chop: offline event log truncation

[matthew-levan]

chop

urbit chop <pier> implements a simple, offline event log truncation¹ tool.

chop gracefully stops the given pier (if running), backs up the current snapshot to <pier>/.urb/bhk, makes sure a current snapshot exists (i.e., is fully written to disk in chk/*.bin with no existing patch files), reads the metadata and the last event from the pier's event log, initializes a fresh event log in the <pier>/.urb/log/chop directory, writes the metadata and last event from the original log into the fresh one, renames the original event log to <pier>/.urb/log/chop/data_<first>_<last>.mdb.bak where first and last are the first and last event numbers from the event log, and exits.

Pilots are then free to move, archive, or delete their .bak event log file, resume normal operation of their ship, and enjoy the many benefits of lowered disk pressure and any reductions in associated hosting costs.

I've tested chop successfully on my own planet ~mastyr-bottec (multiple times), three different comets (all fresh), and multitudes of fake galaxies.

Resolves #122.

Note: knit, which is the "undo" button for chop, is being implemented in its own PR #184.

Footnotes

1. https://roadmap.urbit.org/project/event-log-truncation ↩

[matthew-levan]

This code has now been tested successfully on fake ships, two comets, and my own planet ~mastyr-bottec. Thanks to @mopfel-winrux for helping me clean the code up a little, but it still needs more polish.

Additionally, since chopping one's pier leaves a backup event log file behind (and pilots may indeed chop their piers multiple times throughout its life), we also had the idea to include an urbit knit subcommand in this PR that would accept a list of event log backup files and insert their events back into a single .mdb file of the user's choice (whether that's a new one or the existing data.mdb). This command would allow previously chopped event logs to be restored and replayed, if desired/necessary.

[belisarius222]

This looks good to me.

[mcevoypeter]

Thanks for churning this out so quickly. The overall work looks great, but I have a number of comments and questions on small details.

[joemfb]

This code is clear and makes sense. I've noted a few spots where we'll need more/better error handling:

[matthew-levan]

Here's an example output @joemfb:

matt@mbp14 vere % ./urbit chop zod         
loom: mapped 2048MB
boot: protected loom
live: loaded: MB/139.001.856
boot: installed 652 jets
loom: image backup complete
chop: event log truncation complete
      event log backup written to zod/.urb/log/chop/data_152-152.mdb.bak
      WARNING: ENSURE YOU CAN RESTART YOUR SHIP BEFORE DELETING YOUR EVENT LOG BACKUP FILE!
      if you can't, restore your log by running:
      `mv zod/.urb/log/chop/data_152-152.mdb.bak zod/.urb/log/data.mdb` then try again

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