Http connection not release on 504 error

[tltk90]

It seems that if an 504 error appears in the request to the wms server,
The connection pool does not release the current connection,
And so after 2 responses 504 error from the server,
The connection pool fails to lease a new connection
and the app will be stuck forever,
only restart to gwc webapps will help.

If I add the org.apache.http.impl.conn.PoolingHttpClientConnectionManager with level all to the logger config that is the log I get:

12:10:34 DEBUG  [conn.PoolingHttpClientConnectionManager] - Connection request: [route: {}->http://localhost:8888][total available: 0; route allocated: 0 of 2; total allocated: 0 of 20]
12:10:34 DEBUG  [conn.PoolingHttpClientConnectionManager] - Connection leased: [id: 0][route: {}->http://localhost:8888][total available: 0; route allocated: 1 of 2; total allocated: 1 of 20]
12:10:46 DEBUG  [conn.PoolingHttpClientConnectionManager] - Connection [id: 0][route: {}->http://localhost:8888] can be kept alive for 5.0 seconds
12:10:46 DEBUG  [conn.PoolingHttpClientConnectionManager] - Connection released: [id: 0][route: {}->http://localhost:8888][total available: 1; route allocated: 1 of 2; total allocated: 1 of 20]
12:10:46 DEBUG  [conn.PoolingHttpClientConnectionManager] - Connection request: [route: {}->http://localhost:8888][total available: 1; route allocated: 1 of 2; total allocated: 1 of 20]
12:10:46 DEBUG  [conn.PoolingHttpClientConnectionManager] - Connection leased: [id: 0][route: {}->http://localhost:8888][total available: 0; route allocated: 1 of 2; total allocated: 1 of 20]
12:10:53 WARN   [seed.SeedTask] - Seed failed at ConveyorTile[topp:states EPSG:4326 {0,0,2} image/png] after 1 of 1 attempts. Skipping and continuing with next tile. Total failure count across threads is at: 1
org.geowebcache.GeoWebCacheException: All backends (1) failed. Reason: Unexpected response code from backend: 504 for http://localhost:8888/wms?.  Last request: 'http://localhost:8888/wms?'.
        at org.geowebcache.layer.wms.WMSHttpHelper.makeRequest(WMSHttpHelper.java:154)
        at org.geowebcache.layer.wms.WMSSourceHelper.makeRequest(WMSSourceHelper.java:53)
        at org.geowebcache.layer.wms.WMSLayer.getMetatilingReponse(WMSLayer.java:371)
        at org.geowebcache.layer.wms.WMSLayer.seedTile(WMSLayer.java:303)
        at org.geowebcache.seed.SeedTask.doActionInternal(SeedTask.java:141)
        at org.geowebcache.seed.GWCTask.doAction(GWCTask.java:78)
        at org.geowebcache.seed.MTSeeder.call(MTSeeder.java:35)
        at org.geowebcache.seed.MTSeeder.call(MTSeeder.java:23)
        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
        at java.base/java.lang.Thread.run(Thread.java:835)
12:10:53 DEBUG  [conn.PoolingHttpClientConnectionManager] - Connection request: [route: {}->http://localhost:8888][total available: 0; route allocated: 1 of 2; total allocated: 1 of 20]
12:10:53 DEBUG  [conn.PoolingHttpClientConnectionManager] - Connection leased: [id: 1][route: {}->http://localhost:8888][total available: 0; route allocated: 2 of 2; total allocated: 2 of 20]
12:11:07 WARN   [seed.SeedTask] - Seed failed at ConveyorTile[topp:states EPSG:4326 {0,3,2} image/png] after 1 of 1 attempts. Skipping and continuing with next tile. Total failure count across threads is at: 2
org.geowebcache.GeoWebCacheException: All backends (1) failed. Reason: Unexpected response code from backend: 504 for http://localhost:8888/wms?.  Last request: 'http://localhost:8888/wms?'.
        at org.geowebcache.layer.wms.WMSHttpHelper.makeRequest(WMSHttpHelper.java:154)
        at org.geowebcache.layer.wms.WMSSourceHelper.makeRequest(WMSSourceHelper.java:53)
        at org.geowebcache.layer.wms.WMSLayer.getMetatilingReponse(WMSLayer.java:371)
        at org.geowebcache.layer.wms.WMSLayer.seedTile(WMSLayer.java:303)
        at org.geowebcache.seed.SeedTask.doActionInternal(SeedTask.java:141)
        at org.geowebcache.seed.GWCTask.doAction(GWCTask.java:78)
        at org.geowebcache.seed.MTSeeder.call(MTSeeder.java:35)
        at org.geowebcache.seed.MTSeeder.call(MTSeeder.java:23)
        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
        at java.base/java.lang.Thread.run(Thread.java:835)
12:11:07 DEBUG  [conn.PoolingHttpClientConnectionManager] - Connection request: [route: {}->http://localhost:8888][total available: 0; route allocated: 2 of 2; total allocated: 2 of 20]