From 8d63c983c982f93cc48d3996d6bd097ddeeb327f Mon Sep 17 00:00:00 2001 From: Michael Muré Date: Sun, 3 Jan 2021 23:59:25 +0100 Subject: WIP --- entity/dag/entity.go | 389 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 389 insertions(+) create mode 100644 entity/dag/entity.go (limited to 'entity/dag/entity.go') diff --git a/entity/dag/entity.go b/entity/dag/entity.go new file mode 100644 index 00000000..78347fa0 --- /dev/null +++ b/entity/dag/entity.go @@ -0,0 +1,389 @@ +// Package dag contains the base common code to define an entity stored +// in a chain of git objects, supporting actions like Push, Pull and Merge. +package dag + +import ( + "encoding/json" + "fmt" + "sort" + + "github.com/pkg/errors" + + "github.com/MichaelMure/git-bug/entity" + "github.com/MichaelMure/git-bug/identity" + "github.com/MichaelMure/git-bug/repository" + "github.com/MichaelMure/git-bug/util/lamport" +) + +const refsPattern = "refs/%s/%s" +const creationClockPattern = "%s-create" +const editClockPattern = "%s-edit" + +// Definition hold the details defining one specialization of an Entity. +type Definition struct { + // the name of the entity (bug, pull-request, ...) + typename string + // the namespace in git (bugs, prs, ...) + namespace string + // a function decoding a JSON message into an Operation + operationUnmarshaler func(author identity.Interface, raw json.RawMessage) (Operation, error) + // a function loading an identity.Identity from its Id + identityResolver identity.Resolver + // the expected format version number, that can be used for data migration/upgrade + formatVersion uint +} + +// Entity is a data structure stored in a chain of git objects, supporting actions like Push, Pull and Merge. +type Entity struct { + Definition + + // operations that are already stored in the repository + ops []Operation + // operations not yet stored in the repository + staging []Operation + + // TODO: add here createTime and editTime + + // // TODO: doesn't seems to actually be useful over the topological sort ? Timestamp can be generated from graph depth + // // TODO: maybe EditTime is better because it could spread ops in consecutive groups on the logical timeline --> avoid interleaving + // packClock lamport.Clock + lastCommit repository.Hash +} + +// New create an empty Entity +func New(definition Definition) *Entity { + return &Entity{ + Definition: definition, + // packClock: lamport.NewMemClock(), + } +} + +// Read will read and decode a stored Entity from a repository +func Read(def Definition, repo repository.ClockedRepo, id entity.Id) (*Entity, error) { + if err := id.Validate(); err != nil { + return nil, errors.Wrap(err, "invalid id") + } + + ref := fmt.Sprintf("refs/%s/%s", def.namespace, id.String()) + + return read(def, repo, ref) +} + +// read fetch from git and decode an Entity at an arbitrary git reference. +func read(def Definition, repo repository.ClockedRepo, ref string) (*Entity, error) { + rootHash, err := repo.ResolveRef(ref) + if err != nil { + return nil, err + } + + // Perform a depth-first search to get a topological order of the DAG where we discover the + // parents commit and go back in time up to the chronological root + + stack := make([]repository.Hash, 0, 32) + visited := make(map[repository.Hash]struct{}) + DFSOrder := make([]repository.Commit, 0, 32) + + stack = append(stack, rootHash) + + for len(stack) > 0 { + // pop + hash := stack[len(stack)-1] + stack = stack[:len(stack)-1] + + if _, ok := visited[hash]; ok { + continue + } + + // mark as visited + visited[hash] = struct{}{} + + commit, err := repo.ReadCommit(hash) + if err != nil { + return nil, err + } + + DFSOrder = append(DFSOrder, commit) + + for _, parent := range commit.Parents { + stack = append(stack, parent) + } + } + + // Now, we can reverse this topological order and read the commits in an order where + // we are sure to have read all the chronological ancestors when we read a commit. + + // Next step is to: + // 1) read the operationPacks + // 2) make sure that the clocks causality respect the DAG topology. + + oppMap := make(map[repository.Hash]*operationPack) + var opsCount int + // var packClock = lamport.NewMemClock() + + for i := len(DFSOrder) - 1; i >= 0; i-- { + commit := DFSOrder[i] + isFirstCommit := i == len(DFSOrder)-1 + isMerge := len(commit.Parents) > 1 + + // Verify DAG structure: single chronological root, so only the root + // can have no parents. Said otherwise, the DAG need to have exactly + // one leaf. + if !isFirstCommit && len(commit.Parents) == 0 { + return nil, fmt.Errorf("multiple leafs in the entity DAG") + } + + opp, err := readOperationPack(def, repo, commit) + if err != nil { + return nil, err + } + + err = opp.Validate() + if err != nil { + return nil, err + } + + // Check that the create lamport clock is set (not checked in Validate() as it's optional) + if isFirstCommit && opp.CreateTime <= 0 { + return nil, fmt.Errorf("creation lamport time not set") + } + + // make sure that the lamport clocks causality match the DAG topology + for _, parentHash := range commit.Parents { + parentPack, ok := oppMap[parentHash] + if !ok { + panic("DFS failed") + } + + if parentPack.EditTime >= opp.EditTime { + return nil, fmt.Errorf("lamport clock ordering doesn't match the DAG") + } + + // to avoid an attack where clocks are pushed toward the uint64 rollover, make sure + // that the clocks don't jump too far in the future + // we ignore merge commits here to allow merging after a loooong time without breaking anything, + // as long as there is one valid chain of small hops, it's fine. + if !isMerge && opp.EditTime-parentPack.EditTime > 1_000_000 { + return nil, fmt.Errorf("lamport clock jumping too far in the future, likely an attack") + } + + // TODO: PackTime is not checked + } + + oppMap[commit.Hash] = opp + opsCount += len(opp.Operations) + } + + // The clocks are fine, we witness them + for _, opp := range oppMap { + err = repo.Witness(fmt.Sprintf(creationClockPattern, def.namespace), opp.CreateTime) + if err != nil { + return nil, err + } + err = repo.Witness(fmt.Sprintf(editClockPattern, def.namespace), opp.EditTime) + if err != nil { + return nil, err + } + // err = packClock.Witness(opp.PackTime) + // if err != nil { + // return nil, err + // } + } + + // Now that we know that the topological order and clocks are fine, we order the operationPacks + // based on the logical clocks, entirely ignoring the DAG topology + + oppSlice := make([]*operationPack, 0, len(oppMap)) + for _, pack := range oppMap { + oppSlice = append(oppSlice, pack) + } + sort.Slice(oppSlice, func(i, j int) bool { + // Primary ordering with the dedicated "pack" Lamport time that encode causality + // within the entity + // if oppSlice[i].PackTime != oppSlice[j].PackTime { + // return oppSlice[i].PackTime < oppSlice[i].PackTime + // } + // We have equal PackTime, which means we had a concurrent edition. We can't tell which exactly + // came first. As a secondary arbitrary ordering, we can use the EditTime. It's unlikely to be + // enough but it can give us an edge to approach what really happened. + if oppSlice[i].EditTime != oppSlice[j].EditTime { + return oppSlice[i].EditTime < oppSlice[j].EditTime + } + // Well, what now? We still need a total ordering and the most stable possible. + // As a last resort, we can order based on a hash of the serialized Operations in the + // operationPack. It doesn't carry much meaning but it's unbiased and hard to abuse. + // This is a lexicographic ordering on the stringified ID. + return oppSlice[i].Id() < oppSlice[j].Id() + }) + + // Now that we ordered the operationPacks, we have the order of the Operations + + ops := make([]Operation, 0, opsCount) + for _, pack := range oppSlice { + for _, operation := range pack.Operations { + ops = append(ops, operation) + } + } + + return &Entity{ + Definition: def, + ops: ops, + // packClock: packClock, + lastCommit: rootHash, + }, nil +} + +// Id return the Entity identifier +func (e *Entity) Id() entity.Id { + // id is the id of the first operation + return e.FirstOp().Id() +} + +// Validate check if the Entity data is valid +func (e *Entity) Validate() error { + // non-empty + if len(e.ops) == 0 && len(e.staging) == 0 { + return fmt.Errorf("entity has no operations") + } + + // check if each operations are valid + for _, op := range e.ops { + if err := op.Validate(); err != nil { + return err + } + } + + // check if staging is valid if needed + for _, op := range e.staging { + if err := op.Validate(); err != nil { + return err + } + } + + // Check that there is no colliding operation's ID + ids := make(map[entity.Id]struct{}) + for _, op := range e.Operations() { + if _, ok := ids[op.Id()]; ok { + return fmt.Errorf("id collision: %s", op.Id()) + } + ids[op.Id()] = struct{}{} + } + + return nil +} + +// Operations return the ordered operations +func (e *Entity) Operations() []Operation { + return append(e.ops, e.staging...) +} + +// FirstOp lookup for the very first operation of the Entity +func (e *Entity) FirstOp() Operation { + for _, op := range e.ops { + return op + } + for _, op := range e.staging { + return op + } + return nil +} + +// LastOp lookup for the very last operation of the Entity +func (e *Entity) LastOp() Operation { + if len(e.staging) > 0 { + return e.staging[len(e.staging)-1] + } + if len(e.ops) > 0 { + return e.ops[len(e.ops)-1] + } + return nil +} + +// Append add a new Operation to the Entity +func (e *Entity) Append(op Operation) { + e.staging = append(e.staging, op) +} + +// NeedCommit indicate if the in-memory state changed and need to be commit in the repository +func (e *Entity) NeedCommit() bool { + return len(e.staging) > 0 +} + +// CommitAdNeeded execute a Commit only if necessary. This function is useful to avoid getting an error if the Entity +// is already in sync with the repository. +func (e *Entity) CommitAdNeeded(repo repository.ClockedRepo) error { + if e.NeedCommit() { + return e.Commit(repo) + } + return nil +} + +// Commit write the appended operations in the repository +// TODO: support commit signature +func (e *Entity) Commit(repo repository.ClockedRepo) error { + if !e.NeedCommit() { + return fmt.Errorf("can't commit an entity with no pending operation") + } + + if err := e.Validate(); err != nil { + return errors.Wrapf(err, "can't commit a %s with invalid data", e.Definition.typename) + } + + var author identity.Interface + for _, op := range e.staging { + if author != nil && op.Author() != author { + return fmt.Errorf("operations with different author") + } + author = op.Author() + } + + // increment the various clocks for this new operationPack + // packTime, err := e.packClock.Increment() + // if err != nil { + // return err + // } + editTime, err := repo.Increment(fmt.Sprintf(editClockPattern, e.namespace)) + if err != nil { + return err + } + var creationTime lamport.Time + if e.lastCommit == "" { + creationTime, err = repo.Increment(fmt.Sprintf(creationClockPattern, e.namespace)) + if err != nil { + return err + } + } + + opp := &operationPack{ + Author: author, + Operations: e.staging, + CreateTime: creationTime, + EditTime: editTime, + // PackTime: packTime, + } + + treeHash, err := opp.Write(e.Definition, repo) + if err != nil { + return err + } + + // Write a Git commit referencing the tree, with the previous commit as parent + var commitHash repository.Hash + if e.lastCommit != "" { + commitHash, err = repo.StoreCommit(treeHash, e.lastCommit) + } else { + commitHash, err = repo.StoreCommit(treeHash) + } + if err != nil { + return err + } + + e.lastCommit = commitHash + e.ops = append(e.ops, e.staging...) + e.staging = nil + + // Create or update the Git reference for this entity + // When pushing later, the remote will ensure that this ref update + // is fast-forward, that is no data has been overwritten. + ref := fmt.Sprintf(refsPattern, e.namespace, e.Id().String()) + return repo.UpdateRef(ref, commitHash) +} -- cgit