Mead.Session.Record (mead v0.1.1)

The record framing shared by the session files and the ledger journal, and the few file operations both need.

A record is [4-byte length][4-byte CRC32 of the length][4-byte CRC32 of the payload][term_to_binary payload]. scan/1 reads a whole file: a last record that is cut short or fails its payload checksum is what a crash during a write leaves, so it is a :torn tail and the valid size stops before it; the length has a checksum of its own, so damage anywhere else, or a payload that does not decode, is {:error, :corrupt}.

Payloads are decoded without :safe, for the reasons given in Mead.Session.Store.

Summary

Functions

create_private(path)

Creates an empty file readable only by its owner, unless the path exists, so the content is never readable by anyone else, not even between a write and a later chmod.

encode(term)

Frames term as one record.

remove_leftover(path)

Removes a file; one that is already missing is not an error.

scan(binary)

Reads every record of binary and returns the terms, the size of the valid prefix, and whether the file ended cleanly or in a torn tail.

sync_directory(directory)

Syncs the directory so a rename is durable. Not every platform lets Erlang open a directory, so a refusal is not a failure: the rename has already happened.

truncate(path, size)

Cuts the file at path back to size bytes.

write_synced(path, binary)

Replaces the content of path with binary and syncs it.