Add reader and manifest abstractions for osiris_log
#194
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Note - dialyzer is unhappy with this being opaque :/
We can use this to calculate the CRC32 for remote-tier integrity checks.
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TODO - fully replace the seg_info record with the map?
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This adds two new abstractions into osiris_log which provide hooks necessary for #184.
The simpler one is
osiris_log_reader:open/1,pread/3,sendfile/4andclose/1. The default implementation reads directly from the passed segment file with the functions fromfile. A tiered storage reader would use the default implementation inosiris_log_readerfor local files and otherwise read from the remote tier.Then there is
osiris_log_manifestwhich abstracts over whatever data structure is used to find positions within the log for readers. The default implementation is inosiris_log(in order to minimize the diff of this change) and it lists and sorts segment and index files from the stream's directory. While the manifest concept is meant to enable finding locations in the remote tier, this abstraction could also be used to swap out the current directory listing approach to do something like consult a file instead.The manifest behaviour also has a callback for handling events: currently a segment file being rolled and a chunk being written. The manifest can use these events as hooks to update the backing data structure. This part isn't used by the default implementation.