clintropolis opened a new pull request, #20105:
URL: https://github.com/apache/druid/pull/20105

   ### Description
   This PR fixes partial segment metadata handling to ensure consistent 
metadata header file sizes and guards to ensure that files are complete. First, 
partial entry mounting now writes atomically the metadata header and the bitmap 
that tracks which files have been loaded (instead of writing the metadata and 
then appending the bitmap).
   
   While here, I have simplified partial segment bootstrapping to use the same 
flow that on-demand loading uses, instead of handling it separately, which 
means that bootstrapping a partial entry reserves at the same time as mounting 
instead of reserving everything up front. The switch to using the shared path 
means that in hopefully rare edge cases bootstrap could unintentionally evict 
segments that were already present in cache (though never anything that is held 
under a partial load rule); if this is a problem in practice we could switch to 
a solution that uses the files on disk to use as the estimate instead of the 
config estimate (16 MiB default which is waaaay bigger than most metadatas i've 
seen so far which are usually 1-10kb).
   
   


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