What is it / possible goals

get (early) feedback to changes
knowledge sharing - inform others about changes
catch bugs and issues before landing changes in mainline
let area owners maintain ownership while changed by others
collect approvals (human and automatic) and enforce/support change approval workflow
coordinate and automate landing of changes
clean history while landing - make/ensure it is bisectable

Different aspects / phases of the review workflow and what they can contain

We identify some main areas where there can be different solutions:

how the changes are submitted:

how the changes are shown to reviewers:

diff - unified or side by side, plain text or diff markup or syntax markup or hyperlinked / type information
show individual changesets
aggregated diff since ancestor
merge diff (warning about conflicts)
show meta diff between iterations
static or moving target
web / mail / local native client
- standard for review system client plugins?

how reviewers can communicate

forum style: web click'n'comment, mail inline replies, local native client comments
1 by 1 or batched comments
chat style
constructive contribution, show don't tell, make changes instead of asking for them (using existing process of modifying history or adding changesets, thus iterating)

how reviewers can control

some approval workflow can be specified and be more or less mandatory/enforced
possibly formalized ownership of touched files, suggested or added/informed automatically
different kind of reviews and approvals - architectural, ownership, catch bugs, full or partial, FYI/ACK, stylistic review, product management, release management, CI
may or may not be invalidated by iteration

actual landing of the changes

how changes might change during the process

extra commits with changes
history modification
HOW TO DEAL WITH HOW ALL THAT CAN INFLUENCE COMMENTS - where and whether they apply?
- Heuristics are tough to make accurate
  - Fuzzy match diff context around comment?
- some sort of annotation?
- tracking built in to vcs?
might invalidate reviews / approvals

How different tools and implementations relate to this nomenclature / frame of reference:

Submit: mainly patchbombs, automated with push gateway
Show: patch emails to mailing list, also snarfed by patchwork
Comment: inline replies on mailing list
Approve: trusted people collect patches from mailing list, read informal responses, reject bad parts, might tweak, approve good parts by pushing to pre-landing repo
Land: pre-landed changesets are (currently) re-reviewed, rebased, tweaked and pulled upstream by mpm
Iterate: rejected parts must be "history modified" and re-sent to the mailing list tagged v(n+1)

Submit: repo hosting, fork or shared, PR on live diff
Show: web with unified diff (partly side-by-side), mail listing commit descriptions, notifications on iterations
Comment: web inline comments, notifications
Approve: reviewers are assigned manually - no automatic landing and thus no enforcement
Land: manual offline, shows hint about what to pull and merge
Iterate: static view until iterating (closes old PR and creates new one with additional changes - no good support for modifications)

Tweaked implementation of Kallithea

Submit: shared repo with named branches
Approve: automatic recommentation/review of assigned revieweres based on touched files plus release management plus "auto land request", CI status shows up too
Land: "trunk guardians" are gate keepers and make final meta review, prepare batch merging all possible PRs to verification branch, runs all tests, bisect failures, merge good parts to mainline, bounce PRs causing conflicts or failures
Iterate: append-only history - not clean, only bisectable on mainline, new iteration pretty much invalidates previous approvals ... but negotiable with guardians