TIKA-4809: Map CLIENT_UNAVAILABLE_WITHIN_MS to 429 instead of 503

Author: tballison

tika-server/tika-server-core/src/main/java/org/apache/tika/server/core/resource/PipesParsingHelper.java

@@ -282,8 +282,14 @@ public static Response.Status mapStatusToHttpResponse(PipesResult.RESULT_STATUS
                  EMIT_SUCCESS, EMIT_SUCCESS_PARSE_EXCEPTION, EMIT_SUCCESS_PASSBACK,
                  PARSE_EXCEPTION_NO_EMIT ->
                     Response.Status.OK;
-            case TIMEOUT, OOM, UNSPECIFIED_CRASH, CLIENT_UNAVAILABLE_WITHIN_MS ->
+            case TIMEOUT, OOM, UNSPECIFIED_CRASH ->
                     Response.Status.SERVICE_UNAVAILABLE;
+            // Distinct from the crash statuses above: nothing failed here, the client
+            // pool was simply at capacity for longer than maxWaitForClientMillis. 429
+            // lets monitoring/alerting on HTTP status alone tell "we're at capacity"
+            // (scale up numClients) apart from "a worker is actually crashing" (503).
+            case CLIENT_UNAVAILABLE_WITHIN_MS ->
+                    Response.Status.TOO_MANY_REQUESTS;
             case FETCH_EXCEPTION, EMIT_EXCEPTION,
                  FETCHER_NOT_FOUND, EMITTER_NOT_FOUND,
                  PAYLOAD_LIMIT_EXCEEDED,