Deadlock in multi-threaded use of warped VRT
|Reported by:||Even Rouault||Owned by:||Even Rouault|
Was seen with mapcache, with a GDAL WMS source (with overviews). The issue is that the warped VRT can do writes of block for the alpha band, which is formally a situation of several threads doing writes. The logic added in #6163 to prevent such situations doesn't work well (which shows that the design of the global block cache with multi-threaded write situations is probably a dead end). In the particular case of a warped VRT, we don't actually need the mutex, so let's add a hack to avoid the deadlock...