zenmemes

New post

History rewrite record v1

A history purge rewrites every commit after the first one that touched a purged path, so each of those commits gets a new identity. This record keeps the old and new identities together so /source/blob/<commit>/... and /source/commits/<commit> links written before the purge keep resolving. It is written once per purge, committed with the purge, and never edited. It is instance-local: archives and imports neither export nor accept it.

Canonical path rule

  • records/history-rewrites/history-rewrite-<YYYYMMDD>T<HHMMSS>Z.txt, where the timestamp is Rewritten-At with punctuation removed.

  • A run with more than 100,000 mappings continues in records/history-rewrites/history-rewrite-<timestamp>-<part>.txt for parts 2 and up. All parts of one run share the header values except Part.

  • Two runs cannot share a second; the writer refuses to overwrite.

Headers

All headers are required exactly once, in this order. UTF-8, LF and a final newline are required; each file is at most 16 MiB.

  • Format: history-rewrite-v1

  • Rewritten-At: RFC 3339 UTC timestamp of the rewrite, matching the filename

  • Reason: purge

  • Receipt-Count: number of accepted delete receipts that drove the run, a non-negative integer without sign or leading zeros

  • Part: 1-based part number

  • Parts: total parts in the run; Part must not exceed it

Body

Zero or more mapping blocks, each separated and terminated by a blank line:

  • Old-Commit: the commit identity before the rewrite, 40 lowercase hex

  • New-Commit: the identity after the rewrite, 40 lowercase hex

Within one file Old-Commit values are strictly ascending, no two blocks share a New-Commit, and Old-Commit never equals New-Commit: commits whose identity did not change (everything before the first affected commit) are omitted. A file holds at most 100,000 blocks. Unknown or duplicate headers, header order changes, unknown block fields, uppercase hex, an identity mapping, a repeated Old-Commit or New-Commit, out-of-order blocks, or a filename that disagrees with Rewritten-At and Part are rejected.

Resolution

To resolve an identity, apply every record in ascending Rewritten-At then Part order: whenever the current identity appears as an Old-Commit, continue with its New-Commit. The result is the newest identity; an identity that no record names resolves to itself. Routes consult the map only after Git reports the commit unknown, so a live identity is never redirected.

Example

text
Format: history-rewrite-v1
Rewritten-At: 2026-11-01T10:15:00Z
Reason: purge
Receipt-Count: 2
Part: 1
Parts: 1

Old-Commit: 1111111111111111111111111111111111111111
New-Commit: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

Old-Commit: 2222222222222222222222222222222222222222
New-Commit: bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

Stored at records/history-rewrites/history-rewrite-20261101T101500Z.txt.

Existing records

There are none; this format is introduced with the first purge. A later format version adds a new Format value and a resolver that reads both.