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 "${format_value}"; update koh')54	}55	return value56}5758pub struct OfferReport {59pub mut:60	repo    string61	saves   int62	objects int63	events  int64	head    string65}6667// remember_repo binds a repo to its name on the remote. A steal into a new folder does it at68// once: its folder may be renamed before it ever offers.69pub fn remember_repo(store &storage.Store, name string) ! {70	store.set_state(repo_state_key, name)!71}7273// repo_name is the folder's name at first offer, lowercased so it makes the same74// Kepr URL whatever the folder's case (June, 2026-10-01), and remembered in the75// repo so renaming the folder later changes nothing.76pub fn repo_name(store &storage.Store, mut backend remote.Backend) !string {77	if remembered := store.state(repo_state_key) {78		return follow_rename(store, mut backend, remembered)!79	}80	name := os.base(store.root).to_lower()81	// Before the clash check below: a remote this koh cannot read is a "update82	// koh" answer, not a "rename the folder" one.83	check_format(mut backend, name)!84	existing := backend.get(head_key(name))!85	if existing.found {86		theirs := existing.body.bytestr().trim_space()87		if history.by_id(store, theirs) == none {88			return error('the remote already has "${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// follow_rename follows the marker Kepr leaves when it renames a repo, `.kepr/renamed/<old>`96// holding the new name (§30, 2026-10-01), for as long as the remembered name has no `head`, and97// remembers where it led. A few hops cover a repo renamed more than once.98fn follow_rename(store &storage.Store, mut backend remote.Backend, remembered string) !string {99	mut name := remembered100	for _ in 0 .. 5 {101		if backend.get(head_key(name))!.found {102			break103		}104		marker := backend.get('.kepr/renamed/${name}')!105		next := marker.body.bytestr().trim_space()106		if !marker.found || next == '' || next.contains('/') || next.contains('\\') || next in [107			'.',108			'..',109		] {110			break111		}112		name = next113	}114	if name != remembered {115		store.set_state(repo_state_key, name)!116	}117	return name118}119120// offer sends what the remote lacks and then swaps `head`. Nothing is ever121// queued: the next offer simply sends everything the remote does not have.122pub fn offer(store &storage.Store, mut backend remote.Backend) !OfferReport {123	name := repo_name(store, mut backend)!124	local := history.head(store) or { return error(no_saves) }125	mut report := OfferReport{126		repo: name127		head: local.id128	}129	if check_format(mut backend, name)! == '' {130		// Offered before this key existed: record the layout now.131		put_format(mut backend, name)!132	}133	remote_head := backend.get(head_key(name))!134	// Records offered by a machine that has not applied a strip would put the135	// stripped path back on the remote, where nothing would rewrite it again.136	if unapplied_strips(store, mut backend, name)!.len > 0 {137		return error(strip_not_applied)138	}139	mut uploaded := Uploaded{}140	if remote_head.found {141		theirs := remote_head.body.bytestr().trim_space()142		newer := saves_after(store, local.id, theirs) or {143			return error('remote has new saves — run koh steal')144		}145		for save in newer {146			upload_save(store, mut backend, name, save, mut report, mut uploaded)!147		}148	} else {149		oldest_first := history.line(store)!150		for index := oldest_first.len - 1; index >= 0; index-- {151			upload_save(store, mut backend, name, oldest_first[index], mut report, mut uploaded)!152		}153	}154	upload_events(store, mut backend, name, mut report)!155	// A strip sends its event before it lists the saves to rewrite, so one156	// missed here lists them after these records landed and rewrites them. One157	// seen here may have rewritten before they landed: take back what it reaches.158	late := unapplied_strips(store, mut backend, name)!159	if late.len > 0 {160		take_back(store, mut backend, name, uploaded, late)!161		return error(strip_not_applied)162	}163	if !backend.put_if_match(head_key(name), local.id.bytes(), remote_head.tag)! {164		return error('remote has new saves — run koh steal')165	}166	store.set_state(remote_head_key, local.id)!167	return report168}169170// saves_after lists the local saves newer than `theirs`, oldest first. It fails171// when `theirs` is not on this line, which is the honest answer: we cannot172// offer on top of a save we do not have.173fn saves_after(store &storage.Store, ours string, theirs string) ![]history.Save {174	mut newer := []history.Save{}175	mut cursor := ours176	for cursor != '' && cursor != theirs {177		save := history.by_id(store, cursor) or { return error('save ${cursor} is missing') }178		newer << save179		cursor = save.parent180	}181	if cursor != theirs {182		return error('${theirs} is not on this line')183	}184	newer.reverse_in_place()185	return newer186}187188const strip_not_applied = 'the remote has a strip this repo has not applied — run koh steal'189190// Uploaded is what one offer put on the remote, so a late strip can take it back.191struct Uploaded {192mut:193	saves   []string194	objects map[string]bool195}196197// take_back deletes the records an offer put, which head never reached, and198// every object it put that a strip reaches in one of them.199fn take_back(store &storage.Store, mut backend remote.Backend, repo string, uploaded Uploaded, strips []events.Event) ! {200	for id in uploaded.saves {201		for file in history.files_of(store, id)! {202			if file.hash in uploaded.objects && strips.any(reaches(it, id) && covers(it.path, file.path)) {203				backend.delete(object_key(repo, file.hash))!204			}205		}206		backend.delete(save_key(repo, id))!207	}208}209210// upload_save puts a save's new objects, then its record, so a record on the remote always finds its211// objects there. Both are put_if_absent, so a retry after a partial offer is safe; `uploaded` keeps212// what this call created, which is all take_back may delete.213fn upload_save(store &storage.Store, mut backend remote.Backend, repo string, save history.Save, mut report OfferReport, mut uploaded Uploaded) ! {214	files := history.files_of(store, save.id)!215	// Saves go up oldest first and head moves only once all of them landed, so216	// the parent and its objects are on the remote already: only content new217	// in this save travels, not a round trip per file in the repo.218	mut sent := map[string]bool{}219	if save.parent != '' {220		for file in history.files_of(store, save.parent)! {221			sent[file.hash] = true222		}223	}224	for file in files {225		if file.hash in sent {226			continue227		}228		sent[file.hash] = true229		if backend.put_if_absent(object_key(repo, file.hash), store.object_blob(file.hash)!)! {230			report.objects++231			uploaded.objects[file.hash] = true232		}233	}234	record := save_record(store, save)!235	if backend.put_if_absent(save_key(repo, save.id), encode_save(record).bytes())! {236		report.saves++237		uploaded.saves << save.id238	}239}240241// upload_events sends every event the remote does not have. Events are tiny, so242// listing them costs less than tracking which ones were sent.243fn upload_events(store &storage.Store, mut backend remote.Backend, repo string, mut report OfferReport) ! {244	already := backend.list(events_prefix(repo))!245	mut already_sent := map[string]bool{}246	for key in already {247		already_sent[os.base(key)] = true248	}249	for event in events.all(store)! {250		if event.id in already_sent {251			continue252		}253		if backend.put_if_absent(event_key(repo, event.id), encode_event(event).bytes())! {254			report.events++255		}256	}257}258259// fetched holds everything a steal read from the remote, before any of it is260// written locally. Network work happens first; the database gets one261// transaction, so a failure leaves nothing half-applied.262pub struct Fetched {263pub mut:264	repo_objects map[string][]u8265	saves        []SaveRecord // oldest first266	events_out   []events.Event267	head         string268	base         string // the newest save both sides already shared, '' when none269	unsafe_paths []string270}271272// fetch_back_to_known walks from the remote head along parents until it meets a save on this line273// (`base`), reading only the objects this repo lacks. Nothing is written here: write_fetched lands it.274// A save held locally but off the line, from an earlier steal, is taken from the local copy.275pub fn fetch_back_to_known(store &storage.Store, mut backend remote.Backend, repo string) !Fetched {276	check_format(mut backend, repo)!277	remote_head := backend.get(head_key(repo))!278	if !remote_head.found {279		return error('the remote has no "${repo}" yet — run koh offer here first')280	}281	mut fetched := Fetched{282		head: remote_head.body.bytestr().trim_space()283	}284	mut on_line := map[string]bool{}285	for save in history.line(store)! {286		on_line[save.id] = true287	}288	mut cursor := fetched.head289	for cursor != '' && cursor !in on_line {290		mut record := if saved := history.by_id(store, cursor) {291			save_record(store, saved)!292		} else {293			record_object := backend.get(save_key(repo, cursor))!294			if !record_object.found {295				return error('the remote is missing save ${cursor} — run koh offer there')296			}297			decode_save(record_object.body)!298		}299		fetched.unsafe_paths << drop_unsafe_paths(mut record)300		for file in record.files {301			if !store.has_object(file.hash)! && file.hash !in fetched.repo_objects {302				blob := backend.get(object_key(repo, file.hash))!303				if !blob.found {304					return error('the remote is missing object ${file.hash} that save ${record.id} needs')305				}306				fetched.repo_objects[file.hash] = blob.body307			}308		}309		fetched.saves << record310		cursor = record.parent311	}312	fetched.saves.reverse_in_place()313	fetched.base = cursor314	fetched.events_out = fetch_events(store, mut backend, repo)!315	return fetched316}317318fn fetch_events(store &storage.Store, mut backend remote.Backend, repo string) ![]events.Event {319	have := events.all(store)!320	mut known := map[string]bool{}321	for event in have {322		known[event.id] = true323	}324	mut incoming := []events.Event{}325	for key in backend.list(events_prefix(repo))! {326		id := os.base(key)327		if id in known {328			continue329		}330		body := backend.get(event_key(repo, id))!331		if !body.found {332			continue333		}334		event := decode_event(body.body)!335		if !ignored_strip(event) {336			incoming << event337		}338	}339	return incoming340}341342// write_fetched lands a fetch in one transaction: objects first (verified),343// then the events, then the saves they belong to. A strip bans its path from344// all of history, so it reaches every save, here or arriving, whenever made.345pub fn write_fetched(store &storage.Store, fetched Fetched) ! {346	for hash, blob in fetched.repo_objects {347		store.put_object_blob(hash, blob)!348	}349	mut stripped := false350	for event in fetched.events_out {351		if !events.receive(store, event)! {352			continue353		}354		if event.kind == events.kind_strip {355			remove_path(store, event.path, event.save_id)!356			stripped = true357		}358	}359	strips := known_strips(store, fetched)!360	for record in fetched.saves {361		import_arriving(store, record, strips)!362	}363	if stripped {364		store.drop_unreferenced_objects()!365	}366}367368// write_fetched_records makes network work durable without changing the line.369// Events wait for the combine transaction because a strip can rewrite history.370pub fn write_fetched_records(store &storage.Store, fetched Fetched) ! {371	for hash, blob in fetched.repo_objects {372		store.put_object_blob(hash, blob)!373	}374	strips := known_strips(store, fetched)!375	for record in fetched.saves {376		import_arriving(store, record, strips)!377	}378}379380// known_strips is every strip held here and every one the fetch brings: an381// arriving save loses what any of them reaches, not only what arrived with it.382fn known_strips(store &storage.Store, fetched Fetched) ![]events.Event {383	mut strips := events.strip_events(store)!384	strips << fetched.events_out.filter(it.kind == events.kind_strip)385	return strips386}387388// import_arriving lands one save from the remote, unless it is already here, less every path a389// strip that `reaches` it removed.390fn import_arriving(store &storage.Store, record SaveRecord, strips []events.Event) ! {391	if history.by_id(store, record.id) != none {392		return393	}394	files := to_files(record).filter(fn [strips, record] (file snapshot.FileEntry) bool {395		return snapshot.legal_path(file.path)396			&& !strips.any(reaches(it, record.id) && covers(it.path, file.path))397	})398	history.import_save(store, to_save(record), files)!399}400401// reaches reports whether a strip event applies to a save: its one save, or402// every save there is, whenever it was made.403fn reaches(strip events.Event, save_id string) bool {404	return strip.save_id == '' || strip.save_id == save_id405}