    sql = encodeUtf8 [trimming|
+    SELECT
+      exists(
+        SELECT
+          1
+        FROM pg_catalog.pg_proc      AS p
+        JOIN pg_catalog.pg_depend    AS d
+          ON  d.objid   = p.oid
+          AND d.deptype = 'e'
+        JOIN pg_catalog.pg_extension AS e
+          ON e.oid   = d.refobjid
+        WHERE p.pronamespace   = ANY($$1::regnamespace[] || $$2::regnamespace[])
+          AND p.proname        = 'st_asgeojson'
+          AND e.extname        = 'postgis'
+          AND p.proargtypes[0] = 'record'::regtype
+          AND pg_get_function_result(p.oid) = 'text'
+      );
+    |]
+
 param :: HE.Value a -> HE.Params a
 param = HE.param . HE.nonNullable