How cvs2svn.py Works ===================== A cvs2svn run consists of 5 passes. Every pass but the last saves its data to a file on disk, so that a) we don't hold huge amounts of state in memory, and b) the conversion process is resumable. The final pass makes the actual Subversion commits. Pass 1: ======= The goal of this pass is to get a summary of all the revisions for each file written out to 'cvs2svn-data.revs'; at the end of this stage, revisions will be grouped by RCS file, not by logical commits. We walk over the repository, processing each RCS file with with rcsparse.parse(), using cvs2svn's CollectData class, which is a subclass of rcsparse.Sink(), the parser's callback class. For each RCS file, the first thing the parser encounters is the administrative header, including the head revision, the principal branch, symbolic names, RCS comments, etc. The main thing that happens here is that CollectData.define_tag() is invoked on each symbolic name and its attached revision, so all the tags and branches of this file get collected. Next, the parser hits the revision summary section. That's the part of the RCS file that looks like this: 1.6 date 2002.06.12.04.54.12; author captnmark; state Exp; branches 1.6.2.1; next 1.5; 1.5 date 2002.05.28.18.02.11; author captnmark; state Exp; branches; next 1.4; [...] For each revision summary, CollectData.define_revision() is invoked, recording that revision's metadata in the self.rev_data[] tree. After finishing the revision summaries, the parser invokes CollectData.tree_completed(), which loops over the revisions in self.rev_data, determining if there are instances where a higher revision was committed "before" a lower one (rare, but it can happen when there was clock skew on the repository machine). If there are any, it "resyncs" the timestamp of the higher rev to be just after that of the lower rev, but saves the original timestamp in self.rev_data[blah][3], so we can later write out a record to the resync file indicating that an adjustment was made (this makes it possible to catch the other parts of this commit and resync them similarly, more details below). Next, the parser encounters the *real* revision data, which has the log messages and file contents. For each revision, it invokes CollectData.set_revision_info(), which writes a new line to cvs2svn-data.revs, like this: 3dc32955 5afe9b4ba41843d8eb52ae7db47a43eaa9573254 C 1.2 * 0 0 foo/bar,v The fields are: 1. a fixed-width timestamp 2. a digest of the log message + author 3. the type of change ("C"hange, or "D"elete) 4. the revision number 5. the branch on which this commit happened, or "*" if not on a branch 6. the number of tags rooted at this revision (followed by their names, space-delimited) 7. the number of branches rooted at this revision (followed by their names, space-delimited) 8. the path of the RCS file in the repository (Of course, in the above example, fields 6 and 7 are "0", so they have no additional data.) Also, for resync'd revisions, a line like this is written out to 'cvs2svn-data.resync': 3d6c1329 18a215a05abea1c6c155dcc7283b88ae7ce23502 3d6c1328 The fields are: NEW_TIMESTAMP DIGEST OLD_TIMESTAMP (The resync file will be explained later.) That's it -- the RCS file is done. When every RCS file is done, Pass 1 is complete, and: - cvs2svn-data.revs contains a summary of every RCS file's revisions. All the revisions for a given RCS file are grouped together, but note that the groups are in no particular order. In other words, you can't yet identify the commits from looking at these lines; a multi-file commit will be scattered all over the place. - cvs2svn-data.resync contains a small amount of resync data, in no particular order. Pass 2: ======= This is where the resync file is used. The goal of this pass is to convert cvs2svn-data.revs to a new file, 'cvs2svn-data.c-revs' (clean revs). It's the same as the original file, except for some resync'd timestamps. First, read the whole resync file into a hash table that maps each author+log digest to a list of lists. Each sublist represents one of the timestamp adjustments from Pass 1, and looks like this: [old_time_lower, old_time_upper, new_time] The reason to map each digest to a list of sublists, instead of to one list, is that sometimes you'll get the same digest for unrelated commits (for example, the same author commits many times using the empty log message, or a log message that just says "Doc tweaks."). So each digest may need to "fan out" to cover multiple commits, but without accidentally unifying those commits. Now we loop over cvs2svn-data.revs, writing each line out to 'cvs2svn-data.c-revs'. Most lines are written out unchanged, but those whose digest matches some resync entry, and appear to be part of the same commit as one of the sublists in that entry, get tweaked. The tweak is to adjust the commit time of the line to the new_time, which is taken from the resync hash and results from the adjustment described in Pass 1. The way we figure out whether a given line needs to be tweaked is to loop over all the sublists, seeing if this commit's original time falls within the old<-->new time range for the current sublist. If it does, we tweak the line before writing it out, and then conditionally adjust the sublist's range to account for the timestamp we just adjusted (since it could be an outlier). Note that this could, in theory, result in separate commits being accidentally unified, since we might gradually the two sides of the range such that they are eventually more than COMMIT_THRESHOLD seconds apart. However, this is really a case of CVS not recording enough information to disambiguate the commits; we'd know we have a time range that exceeds the COMMIT_THRESHOLD, but we wouldn't necessarily know where to divide it up. We could try some clever heuristic, but for now it's not important -- after all, we're talking about commits that weren't important enough to have a distinctive log message anyway, so does it really matter if a couple of them accidentally get unified? Probably not. Pass 3: ======= This is where we deduce the changesets, that is, the grouping of file changes into single commits. It's very simple -- run 'sort' on cvs2svn-data.c-revs, converting it to 'cvs2svn-data.s-revs'. Because of the way the data is laid out, this causes commits with the same digest (that is, the same author and log message) to be grouped together. Poof! We now have the CVS changes grouped by logical commit. In some cases, the changes in a given commit may be interleaved with other commits that went on at the same time, because the sort gives precedence to date before log digest. However, Pass 4 detects this by seeing that the log digest is different, and reseparates the commits. Pass 4: ======= Walk through cvs2svn-data.s-revs and print the commits to a Subversion dumpfile (a file intended for 'svnadmin load'). The dumpfile is the data's last static stage: last chance to check over the data, run it through svndumpfilter, move the dumpfile to another machine, etc. =============================== Branches and Tags Plan. =============================== This pass is also where tag and branch creation is done. Since subversion does tags and branches by copying from existing revisions (then maybe editing the copy, making subcopies underneath, etc), the big question for cvs2svn is how to achieve the minimum number of operations per creation. For example, if it's possible to get the right tag by just copying revision 53, then it's better to do that than, say, copying revision 51 and then sub-copying in bits of revision 52 and 53. Also, since CVS does not version symbolic names, there is the secondary question of *when* to create a particular tag or branch. For example, a tag might have been made at any time after the youngest commit included in it, or might even have been made piecemeal; and the same is true for a branch, with the added constraint that for any particular file, the branch must have been created before the first commit on the branch. Answering the second question first: cvs2svn creates tags and branches as late as possible. For branches, this is "just in time" creation -- the moment it sees the first commit on a branch, it snaps the entire branch into existence (or as much of it as possible), and then outputs the branch commit. The reason we say "as much of it as possible" is that it's possible to have a branch where some files have branch commits occuring earlier than the other files even have the source revisions from which the branch sprouts (this can happen if the branch was created piecemeal, for example). In this case, we create as much of the branch as we can, that is, as much of it as there are source revisions available to copy, and leave the rest for later. "Later" might mean just until other branch commits come in, or else during a cleanup stage that happens at the end of this pass (about which more later). All tags are created during the cleanup stage, after all regular commits have been made. That way there's no need to worry whether all the required revisions for a particular tag have been committed yet, and it's as correct as any other time, since no one can tell when a tag was made anyway. How just-in-time branch creation works: In order to make the "best" set of copies/deletes when creating a branch, cvs2svn keeps track of two sets of trees while it's making commits: 1. A skeleton mirror of the subversion repository, that is, an array of revisions, with a tree hanging off each revision. (The "array" is actually implemented as an anydbm database itself, mapping string representations of numbers to root keys.) 2. A tree for each CVS symbolic name, and the svn file/directory revisions from which various parts of that tree could be copied. Both tree sets live in anydbm databases, using the same basic schema: unique keys map to marshal.dumps() representations of dictionaries, which in turn map entry names to other unique keys: root_key ==> { entryname1 : entrykey1, entryname2 : entrykey2, ... } entrykey1 ==> { entrynameX : entrykeyX, ... } entrykey2 ==> { entrynameY : entrykeyY, ... } entrykeyX ==> { etc, etc ...} entrykeyY ==> { etc, etc ...} (The leaf nodes -- files -- are also dictionaries, for simplicity.) Both file and directory dictionaries store metadata under special keys whose names start with "/", so they can always be distinguished from entries (for example, search for "/mutable", "/openings", or "/closings" in cvs2svn.py). The repository mirror allows cvs2svn to remember what paths exist in what revisions. For each file path in a revision, it records what tags and branches can sprout from that revision; when the file changes, these attributes do not propagate to the new revision, since the symbolic name isn't based on that revision. The symbolic name trees are all stored in one db file, as paths, where the first element in each path is the symbolic name, and the rest is the full Subversion path to the file in question. For example, if the Subversion revision 7 is the root of branch 'Rel_1', this fact would be recorded under the path '/Rel_1/myproj/trunk/lib/driver.c' (the exact layout is dependent on the make_path() function in cvs2svn.py, which may change). root_key ==> { 'Rel_1' : 'a', ... } 'a' ==> { 'myproj' : 'b', ... } 'b' ==> { 'trunk : 'c', ... } 'c' ==> { 'lib' : 'd', ... } 'd' ==> { 'driver.c' : 'e', ... } 'e' ==> { } The source revision is stored in the leaf node, and also in all the parent nodes, in the manner described in the class documentation for 'SymbolicNameTracker'. The special entries "/opening" and "/closing" are not shown above, for brevity, but their values are where the revision ranges are stored (that is, the ranges indicating when this path could be copied from to produce the tag or branch in question). When it's time to create a branch or tag, cvs2svn.py walks the appropriate symbolic name tree, calculating the ideal source revision for each subpath (see 'SymbolicNameTracker' for the exact algorithm) and emitting the minimum number of copies to the dumpfile and to the skeleton repository mirror. As it goes, it marks each path as emitted, so that we don't redo the same copies during the cleanup phase later on. At this point, the entire branch is done except for: 1. Any source revisions that haven't yet been committed (this is a rare situation, but anyway such revisions will automatically be handled later by the same algorithm, invoked either due to another commit on the branch, or in the cleanup phase), and 2. Files that were accidentally copied onto the branch as part of a subtree, but which don't actually belong on the branch, because the corresponding CVS file doesn't contain that tag. We handle (2) by doing tree diffs between the newly copied tree in the skeleton repository mirror, and the corresponding portion of the symbolic name tree. If the skeleton mirror has a file that's not in the symbolic name tree, we emit a delete to the dumpfile and remove that path from the skeleton mirror. The cleanup phase happens after all regular changes have been processed. Just loop over the "root directory" of the symbolic name tree, running the same creation algorithm on each name (we'll have to distinguish between branches and tags, probably through a special entry on the directory object), skipping parts of the tree already marked as copied. Pass 5: ======= Load the dumpfile into Subversion. Voilą. -*- -*- -*- -*- -*- -*- -*- -*- -*- -*- -*- -*- -*- -*- -*- -*- -*- -*- Some older notes and ideas about cvs2svn. Not deleted, because they may contain suggestions for future improvements in design. ----------------------------------------------------------------------- An email from John Gardiner Myers about some considerations for the tool. ------ From: John Gardiner Myers Subject: Thoughts on CVS to SVN conversion To: gstein@lyra.org Date: Sun, 15 Apr 2001 17:47:10 -0700 Some things you may want to consider for a CVS to SVN conversion utility: If converting a CVS repository to SVN takes days, it would be good for the conversion utility to keep its progress state on disk. If the conversion fails halfway through due to a network outage or power failure, that would allow the conversion to be resumed where it left off instead of having to start over from an empty SVN repository. It is a short step from there to allowing periodic updates of a read-only SVN repository from a read/write CVS repository. This allows the more relaxed conversion procedure: 1) Create SVN repository writable only by the conversion tool. 2) Update SVN repository from CVS repository. 3) Announce the time of CVS to SVN cutover. 4) Repeat step (2) as needed. 5) Disable commits to CVS repository, making it read-only. 6) Repeat step (2). 7) Enable commits to SVN repository. 8) Wait for developers to move their workspaces to SVN. 9) Decomission the CVS repository. You may forward this message or parts of it as you seem fit. ------ ----------------------------------------------------------------------- Further design thoughts from Greg Stein * timestamp the beginning of the process. ignore any commits that occur after that timestamp; otherwise, you could miss portions of a commit (e.g. scan A; commit occurs to A and B; scan B; create SVN revision for items in B; we missed A) * the above timestamp can also be used for John's "grab any updates that were missed in the previous pass." * for each file processed, watch out for simultaneous commits. this may cause a problem during the reading/scanning/parsing of the file, or the parse succeeds but the results are garbaged. this could be fixed with a CVS lock, but I'd prefer read-only access. algorithm: get the mtime before opening the file. if an error occurs during reading, and the mtime has changed, then restart the file. if the read is successful, but the mtime changed, then restart the file. * use a separate log to track unique branches and non-branched forks of revision history (Q: is it possible to create, say, 1.4.1.3 without a "real" branch?). this log can then be used to create a /branches/ directory in the SVN repository. Note: we want to determine some way to coalesce branches across files. It can't be based on name, though, since the same branch name could be used in multiple places, yet they are semantically different branches. Given files R, S, and T with branch B, we can tie those files' branch B into a "semantic group" whenever we see commit groups on a branch touching multiple files. Files that are have a (named) branch but no commits on it are simply ignored. For each "semantic group" of a branch, we'd create a branch based on their common ancestor, then make the changes on the children as necessary. For single-file commits to a branch, we could use heuristics (pathname analysis) to add these to a group (and log what we did), or we could put them in a "reject" kind of file for a human to tell us what to do (the human would edit a config file of some kind to instruct the converter). * if we have access to the CVSROOT/history, then we could process tags properly. otherwise, we can only use heuristics or configuration info to group up tags (branches can use commits; there are no commits associated with tags) * ideally, we store every bit of data from the ,v files to enable a complete restoration of the CVS repository. this could be done by storing properties with CVS revision numbers and stuff (i.e. all metadata not already embodied by SVN would go into properties) * how do we track the "states"? I presume "dead" is simply deleting the entry from SVN. what are the other legal states, and do we need to do anything with them? * where do we put the "description"? how about locks, access list, keyword flags, etc. * note that using something like the SourceForge repository will be an ideal test case. people *move* their repositories there, which means that all kinds of stuff can be found in those repositories, from wherever people used to run them, and under whatever development policies may have been used. For example: I found one of the projects with a "permissions 644;" line in the "gnuplot" repository. Most RCS releases issue warnings about that (although they properly handle/skip the lines).