exchange.v

1//! Sync: offer and steal.2//!3//! All the cleverness sits here, above the five-operation remote interface, so4//! the backends stay thin and this logic is written once and tested offline.5//!6//! The remote is a line of saves, the same shape as the local one. `offer`7//! sends local saves the remote lacks; `steal` takes remote saves and, when8//! both sides moved, replays the local work on top of the remote's. Divergence9//! is resolved per file, never per line.1011module exchange1213import os14import src.events15import src.history16import src.remote17import src.snapshot18import src.storage1920// Where the repo's name on the remote is remembered.21const repo_state_key = 'remote_repo'2223// The remote head as of this machine's last successful offer: a record for the24// person or a future feature. Nothing depends on it.25const remote_head_key = 'remote_head'2627pub const no_saves = 'this repo has no saves yet — nothing to offer'2829// The only remote layout this koh reads or writes. A repo offered before the30// key existed has no `format` key and counts as this version.31const format_value = 'koh-remote 1'3233// put_format records the layout on a repo that has none. put_if_absent never34// overwrites, so a machine that wrote one while this one read wins the race:35// its value is read back and checked, never assumed.36fn put_format(mut backend remote.Backend, repo string) ! {37	if backend.put_if_absent(format_key(repo), format_value.bytes())! {38		return39	}40	check_format(mut backend, repo)!41}4243// check_format reads <repo>/format and refuses a remote written by a koh that44// speaks another layout, so two versions never half-understand one bucket. An45// empty answer means the key is missing: version 1, which `offer` records.46fn check_format(mut backend remote.Backend, repo string) !string {47	existing := backend.get(format_key(repo))!48	if !existing.found {49		return ''50	}51	value := existing.body.bytestr().trim_space()52	if value != format_value {53		return error('the remote "${repo}" is "${value}" — this koh speaks "${format_value}"; update koh')54	}55	return value56}5758pub struct OfferReport {59pub mut:60	repo    string61	saves   int62	objects int63	events  int64	head    string65}6667// remember_repo binds a repo to its name on the remote. A steal into a new folder does it at68// once: its folder may be renamed before it ever offers.69pub fn remember_repo(store &storage.Store, name string) ! {70	store.set_state(repo_state_key, name)!71}7273// repo_name is the folder's name at first offer, lowercased so it makes the same74// Kepr URL whatever the folder's case (June, 2026-10-01), and remembered in the75// repo so renaming the folder later changes nothing.76pub fn repo_name(store &storage.Store, mut backend remote.Backend) !string {77	if remembered := store.state(repo_state_key) {78		return remembered79	}80	name := os.base(store.root).to_lower()81	// Before the clash check below: a remote this koh cannot read is a "update82	// koh" answer, not a "rename the folder" one.83	check_format(mut backend, name)!84	existing := backend.get(head_key(name))!85	if existing.found {86		theirs := existing.body.bytestr().trim_space()87		if history.by_id(store, theirs) == none {88			return error('the remote already has "${name}" with a history this repo does not share — rename the folder, or use another remote')89		}90	}91	store.set_state(repo_state_key, name)!92	return name93}9495// offer sends what the remote lacks and then swaps `head`. Nothing is ever96// queued: the next offer simply sends everything the remote does not have.97pub fn offer(store &storage.Store, mut backend remote.Backend) !OfferReport {98	name := repo_name(store, mut backend)!99	local := history.head(store) or { return error(no_saves) }100	mut report := OfferReport{101		repo: name102		head: local.id103	}104	if check_format(mut backend, name)! == '' {105		// Offered before this key existed: record the layout now.106		put_format(mut backend, name)!107	}108	remote_head := backend.get(head_key(name))!109	// Records offered by a machine that has not applied a strip would put the110	// stripped path back on the remote, where nothing would rewrite it again.111	if unapplied_strips(store, mut backend, name)!.len > 0 {112		return error(strip_not_applied)113	}114	mut uploaded := Uploaded{}115	if remote_head.found {116		theirs := remote_head.body.bytestr().trim_space()117		newer := saves_after(store, local.id, theirs) or {118			return error('remote has new saves — run koh steal')119		}120		for save in newer {121			upload_save(store, mut backend, name, save, mut report, mut uploaded)!122		}123	} else {124		oldest_first := history.line(store)!125		for index := oldest_first.len - 1; index >= 0; index-- {126			upload_save(store, mut backend, name, oldest_first[index], mut report, mut uploaded)!127		}128	}129	upload_events(store, mut backend, name, mut report)!130	// A strip sends its event before it lists the saves to rewrite, so one131	// missed here lists them after these records landed and rewrites them. One132	// seen here may have rewritten before they landed: take back what it reaches.133	late := unapplied_strips(store, mut backend, name)!134	if late.len > 0 {135		take_back(store, mut backend, name, uploaded, late)!136		return error(strip_not_applied)137	}138	if !backend.put_if_match(head_key(name), local.id.bytes(), remote_head.tag)! {139		return error('remote has new saves — run koh steal')140	}141	store.set_state(remote_head_key, local.id)!142	return report143}144145// saves_after lists the local saves newer than `theirs`, oldest first. It fails146// when `theirs` is not on this line, which is the honest answer: we cannot147// offer on top of a save we do not have.148fn saves_after(store &storage.Store, ours string, theirs string) ![]history.Save {149	mut newer := []history.Save{}150	mut cursor := ours151	for cursor != '' && cursor != theirs {152		save := history.by_id(store, cursor) or { return error('save ${cursor} is missing') }153		newer << save154		cursor = save.parent155	}156	if cursor != theirs {157		return error('${theirs} is not on this line')158	}159	newer.reverse_in_place()160	return newer161}162163const strip_not_applied = 'the remote has a strip this repo has not applied — run koh steal'164165// Uploaded is what one offer put on the remote, so a late strip can take it back.166struct Uploaded {167mut:168	saves   []string169	objects map[string]bool170}171172// take_back deletes the records an offer put, which head never reached, and173// every object it put that a strip reaches in one of them.174fn take_back(store &storage.Store, mut backend remote.Backend, repo string, uploaded Uploaded, strips []events.Event) ! {175	for id in uploaded.saves {176		for file in history.files_of(store, id)! {177			if file.hash in uploaded.objects && strips.any(reaches(it, id) && covers(it.path, file.path)) {178				backend.delete(object_key(repo, file.hash))!179			}180		}181		backend.delete(save_key(repo, id))!182	}183}184185// upload_save puts a save's new objects, then its record, so a record on the remote always finds its186// objects there. Both are put_if_absent, so a retry after a partial offer is safe; `uploaded` keeps187// what this call created, which is all take_back may delete.188fn upload_save(store &storage.Store, mut backend remote.Backend, repo string, save history.Save, mut report OfferReport, mut uploaded Uploaded) ! {189	files := history.files_of(store, save.id)!190	// Saves go up oldest first and head moves only once all of them landed, so191	// the parent and its objects are on the remote already: only content new192	// in this save travels, not a round trip per file in the repo.193	mut sent := map[string]bool{}194	if save.parent != '' {195		for file in history.files_of(store, save.parent)! {196			sent[file.hash] = true197		}198	}199	for file in files {200		if file.hash in sent {201			continue202		}203		sent[file.hash] = true204		if backend.put_if_absent(object_key(repo, file.hash), store.object_blob(file.hash)!)! {205			report.objects++206			uploaded.objects[file.hash] = true207		}208	}209	record := save_record(store, save)!210	if backend.put_if_absent(save_key(repo, save.id), encode_save(record).bytes())! {211		report.saves++212		uploaded.saves << save.id213	}214}215216// upload_events sends every event the remote does not have. Events are tiny, so217// listing them costs less than tracking which ones were sent.218fn upload_events(store &storage.Store, mut backend remote.Backend, repo string, mut report OfferReport) ! {219	already := backend.list(events_prefix(repo))!220	mut already_sent := map[string]bool{}221	for key in already {222		already_sent[os.base(key)] = true223	}224	for event in events.all(store)! {225		if event.id in already_sent {226			continue227		}228		if backend.put_if_absent(event_key(repo, event.id), encode_event(event).bytes())! {229			report.events++230		}231	}232}233234// fetched holds everything a steal read from the remote, before any of it is235// written locally. Network work happens first; the database gets one236// transaction, so a failure leaves nothing half-applied.237pub struct Fetched {238pub mut:239	repo_objects map[string][]u8240	saves        []SaveRecord // oldest first241	events_out   []events.Event242	head         string243	base         string // the newest save both sides already shared, '' when none244	unsafe_paths []string245}246247// fetch_back_to_known walks from the remote head along parents until it meets a save on this line248// (`base`), reading only the objects this repo lacks. Nothing is written here: write_fetched lands it.249// A save held locally but off the line, from an earlier steal, is taken from the local copy.250pub fn fetch_back_to_known(store &storage.Store, mut backend remote.Backend, repo string) !Fetched {251	check_format(mut backend, repo)!252	remote_head := backend.get(head_key(repo))!253	if !remote_head.found {254		return error('the remote has no "${repo}" yet — run koh offer here first')255	}256	mut fetched := Fetched{257		head: remote_head.body.bytestr().trim_space()258	}259	mut on_line := map[string]bool{}260	for save in history.line(store)! {261		on_line[save.id] = true262	}263	mut cursor := fetched.head264	for cursor != '' && cursor !in on_line {265		mut record := if saved := history.by_id(store, cursor) {266			save_record(store, saved)!267		} else {268			record_object := backend.get(save_key(repo, cursor))!269			if !record_object.found {270				return error('the remote is missing save ${cursor} — run koh offer there')271			}272			decode_save(record_object.body)!273		}274		fetched.unsafe_paths << drop_unsafe_paths(mut record)275		for file in record.files {276			if !store.has_object(file.hash)! && file.hash !in fetched.repo_objects {277				blob := backend.get(object_key(repo, file.hash))!278				if !blob.found {279					return error('the remote is missing object ${file.hash} that save ${record.id} needs')280				}281				fetched.repo_objects[file.hash] = blob.body282			}283		}284		fetched.saves << record285		cursor = record.parent286	}287	fetched.saves.reverse_in_place()288	fetched.base = cursor289	fetched.events_out = fetch_events(store, mut backend, repo)!290	return fetched291}292293fn fetch_events(store &storage.Store, mut backend remote.Backend, repo string) ![]events.Event {294	have := events.all(store)!295	mut known := map[string]bool{}296	for event in have {297		known[event.id] = true298	}299	mut incoming := []events.Event{}300	for key in backend.list(events_prefix(repo))! {301		id := os.base(key)302		if id in known {303			continue304		}305		body := backend.get(event_key(repo, id))!306		if !body.found {307			continue308		}309		event := decode_event(body.body)!310		if !ignored_strip(event) {311			incoming << event312		}313	}314	return incoming315}316317// write_fetched lands a fetch in one transaction: objects first (verified),318// then the events, then the saves they belong to. A strip bans its path from319// all of history, so it reaches every save, here or arriving, whenever made.320pub fn write_fetched(store &storage.Store, fetched Fetched) ! {321	for hash, blob in fetched.repo_objects {322		store.put_object_blob(hash, blob)!323	}324	mut stripped := false325	for event in fetched.events_out {326		if !events.receive(store, event)! {327			continue328		}329		if event.kind == events.kind_strip {330			remove_path(store, event.path, event.save_id)!331			stripped = true332		}333	}334	strips := known_strips(store, fetched)!335	for record in fetched.saves {336		import_arriving(store, record, strips)!337	}338	if stripped {339		store.drop_unreferenced_objects()!340	}341}342343// write_fetched_records makes network work durable without changing the line.344// Events wait for the combine transaction because a strip can rewrite history.345pub fn write_fetched_records(store &storage.Store, fetched Fetched) ! {346	for hash, blob in fetched.repo_objects {347		store.put_object_blob(hash, blob)!348	}349	strips := known_strips(store, fetched)!350	for record in fetched.saves {351		import_arriving(store, record, strips)!352	}353}354355// known_strips is every strip held here and every one the fetch brings: an356// arriving save loses what any of them reaches, not only what arrived with it.357fn known_strips(store &storage.Store, fetched Fetched) ![]events.Event {358	mut strips := events.strip_events(store)!359	strips << fetched.events_out.filter(it.kind == events.kind_strip)360	return strips361}362363// import_arriving lands one save from the remote, unless it is already here, less every path a364// strip that `reaches` it removed.365fn import_arriving(store &storage.Store, record SaveRecord, strips []events.Event) ! {366	if history.by_id(store, record.id) != none {367		return368	}369	files := to_files(record).filter(fn [strips, record] (file snapshot.FileEntry) bool {370		return snapshot.legal_path(file.path)371			&& !strips.any(reaches(it, record.id) && covers(it.path, file.path))372	})373	history.import_save(store, to_save(record), files)!374}375376// reaches reports whether a strip event applies to a save: its one save, or377// every save there is, whenever it was made.378fn reaches(strip events.Event, save_id string) bool {379	return strip.save_id == '' || strip.save_id == save_id380}