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docs/site/content/docs/concepts/horizons.md
— part of check-in
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2026-10-24 00:02:06
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— feat: clone a horizon by name and depth
CloneOpts.HorizonName and HorizonDepth, and repo clone --horizon NAME --depth N, ask the origin for the horizon by name; CloneResult reports the resolved check-in, the steps taken and whether the clone resolved it locally. A full hash with no depth is still sent as the hash itself. Docs and the generated CLI reference and conformance data follow. (user: [email protected], size: 15734) [annotate] [blame] [check-ins using]
--- title: Horizons weight: 70 --- A horizon cuts a store by time. It names a check-in **C**, a rule, and a depth. The check-ins the rule keeps are the **held set**. A blob is elided when no check-in in the held set names it. Structure (manifests, tags, control artifacts) is never elided by a content horizon. A horizon is a member of a store cone, so everything on [Sparse cones](/docs/concepts/sparse-cones/) about store cones, elided artifacts and compaction applies to it too. The behaviour follows `draft-fossil-horizon-00`, implemented in step with zit. The held set is computed from the check-in graph the repository holds. It is stored nowhere (§3.1). ## Rules - **descendants** (the default): C and every check-in descended from it, through any parent, merges included. - **leaves**: the descendants, plus, for every childless check-in that does not descend from C, every descendant of each check-in where its ancestry meets C's. A check-in is childless when no other check-in names it as a parent, open or closed. A branch tip merged into another branch has a child, so it is not childless. Under the leaves rule, any two childless check-ins have their merge base in the held set, so merging branches that are still being extended never crosses the horizon. Both rules give a set that includes every child of every check-in in it. The leaves rule has a cost you should check before using it. An old branch that was never merged forks from C's ancestry near the root, and the rule then holds every descendant of that fork. On fossil-scm.org's repository it held 20,907 of 21,008 check-ins at every depth tried. The descendants rule, with C 10, 100, 426 and 1000 check-ins back from the tip, saves 97, 86, 56 and 41 percent of that history (§3.4). Use the leaves rule when the repository keeps few old unmerged branches. `repo cone cost <check-in>` (`Repo.HorizonCost`) reports what each rule would hold at a check-in, read from a repository that has the history: ``` $ libfossil -R repo.fossil repo cone cost trunk horizon at trunk descendants 1 check-ins, 3 blobs, 6 B expanded, 57 B stored leaves 1 check-ins, 3 blobs, 6 B expanded, 57 B stored ``` ## Depth - **content** (the default): which blobs are held. Every check-in's structure is still held (§4). - **structural** (`Horizon.Structural`, `--structural`): also cuts the manifests below C (§5). It needs a seed, a control artifact the origin mints so a clone can derive tag and branch state without C's ancestry. A content horizon keeps all structure and elides only blobs; a structural horizon also leaves out the manifests below C. ## Signed seeds An origin with the `clearsign` setting on (and the legacy `omitsign` off) signs each seed it mints with its `pgp-command`, the same settings fossil and zit sign check-ins with (§5.2.4). It keeps one signature per seed, so a seed keeps its name while the tag state it records is unchanged. A clone given the origin's public key verifies every seed against it: at clone time, on every pull, and when it rebuilds. A seed that is unsigned, or signed by another key, is discarded and the clone falls back to the content horizon. A clone with no key accepts an unsigned seed. The key is the clone's own `horizon-seed-key` setting, never something the origin sends. `CloneOpts.SeedKey` and `repo clone --seed-key` set it when the clone is made; `Repo.SetConfig` refuses a value that is neither an SSH public key nor an OpenPGP fingerprint. - **SSH:** the origin signs with `ssh-keygen -q -Y sign -n fossilscm -f KEY`; the clone's key is the public key line, verified in process. - **PGP:** the origin signs with `gpg --clearsign -o`, fossil's default; the clone's key is the key's 40 or 64 digit fingerprint, verified by `gpg` against the user's keyring. An origin signs only for a requester that may read, after the request is authorized, and keeps the signature in its own table; a store that cannot write, such as a read-only mirror, signs again for every request. Setting the origin's own `horizon-seed-key` to its public key makes it check a kept signature before serving it, so replacing the key behind an unchanged `pgp-command` takes effect at once. A clone that cannot run its verifier (gpg missing, or timed out) keeps the seed it holds and reports the error rather than falling back. Settings are read from the repository only: go-libfossil reads no global fossil configuration, so `clearsign` and `pgp-command` set with `fossil settings --global` do not apply. [Signing](/docs/concepts/signing/) covers the signing settings and the verifier in more detail. ## Moving the horizon Moving C earlier and pulling fetches what the deeper horizon keeps. Moving it later keeps what you hold until you compact. A horizon store cannot be a checkout's cone: a horizon governs what the repository holds, not which paths a checkout writes. ## Library usage ### Build a horizon cone ```go h := libfossil.Horizon{ Checkin: "9f2c...", // full 40 or 64 digit hash Rule: libfossil.HorizonDescendants, // or HorizonLeaves Structural: false, // true for the structural cut } // Horizon only: every path, cut by time. hc, err := libfossil.NewHorizonCone(nil, nil, h) // Horizon plus paths. hc, err = libfossil.NewHorizonCone([]string{"src"}, nil, h) got, ok := hc.Horizon() // read it back noHorizon, err := hc.WithHorizon(nil) // drop it, keep the paths ``` `Checkin` must be a full hash, and it is lower-cased. Dropping the horizon of a horizon-only cone is an error, since nothing would be left; use `SetStoreCone(nil)` to make the store dense. ### Clone into a horizon store ```go t := libfossil.NewHTTPTransport("https://example.com/repo") repo, res, err := libfossil.Clone(ctx, "repo.fossil", t, libfossil.CloneOpts{ StoreCone: hc, SeedKey: "ssh-ed25519 AAAA...", // optional; structural only }) if err != nil { log.Fatal(err) } defer repo.Close() fmt.Println(res.Elided, res.HorizonServed, res.StructuralServed, res.HorizonFallback) ``` - `HorizonServed`: replies where the server cut at the horizon (`pragma horizon-served`, §6.2.5). The other rounds were classified locally. - `StructuralServed`: those served at the structural level. - `HorizonFallback`: a structural clone kept the content horizon instead (§6.3). `SyncResult` has the same fields. ### Clone by name or depth A clone has no history yet, so it cannot turn a branch name or a count into C. It asks the origin, which resolves the name once, in its first reply (§6.2.7): ```go repo, res, err := libfossil.Clone(ctx, "repo.fossil", t, libfossil.CloneOpts{ HorizonName: "trunk", // "tip", a branch or tag, or a full hash HorizonDepth: 100, // step back 100 check-ins along primary parents }) fmt.Println(res.HorizonResolved, res.HorizonSteps, res.HorizonResolvedLocally) ``` - `tip` is the most recent check-in. A full hash names that check-in. Any other name is the most recent check-in carrying the tag `sym-NAME`, so branches and tags both work. Hash prefixes are not resolved. - `HorizonDepth` steps back along primary parents. If history runs out first, C is the root it stopped at and `HorizonSteps` is less than the depth asked for. - The store records only the resolved hash. Every later request of the clone, and every later pull, sync or deepen, sends that hash, never the name. - A name the origin cannot resolve fails the clone with `horizon name not found: NAME`, and nothing is left on disk. Without a horizon in `StoreCone`, the clone takes a content horizon under the descendants rule, over `StoreCone`'s path members if it has any. To ask for the leaves rule or the structural cut by name, pass a horizon `StoreCone`: its rule and depth stand, and its `Checkin` is only a stand-in until the name resolves. A structural clone by name gets the seed the origin mints for the resolved C. An origin that does not answer the name (stock fossil, zit 0.7.0 and earlier) serves the clone densely. The clone then resolves the name itself over what it received, classifies locally at the content horizon, and reports `HorizonResolvedLocally`; a structural request falls back to the content horizon and reports `HorizonFallback` (§6.3). `HorizonHistoryURL`, on both results, is the address the origin gives for querying history below the horizon outside the sync protocol, when it gives one. An origin sets it with `repo config set horizon-history-url https://...` (`SettingHorizonHistoryURL`); only one http or https URL with no spaces or credentials is accepted. `repo clone` prints it as `History:`. ### Deepen, merge across, and verify the seed Setting, replacing and dropping a horizon on an existing store use `SetStoreCone` and `SetStoreConeContext`, as for any [store cone](/docs/concepts/sparse-cones/). ```go // Move the horizon down far enough to reach a check-in, which may lie // below it. C moves to that check-in when it is an ancestor of C, else to // a common ancestor of the two, asked of the origin when the store cannot // see one. The rule, the depth and the path members stay. sr, err := repo.DeepenHorizon(ctx, target, libfossil.RequiredContentOpts{Peer: t}) // Merge. With a Peer, a merge where either side's ancestry crosses a // structural horizon asks the origin for the base, deepening to it when it // lies below. Without one it fails with an error matching // libfossil.ErrBelowHorizon, even when the store finds a base it holds. mr, err := co.MergeContext(ctx, libfossil.CheckoutMergeOpts{Version: rid}, libfossil.RequiredContentOpts{Peer: t}) // Check the seed against the history it summarizes. This fetches every // manifest below C and leaves a content-horizon store. v, err := repo.VerifySeed(ctx, libfossil.RequiredContentOpts{Peer: t}) ``` Deepening a structural store discards its seed before it fetches the new one, so the old seed cannot shadow the new one. If the transfer fails, the old horizon and seed are put back (§5.8). A merge never runs against a base the store does not hold. A bisection whose good end lies below the horizon is refused the same way, naming the horizon and the deepen. An origin checks each pushed check-in that creates a branch: if the branch it cancels is not the one this origin derives for the parent, the clone's seed was stale, and the whole push is refused with `stale seed: ...` (§5.7). Pull, push a control artifact that cancels the parent's real branch tag on the check-in, and push again. ## Command line ### Clone a horizon store ``` libfossil repo clone URL repo.fossil --horizon <full hash> libfossil repo clone URL repo.fossil --horizon trunk --depth 100 libfossil repo clone URL repo.fossil --horizon tip --leaves --cone src libfossil repo clone URL repo.fossil --horizon release --structural --seed-key "ssh-ed25519 AAAA..." ``` - `--horizon=STRING`: clone a horizon store: elide blobs below this check-in, given as `tip`, a branch or tag, or its full 40 or 64 digit hash. - `--depth=N`: with `--horizon`, step the check-in back N check-ins along primary parents, stopping at a root. - `--leaves`: with `--horizon`, also hold every open leaf's ancestry back to the check-in. - `--structural`: with `--horizon`, also elide the manifests below the check-in, deriving their branch and tag state from the origin's seed. - `--seed-key=STRING`: the origin's public key, an SSH public key line or an OpenPGP fingerprint. With `--structural`, a seed not signed by it is discarded and the clone falls back to the content horizon. It is kept as the `horizon-seed-key` setting. `--cone` and `--cone-not` combine with these. The origin resolves a name, since nothing is local yet to resolve it against, and the clone prints the check-in it resolved to as `Horizon at:`. A full hash with no `--depth` is sent as the hash itself, which every horizon origin reads. ### Change the horizon ``` libfossil -R repo.fossil repo cone store --horizon <check-in> libfossil -R repo.fossil repo cone store --horizon <check-in> --leaves libfossil -R repo.fossil repo cone store --horizon <check-in> --structural libfossil -R repo.fossil repo cone store --no-horizon libfossil -R repo.fossil repo cone deepen <check-in> libfossil -R repo.fossil repo cone cost <check-in> ``` - `--horizon=STRING`: elide blobs below this check-in (name, branch or hash prefix), fetching what a deeper horizon keeps. Unlike `clone`, a local name is fine. - `--leaves`: with `--horizon`, also hold every open leaf's ancestry back to the check-in. - `--structural`: with `--horizon`, also elide the manifests below the check-in; the next pull fetches the origin's seed. - `--no-horizon`: drop the horizon, keeping the path members, and fetch what it elided. - `cone deepen`: move the horizon down to reach the check-in, fetching what the deeper horizon keeps. - `cone cost`: report what each rule would hold at the check-in, from this repository. ### Merge and verify ``` libfossil -R repo.fossil repo merge <version> -d DIR libfossil -R repo.fossil repo verify --seed ``` - `merge`: a merge where either side's ancestry crosses a structural horizon asks the remote for the base, and deepens to it when it lies below. A merge whose ancestry stays above the horizon contacts no peer. - `verify --seed`: verify the structural seed by fetching the history below the horizon. The store stays a content-horizon store. `store`, `deepen`, `merge` and `verify` take `--remote` (default: the saved last-sync-url), `--sync-user`, `--sync-pass` (never saved), `--sync-private` and `--timeout` (default 10m; 0 disables it). ## Interoperability **zit.** The horizon format matches zit's byte for byte. Horizon sync works in both directions against zit's `horizon-rfc` revision. **Stock fossil 2.28.** A horizon store refuses a dense clone, because the clone would be silently incomplete. **zit 0.7.1** resolves horizon names and depths as go-libfossil's origin does. Against zit 0.7.0 and earlier a clone by name falls back to resolving locally. **A peer that knows nothing of horizons.** A horizon clone from stock Fossil works, by hash or by name. The client asks for nothing below the horizon, records the below-horizon blobs it is sent as elided instead of storing them, and classifies locally. `HorizonServed` stays 0. Stock Fossil can announce a new check-in only inside a cluster. A horizon store holds no clusters, so it requests the cluster members as transit and classifies what arrives (§4.5). Without that a new check-in would not be learned until a later one named it. ## Limits - **The structural horizon needs an origin that serves it.** Against a peer that does not answer `horizon-served structural` (stock fossil, or a server without a current seed) the clone falls back to the content horizon and reports `HorizonFallback` (§6.3). - **PGP verification needs gpg.** A PGP-signed seed is verified by running `gpg`, so a clone keyed with a fingerprint needs gpg installed and the key in its keyring; on js/wasm only SSH keys verify. gpg-agent hangs at start under a very large open-file limit, which a macOS shell can set to 2^31 (`ulimit -n`): lower it, to 4096 say, in the environment that signs or verifies with gpg. - **The horizon cursor needs a transport that names its peer.** The cursor (`pragma horizon-cursor`, §6.2.6) lets an exchange skip re-announcing what both sides hold. The HTTP transport keys it by URL; a custom `Transport` keeps none and announces in full. - **A horizon is not a checkout cone.** Give a checkout a horizon and you get `ErrHorizonCheckout`. - **Compaction is irreversible locally.** `repo cone compact` drops the held blobs below the horizon; deepening later needs a peer that still has them.