rolling tape optimizations

[redsh4de]

Rolling tapes:

• Look up files by index, not by deque position. Reads, writer() and commits were O(n) in the number of files. They are now O(1).
• Commits visit only the files that go out of range, not every file.
• A truncate below the oldest file no longer creates every file in between. The tape creates each file when it writes to it.
• Old files past the new end stay on disk, but the tape no longer keeps them open. It reuses them when it grows back.
• Moving the start forward marks old files for deletion. A truncate that moves the start back now unmarks every file it needs again. Before, it unmarked only one, so the tape could delete files that held new data.
• read_bytes returns an error if a file in the range does not exist. Before, it returned Ok and left part of the buffer unchanged.
• Reserve the tape name metadata. A rolling tape with this name could delete the database metadata.

Other:

• Reject a fixed-sized tape whose start is not a multiple of the entry size.
• Cached tape: start the cache at start_index for a new tape. Before, the cache started at 0, which could cause a panic.
• Cached tape: release the cache lock before a disk read, so appends do not wait for the read.

Behaviour changes:

• A commit can delete the last file when the start moves past it. Before, the tape always kept the last file.
• The name metadata is rejected for every rolling tape, also when tape has its own directory.
• A fixed-sized tape whose stored start is not a multiple of the entry size does not open. Before, it opened, but its entries could not be read.

[Boog900]

Just a nit and a simplification

[Boog900]

This confused me, couldn't we just do first_file_touched: current_file_idx without all this?

[redsh4de]

This ensures that if the current file is missing, we sync the previous one as it might not have been synced yet

Made it easier to reason about

[Boog900]

Couldn't there be more than 1 previous file that we haven't synced yet? If we write 2 files but only buffer their writes then this will only sync one of the 2 right?

[Boog900]

I'll merge this anyway as it is harmless