Mercurial > hg > mercurial-crew-with-dirclash
annotate doc/FAQ.txt @ 710:ca9353f43345
Use dirstate to check if .hgtags needs to be added.
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
Use dirstate to check if .hgtags needs to be added.
manifest hash: f90af1553754eb703fc457ffdd5ded0de6fa47e0
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.1 (GNU/Linux)
iD8DBQFC15i1W7P1GVgWeRoRArDvAKCSxWDCbwU12ICwwVUoe753H8GYIwCfZGs0
HcL8cOp+eHz85AMJJE/xfck=
=1xKK
-----END PGP SIGNATURE-----
author | Thomas Arendsen Hein <thomas@intevation.de> |
---|---|
date | Fri, 15 Jul 2005 12:06:29 +0100 |
parents | a287f6cd9c6b |
children | 7dae73778114 c6b912f8b5b2 |
rev | line source |
---|---|
446 | 1 Mercurial Frequently Asked Questions |
449 | 2 ==================================== |
446 | 3 |
4 Section 1: General Usage | |
5 ------------------------ | |
6 | |
449 | 7 .Q. I did an "hg pull" and my working directory is empty! |
446 | 8 |
9 There are two parts to Mercurial: the repository and the working | |
449 | 10 directory. "hg pull" pulls all new changes from a remote repository |
446 | 11 into the local one but doesn't alter the working directory. |
12 | |
13 This keeps you from upsetting your work in progress, which may not be | |
14 ready to merge with the new changes you've pulled and also allows you | |
15 to manage merging more easily (see below about best practices). | |
16 | |
449 | 17 To update your working directory, run "hg update". If you're sure you |
18 want to update your working directory on a pull, you can also use "hg | |
19 pull -u". This will refuse to merge or overwrite local changes. | |
446 | 20 |
21 | |
449 | 22 .Q. What are revision numbers, changeset IDs, and tags? |
446 | 23 |
24 Mercurial will generally allow you to refer to a revision in three | |
25 ways: by revision number, by changeset ID, and by tag. | |
26 | |
27 A revision number is a simple decimal number that corresponds with the | |
28 ordering of commits in the local repository. It is important to | |
29 understand that this ordering can change from machine to machine due | |
30 to Mercurial's distributed, decentralized architecture. | |
31 | |
32 This is where changeset IDs come in. A changeset ID is a 160-bit | |
33 identifier that uniquely describes a changeset and its position in the | |
34 change history, regardless of which machine it's on. This is | |
35 represented to the user as a 40 digit hexadecimal number. As that | |
36 tends to be unwieldy, Mercurial will accept any unambiguous substring | |
37 of that number when specifying versions. It will also generally print | |
38 these numbers in "short form", which is the first 12 digits. | |
39 | |
40 You should always use some form of changeset ID rather than the local | |
41 revision number when discussing revisions with other Mercurial users | |
42 as they may have different revision numbering on their system. | |
43 | |
44 Finally, a tag is an arbitrary string that has been assigned a | |
45 correspondence to a changeset ID. This lets you refer to revisions | |
46 symbolically. | |
47 | |
48 | |
449 | 49 .Q. What are branches, heads, and the tip? |
446 | 50 |
51 The central concept of Mercurial is branching. A 'branch' is simply | |
52 an independent line of development. In most other version control | |
53 systems, all users generally commit to the same line of development | |
54 called 'the trunk' or 'the main branch'. In Mercurial, every developer | |
55 effectively works on a private branch and there is no internal concept | |
56 of 'the main branch'. | |
57 | |
58 Thus Mercurial works hard to make repeated merging between branches | |
449 | 59 easy. Simply run "hg pull" and "hg update -m" and commit the result. |
446 | 60 |
61 'Heads' are simply the most recent commits on a branch. Technically, | |
62 they are changesets which have no children. Merging is the process of | |
63 joining points on two branches into one, usually at their current | |
449 | 64 heads. Use "hg heads" to find the heads in the current repository. |
446 | 65 |
66 The 'tip' is the most recently changed head, and also the highest | |
67 numbered revision. If you have just made a commit, that commit will be | |
68 the head. Alternately, if you have just pulled from another | |
69 repository, the tip of that repository becomes the current tip. | |
70 | |
71 The 'tip' is the default revision for many commands such as update, | |
72 and also functions as a special symbolic tag. | |
73 | |
74 | |
449 | 75 .Q. How does merging work? |
446 | 76 |
77 The merge process is simple. Usually you will want to merge the tip | |
449 | 78 into your working directory. Thus you run "hg update -m" and Mercurial |
446 | 79 will incorporate the changes from tip into your local changes. |
80 | |
81 The first step of this process is tracing back through the history of | |
82 changesets and finding the 'common ancestor' of the two versions that | |
83 are being merged. This is done on a project-wide and a file by file | |
84 basis. | |
85 | |
86 For files that have been changed in both projects, a three-way merge | |
87 is attempted to add the changes made remotely into the changes made | |
88 locally. If there are conflicts between these changes, the user is | |
89 prompted to interactively resolve them. | |
90 | |
91 Mercurial uses a helper tool for this, which is usually found by the | |
92 hgmerge script. Example tools include tkdiff, kdiff3, and the classic | |
93 RCS merge. | |
94 | |
95 After you've completed the merge and you're satisfied that the results | |
96 are correct, it's a good idea to commit your changes. Mercurial won't | |
97 allow you to perform another merge until you've done this commit as | |
98 that would lose important history that will be needed for future | |
99 merges. | |
100 | |
101 | |
449 | 102 .Q. How do tags work in Mercurial? |
446 | 103 |
104 Tags work slightly differently in Mercurial than most revision | |
105 systems. The design attempts to meet the following requirements: | |
106 | |
107 - be version controlled and mergeable just like any other file | |
108 - allow signing of tags | |
109 - allow adding a tag to an already committed changeset | |
110 - allow changing tags in the future | |
111 | |
112 Thus Mercurial stores tags as a file in the working dir. This file is | |
113 called .hgtags and consists of a list of changeset IDs and their | |
114 corresponding tags. To add a tag to the system, simply add a line to | |
449 | 115 this file and then commit it for it to take effect. The "hg tag" |
116 command will do this for you and "hg tags" will show the currently | |
446 | 117 effective tags. |
118 | |
119 Note that because tags refer to changeset IDs and the changeset ID is | |
120 effectively the sum of all the contents of the repository for that | |
121 change, it is impossible in Mercurial to simultaneously commit and add | |
122 a tag. Thus tagging a revision must be done as a second step. | |
123 | |
449 | 124 |
455 | 125 .Q. What if I want to just keep local tags? |
126 | |
631
a287f6cd9c6b
Update documentation of hg tag
Radoslaw Szkodzinski <astralstorm@gorzow.mm.pl>
parents:
456
diff
changeset
|
127 You can use "hg tag" command with an option "-l" or "--local". This |
a287f6cd9c6b
Update documentation of hg tag
Radoslaw Szkodzinski <astralstorm@gorzow.mm.pl>
parents:
456
diff
changeset
|
128 will store the tag in the file .hg/localtags, which will not be |
a287f6cd9c6b
Update documentation of hg tag
Radoslaw Szkodzinski <astralstorm@gorzow.mm.pl>
parents:
456
diff
changeset
|
129 distributed or versioned. The format of this file is identical to the |
a287f6cd9c6b
Update documentation of hg tag
Radoslaw Szkodzinski <astralstorm@gorzow.mm.pl>
parents:
456
diff
changeset
|
130 one of .hgtags and the tags stored there are handled the same. |
455 | 131 |
132 | |
449 | 133 .Q. How do tags work with multiple heads? |
446 | 134 |
135 The tags that are in effect at any given time are the tags specified | |
455 | 136 in each head, with heads closer to the tip taking precedence. Local |
137 tags override all other tags. | |
446 | 138 |
139 | |
449 | 140 .Q. What are some best practices for distributed development with Mercurial? |
446 | 141 |
142 First, merge often! This makes merging easier for everyone and you | |
143 find out about conflicts (which are often rooted in incompatible | |
144 design decisions) earlier. | |
145 | |
146 Second, don't hesitate to use multiple trees locally. Mercurial makes | |
147 this fast and light-weight. Typical usage is to have an incoming tree, | |
148 an outgoing tree, and a separate tree for each area being worked on. | |
149 | |
150 The incoming tree is best maintained as a pristine copy of the | |
151 upstream repository. This works as a cache so that you don't have to | |
152 pull multiple copies over the network. No need to check files out here | |
153 as you won't be changing them. | |
154 | |
155 The outgoing tree contains all the changes you intend for merger into | |
449 | 156 upsteam. Publish this tree with 'hg serve" or hgweb.cgi or use 'hg |
157 push" to push it to another publicly availabe repository. | |
446 | 158 |
159 Then, for each feature you work on, create a new tree. Commit early | |
160 and commit often, merge with incoming regularly, and once you're | |
161 satisfied with your feature, pull the changes into your outgoing tree. | |
162 | |
163 | |
449 | 164 .Q. How do I import from a repository created in a different SCM? |
446 | 165 |
166 Take a look at contrib/convert-repo. This is an extensible | |
167 framework for converting between repository types. | |
168 | |
169 | |
449 | 170 .Q. What about Windows support? |
446 | 171 |
172 Patches to support Windows are being actively integrated, a fully | |
173 working Windows version is probably not far off | |
174 | |
175 | |
456 | 176 Section 2: Bugs and Features |
177 ---------------------------- | |
178 | |
179 .Q. I found a bug, what do I do? | |
180 | |
181 Report it to the mercurial mailing list, mercurial@selenic.com. | |
182 | |
183 | |
184 .Q. What should I include in my bug report? | |
185 | |
186 Enough information to reproduce or diagnose the bug. If you can, try | |
187 using the hg -v and hg -d switches to figure out exactly what | |
188 Mercurial is doing. | |
189 | |
190 If you can reproduce the bug in a simple repository, that is very | |
191 helpful. The best is to create a simple shell script to automate this | |
192 process, which can then be added to our test suite. | |
193 | |
194 | |
195 .Q. Can Mercurial do <x>? | |
196 | |
197 If you'd like to request a feature, send your request to | |
198 mercurial@selenic.com. As Mercurial is still very new, there are | |
199 certainly features it is missing and you can give up feedback on how | |
200 best to implement them. | |
201 | |
202 | |
203 Section 3: Technical | |
446 | 204 -------------------- |
205 | |
449 | 206 .Q. What limits does Mercurial have? |
446 | 207 |
208 Mercurial currently assumes that single files, indices, and manifests | |
209 can fit in memory for efficiency. | |
210 | |
211 Offsets in revlogs are currently tracked with 32 bits, so a revlog for | |
212 a single file can currently not grow beyond 4G. | |
213 | |
214 There should otherwise be no limits on file name length, file size, | |
215 file contents, number of files, or number of revisions. | |
216 | |
217 The network protocol is big-endian. | |
218 | |
219 File names cannot contain the null character. Committer addresses | |
220 cannot contain newlines. | |
221 | |
222 Mercurial is primarily developed for UNIX systems, so some UNIXisms | |
223 may be present in ports. | |
224 | |
225 | |
455 | 226 .Q. How does Mercurial store its data? |
227 | |
228 The fundamental storage type in Mercurial is a "revlog". A revlog is | |
229 the set of all revisions of a named object. Each revision is either | |
230 stored compressed in its entirety or as a compressed binary delta | |
231 against the previous version. The decision of when to store a full | |
232 version is made based on how much data would be needed to reconstruct | |
233 the file. This lets us ensure that we never need to read huge amounts | |
234 of data to reconstruct a object, regardless of how many revisions of it | |
235 we store. | |
236 | |
237 In fact, we should always be able to do it with a single read, | |
238 provided we know when and where to read. This is where the index comes | |
239 in. Each revlog has an index containing a special hash (nodeid) of the | |
240 text, hashes for its parents, and where and how much of the revlog | |
241 data we need to read to reconstruct it. Thus, with one read of the | |
242 index and one read of the data, we can reconstruct any version in time | |
243 proportional to the object size. | |
244 | |
245 Similarly, revlogs and their indices are append-only. This means that | |
246 adding a new version is also O(1) seeks. | |
247 | |
248 Revlogs are used to represent all revisions of files, manifests, and | |
249 changesets. Compression for typical objects with lots of revisions can | |
250 range from 100 to 1 for things like project makefiles to over 2000 to | |
251 1 for objects like the manifest. | |
252 | |
253 | |
254 .Q. How are manifests and changesets stored? | |
255 | |
256 A manifest is simply a list of all files in a given revision of a | |
257 project along with the nodeids of the corresponding file revisions. So | |
258 grabbing a given version of the project means simply looking up its | |
259 manifest and reconstruction all the file revisions pointed to by it. | |
446 | 260 |
455 | 261 A changeset is a list of all files changed in a check-in along with a |
262 change description and some metadata like user and date. It also | |
263 contains a nodeid to the relevent revision of the manifest. | |
264 | |
265 | |
266 .Q. How do Mercurial hashes get calculated? | |
267 | |
268 Mercurial hashes both the contents of an object and the hash of its | |
269 parents to create an identifier that uniquely identifies an object's | |
270 contents and history. This greatly simplifies merging of histories | |
271 because it avoid graph cycles that can occur when a object is reverted | |
272 to an earlier state. | |
273 | |
274 All file revisions have an associated hash value. These are listed in | |
275 the manifest of a given project revision, and the manifest hash is | |
276 listed in the changeset. The changeset hash is again a hash of the | |
277 changeset contents and its parents, so it uniquely identifies the | |
278 entire history of the project to that point. | |
279 | |
446 | 280 |
455 | 281 .Q. What checks are there on repository integrity? |
282 | |
283 Every time a revlog object is retrieved, it is checked against its | |
284 hash for integrity. It is also incidentally doublechecked by the | |
285 Adler32 checksum used by the underlying zlib compression. | |
286 | |
287 Running 'hg verify' decompresses and reconstitutes each revision of | |
288 each object in the repository and cross-checks all of the index | |
289 metadata with those contents. | |
290 | |
291 But this alone is not enough to ensure that someone hasn't tampered | |
292 with a repository. For that, you need cryptographic signing. | |
293 | |
294 | |
295 .Q. How does signing work with Mercurial? | |
296 | |
297 Take a look at the hgeditor script for an example. The basic idea is | |
298 to use GPG to sign the manifest ID inside that changelog entry. The | |
299 manifest ID is a recursive hash of all of the files in the system and | |
300 their complete history, and thus signing the manifest hash signs the | |
301 entire project contents. | |
446 | 302 |
303 | |
449 | 304 .Q. What about hash collisions? What about weaknesses in SHA1? |
446 | 305 |
306 The SHA1 hashes are large enough that the odds of accidental hash collision | |
307 are negligible for projects that could be handled by the human race. | |
308 The known weaknesses in SHA1 are currently still not practical to | |
309 attack, and Mercurial will switch to SHA256 hashing before that | |
310 becomes a realistic concern. | |
311 | |
312 Collisions with the "short hashes" are not a concern as they're always | |
313 checked for ambiguity and are still long enough that they're not | |
314 likely to happen for reasonably-sized projects (< 1M changes). | |
455 | 315 |
316 | |
317 |