1// Package dag contains the base common code to define an entity stored
2// in a chain of git objects, supporting actions like Push, Pull and Merge.
3package dag
4
5import (
6 "encoding/json"
7 "fmt"
8 "sort"
9
10 "github.com/pkg/errors"
11
12 "github.com/MichaelMure/git-bug/entity"
13 "github.com/MichaelMure/git-bug/identity"
14 "github.com/MichaelMure/git-bug/repository"
15 "github.com/MichaelMure/git-bug/util/lamport"
16)
17
18const refsPattern = "refs/%s/%s"
19const creationClockPattern = "%s-create"
20const editClockPattern = "%s-edit"
21
22// Definition hold the details defining one specialization of an Entity.
23type Definition struct {
24 // the name of the entity (bug, pull-request, ...), for human consumption
25 Typename string
26 // the Namespace in git references (bugs, prs, ...)
27 Namespace string
28 // a function decoding a JSON message into an Operation
29 OperationUnmarshaler func(author identity.Interface, raw json.RawMessage) (Operation, error)
30 // the expected format version number, that can be used for data migration/upgrade
31 FormatVersion uint
32}
33
34// Entity is a data structure stored in a chain of git objects, supporting actions like Push, Pull and Merge.
35type Entity struct {
36 // A Lamport clock is a logical clock that allow to order event
37 // inside a distributed system.
38 // It must be the first field in this struct due to https://github.com/golang/go/issues/36606
39 createTime lamport.Time
40 editTime lamport.Time
41
42 Definition
43
44 // operations that are already stored in the repository
45 ops []Operation
46 // operations not yet stored in the repository
47 staging []Operation
48
49 lastCommit repository.Hash
50}
51
52// New create an empty Entity
53func New(definition Definition) *Entity {
54 return &Entity{
55 Definition: definition,
56 }
57}
58
59// Read will read and decode a stored local Entity from a repository
60func Read(def Definition, repo repository.ClockedRepo, resolver identity.Resolver, id entity.Id) (*Entity, error) {
61 if err := id.Validate(); err != nil {
62 return nil, errors.Wrap(err, "invalid id")
63 }
64
65 ref := fmt.Sprintf("refs/%s/%s", def.Namespace, id.String())
66
67 return read(def, repo, resolver, ref)
68}
69
70// readRemote will read and decode a stored remote Entity from a repository
71func readRemote(def Definition, repo repository.ClockedRepo, resolver identity.Resolver, remote string, id entity.Id) (*Entity, error) {
72 if err := id.Validate(); err != nil {
73 return nil, errors.Wrap(err, "invalid id")
74 }
75
76 ref := fmt.Sprintf("refs/remotes/%s/%s/%s", def.Namespace, remote, id.String())
77
78 return read(def, repo, resolver, ref)
79}
80
81// read fetch from git and decode an Entity at an arbitrary git reference.
82func read(def Definition, repo repository.ClockedRepo, resolver identity.Resolver, ref string) (*Entity, error) {
83 rootHash, err := repo.ResolveRef(ref)
84 if err != nil {
85 return nil, err
86 }
87
88 // Perform a breadth-first search to get a topological order of the DAG where we discover the
89 // parents commit and go back in time up to the chronological root
90
91 queue := make([]repository.Hash, 0, 32)
92 visited := make(map[repository.Hash]struct{})
93 BFSOrder := make([]repository.Commit, 0, 32)
94
95 queue = append(queue, rootHash)
96 visited[rootHash] = struct{}{}
97
98 for len(queue) > 0 {
99 // pop
100 hash := queue[0]
101 queue = queue[1:]
102
103 commit, err := repo.ReadCommit(hash)
104 if err != nil {
105 return nil, err
106 }
107
108 BFSOrder = append(BFSOrder, commit)
109
110 for _, parent := range commit.Parents {
111 if _, ok := visited[parent]; !ok {
112 queue = append(queue, parent)
113 // mark as visited
114 visited[parent] = struct{}{}
115 }
116 }
117 }
118
119 // Now, we can reverse this topological order and read the commits in an order where
120 // we are sure to have read all the chronological ancestors when we read a commit.
121
122 // Next step is to:
123 // 1) read the operationPacks
124 // 2) make sure that clocks causality respect the DAG topology.
125
126 oppMap := make(map[repository.Hash]*operationPack)
127 var opsCount int
128
129 for i := len(BFSOrder) - 1; i >= 0; i-- {
130 commit := BFSOrder[i]
131 isFirstCommit := i == len(BFSOrder)-1
132 isMerge := len(commit.Parents) > 1
133
134 // Verify DAG structure: single chronological root, so only the root
135 // can have no parents. Said otherwise, the DAG need to have exactly
136 // one leaf.
137 if !isFirstCommit && len(commit.Parents) == 0 {
138 return nil, fmt.Errorf("multiple leafs in the entity DAG")
139 }
140
141 opp, err := readOperationPack(def, repo, resolver, commit)
142 if err != nil {
143 return nil, err
144 }
145
146 err = opp.Validate()
147 if err != nil {
148 return nil, err
149 }
150
151 if isMerge && len(opp.Operations) > 0 {
152 return nil, fmt.Errorf("merge commit cannot have operations")
153 }
154
155 // Check that the create lamport clock is set (not checked in Validate() as it's optional)
156 if isFirstCommit && opp.CreateTime <= 0 {
157 return nil, fmt.Errorf("creation lamport time not set")
158 }
159
160 // make sure that the lamport clocks causality match the DAG topology
161 for _, parentHash := range commit.Parents {
162 parentPack, ok := oppMap[parentHash]
163 if !ok {
164 panic("DFS failed")
165 }
166
167 if parentPack.EditTime >= opp.EditTime {
168 return nil, fmt.Errorf("lamport clock ordering doesn't match the DAG")
169 }
170
171 // to avoid an attack where clocks are pushed toward the uint64 rollover, make sure
172 // that the clocks don't jump too far in the future
173 // we ignore merge commits here to allow merging after a loooong time without breaking anything,
174 // as long as there is one valid chain of small hops, it's fine.
175 if !isMerge && opp.EditTime-parentPack.EditTime > 1_000_000 {
176 return nil, fmt.Errorf("lamport clock jumping too far in the future, likely an attack")
177 }
178 }
179
180 oppMap[commit.Hash] = opp
181 opsCount += len(opp.Operations)
182 }
183
184 // The clocks are fine, we witness them
185 for _, opp := range oppMap {
186 err = repo.Witness(fmt.Sprintf(creationClockPattern, def.Namespace), opp.CreateTime)
187 if err != nil {
188 return nil, err
189 }
190 err = repo.Witness(fmt.Sprintf(editClockPattern, def.Namespace), opp.EditTime)
191 if err != nil {
192 return nil, err
193 }
194 }
195
196 // Now that we know that the topological order and clocks are fine, we order the operationPacks
197 // based on the logical clocks, entirely ignoring the DAG topology
198
199 oppSlice := make([]*operationPack, 0, len(oppMap))
200 for _, pack := range oppMap {
201 oppSlice = append(oppSlice, pack)
202 }
203 sort.Slice(oppSlice, func(i, j int) bool {
204 // Primary ordering with the EditTime.
205 if oppSlice[i].EditTime != oppSlice[j].EditTime {
206 return oppSlice[i].EditTime < oppSlice[j].EditTime
207 }
208 // We have equal EditTime, which means we have concurrent edition over different machines and we
209 // can't tell which one came first. So, what now? We still need a total ordering and the most stable possible.
210 // As a secondary ordering, we can order based on a hash of the serialized Operations in the
211 // operationPack. It doesn't carry much meaning but it's unbiased and hard to abuse.
212 // This is a lexicographic ordering on the stringified ID.
213 return oppSlice[i].Id() < oppSlice[j].Id()
214 })
215
216 // Now that we ordered the operationPacks, we have the order of the Operations
217
218 ops := make([]Operation, 0, opsCount)
219 var createTime lamport.Time
220 var editTime lamport.Time
221 for _, pack := range oppSlice {
222 for _, operation := range pack.Operations {
223 ops = append(ops, operation)
224 }
225 if pack.CreateTime > createTime {
226 createTime = pack.CreateTime
227 }
228 if pack.EditTime > editTime {
229 editTime = pack.EditTime
230 }
231 }
232
233 return &Entity{
234 Definition: def,
235 ops: ops,
236 lastCommit: rootHash,
237 createTime: createTime,
238 editTime: editTime,
239 }, nil
240}
241
242type StreamedEntity struct {
243 Entity *Entity
244 Err error
245}
246
247// ReadAll read and parse all local Entity
248func ReadAll(def Definition, repo repository.ClockedRepo, resolver identity.Resolver) <-chan StreamedEntity {
249 out := make(chan StreamedEntity)
250
251 go func() {
252 defer close(out)
253
254 refPrefix := fmt.Sprintf("refs/%s/", def.Namespace)
255
256 refs, err := repo.ListRefs(refPrefix)
257 if err != nil {
258 out <- StreamedEntity{Err: err}
259 return
260 }
261
262 for _, ref := range refs {
263 e, err := read(def, repo, resolver, ref)
264
265 if err != nil {
266 out <- StreamedEntity{Err: err}
267 return
268 }
269
270 out <- StreamedEntity{Entity: e}
271 }
272 }()
273
274 return out
275}
276
277// Id return the Entity identifier
278func (e *Entity) Id() entity.Id {
279 // id is the id of the first operation
280 return e.FirstOp().Id()
281}
282
283// Validate check if the Entity data is valid
284func (e *Entity) Validate() error {
285 // non-empty
286 if len(e.ops) == 0 && len(e.staging) == 0 {
287 return fmt.Errorf("entity has no operations")
288 }
289
290 // check if each operations are valid
291 for _, op := range e.ops {
292 if err := op.Validate(); err != nil {
293 return err
294 }
295 }
296
297 // check if staging is valid if needed
298 for _, op := range e.staging {
299 if err := op.Validate(); err != nil {
300 return err
301 }
302 }
303
304 // Check that there is no colliding operation's ID
305 ids := make(map[entity.Id]struct{})
306 for _, op := range e.Operations() {
307 if _, ok := ids[op.Id()]; ok {
308 return fmt.Errorf("id collision: %s", op.Id())
309 }
310 ids[op.Id()] = struct{}{}
311 }
312
313 return nil
314}
315
316// Operations return the ordered operations
317func (e *Entity) Operations() []Operation {
318 return append(e.ops, e.staging...)
319}
320
321// FirstOp lookup for the very first operation of the Entity
322func (e *Entity) FirstOp() Operation {
323 for _, op := range e.ops {
324 return op
325 }
326 for _, op := range e.staging {
327 return op
328 }
329 return nil
330}
331
332// LastOp lookup for the very last operation of the Entity
333func (e *Entity) LastOp() Operation {
334 if len(e.staging) > 0 {
335 return e.staging[len(e.staging)-1]
336 }
337 if len(e.ops) > 0 {
338 return e.ops[len(e.ops)-1]
339 }
340 return nil
341}
342
343// Append add a new Operation to the Entity
344func (e *Entity) Append(op Operation) {
345 e.staging = append(e.staging, op)
346}
347
348// NeedCommit indicate if the in-memory state changed and need to be commit in the repository
349func (e *Entity) NeedCommit() bool {
350 return len(e.staging) > 0
351}
352
353// CommitAsNeeded execute a Commit only if necessary. This function is useful to avoid getting an error if the Entity
354// is already in sync with the repository.
355func (e *Entity) CommitAsNeeded(repo repository.ClockedRepo) error {
356 if e.NeedCommit() {
357 return e.Commit(repo)
358 }
359 return nil
360}
361
362// Commit write the appended operations in the repository
363func (e *Entity) Commit(repo repository.ClockedRepo) error {
364 if !e.NeedCommit() {
365 return fmt.Errorf("can't commit an entity with no pending operation")
366 }
367
368 err := e.Validate()
369 if err != nil {
370 return errors.Wrapf(err, "can't commit a %s with invalid data", e.Definition.Typename)
371 }
372
373 for len(e.staging) > 0 {
374 var author identity.Interface
375 var toCommit []Operation
376
377 // Split into chunks with the same author
378 for len(e.staging) > 0 {
379 op := e.staging[0]
380 if author != nil && op.Author().Id() != author.Id() {
381 break
382 }
383 author = e.staging[0].Author()
384 toCommit = append(toCommit, op)
385 e.staging = e.staging[1:]
386 }
387
388 e.editTime, err = repo.Increment(fmt.Sprintf(editClockPattern, e.Namespace))
389 if err != nil {
390 return err
391 }
392
393 opp := &operationPack{
394 Author: author,
395 Operations: toCommit,
396 EditTime: e.editTime,
397 }
398
399 if e.lastCommit == "" {
400 e.createTime, err = repo.Increment(fmt.Sprintf(creationClockPattern, e.Namespace))
401 if err != nil {
402 return err
403 }
404 opp.CreateTime = e.createTime
405 }
406
407 var parentCommit []repository.Hash
408 if e.lastCommit != "" {
409 parentCommit = []repository.Hash{e.lastCommit}
410 }
411
412 commitHash, err := opp.Write(e.Definition, repo, parentCommit...)
413 if err != nil {
414 return err
415 }
416
417 e.lastCommit = commitHash
418 e.ops = append(e.ops, toCommit...)
419 }
420
421 // not strictly necessary but make equality testing easier in tests
422 e.staging = nil
423
424 // Create or update the Git reference for this entity
425 // When pushing later, the remote will ensure that this ref update
426 // is fast-forward, that is no data has been overwritten.
427 ref := fmt.Sprintf(refsPattern, e.Namespace, e.Id().String())
428 return repo.UpdateRef(ref, e.lastCommit)
429}
430
431// CreateLamportTime return the Lamport time of creation
432func (e *Entity) CreateLamportTime() lamport.Time {
433 return e.createTime
434}
435
436// EditLamportTime return the Lamport time of the last edition
437func (e *Entity) EditLamportTime() lamport.Time {
438 return e.editTime
439}