tags: improve documentation

The documentation for tags.py was making comprehension difficult.
This patch rewrites most of the comments in the file to make it easier
for mere mortals to understand what's going on.

mercurial/tags.py

@@ -1,348 +1,391
 # tags.py - read tag info from local repository
 #
 # Copyright 2009 Matt Mackall <[e-mail]>
 # Copyright 2009 Greg Ward <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 # Currently this module only deals with reading and caching tags.
 # Eventually, it could take care of updating (adding/removing/moving)
 # tags too.
 
 from node import nullid, bin, hex, short
 from i18n import _
 import util
 import encoding
 import error
 import errno
 import time
 
+# The tags cache stores information about heads and the history of tags.
+#
+# The cache file consists of two parts. The first part maps head nodes
+# to .hgtags filenodes. The second part is a history of tags. The two
+# parts are separated by an empty line.
+#
+# The first part consists of lines of the form:
+#
+#   <headrev> <headnode> [<hgtagsnode>]
+#
+# <headrev> is an integer revision and <headnode> is a 40 character hex
+# node for that changeset. These redundantly identify a repository
+# head from the time the cache was written.
+#
+# <tagnode> is the filenode of .hgtags on that head. Heads with no .hgtags
+# file will have no <hgtagsnode> (just 2 values per line).
+#
+# The filenode cache is ordered from tip to oldest (which is part of why
+# <headrev> is there: a quick check of the tip from when the cache was
+# written against the current tip is all that is needed to check whether
+# the cache is up to date).
+#
+# The purpose of the filenode cache is to avoid the most expensive part
+# of finding global tags, which is looking up the .hgtags filenode in the
+# manifest for each head. This can take over a minute on repositories
+# that have large manifests and many heads.
+#
+# The second part of the tags cache consists of lines of the form:
+#
+#   <node> <tag>
+#
+# (This format is identical to that of .hgtags files.)
+#
+# <tag> is the tag name and <node> is the 40 character hex changeset
+# the tag is associated with.
+#
+# Tags are written sorted by tag name.
+#
+# Tags associated with multiple changesets have an entry for each changeset.
+# The most recent changeset (in terms of revlog ordering for the head
+# setting it) for each tag is last.
+
 def findglobaltags(ui, repo, alltags, tagtypes):
-    '''Find global tags in repo by reading .hgtags from every head that
-    has a distinct version of it, using a cache to avoid excess work.
-    Updates the dicts alltags, tagtypes in place: alltags maps tag name
-    to (node, hist) pair (see _readtags() below), and tagtypes maps tag
-    name to tag type ("global" in this case).'''
+    '''Find global tags in a repo.
+
+    "alltags" maps tag name to (node, hist) 2-tuples.
+
+    "tagtypes" maps tag name to tag type. Global tags always have the
+    "global" tag type.
+
+    The "alltags" and "tagtypes" dicts are updated in place. Empty dicts
+    should be passed in.
+
+    The tags cache is read and updated as a side-effect of calling.
+    '''
     # This is so we can be lazy and assume alltags contains only global
     # tags when we pass it to _writetagcache().
     assert len(alltags) == len(tagtypes) == 0, \
            "findglobaltags() should be called first"
 
     (heads, tagfnode, cachetags, shouldwrite) = _readtagcache(ui, repo)
     if cachetags is not None:
         assert not shouldwrite
         # XXX is this really 100% correct?  are there oddball special
         # cases where a global tag should outrank a local tag but won't,
         # because cachetags does not contain rank info?
         _updatetags(cachetags, 'global', alltags, tagtypes)
         return
 
-    seen = set()                    # set of fnode
+    seen = set()  # set of fnode
     fctx = None
-    for head in reversed(heads):        # oldest to newest
+    for head in reversed(heads):  # oldest to newest
         assert head in repo.changelog.nodemap, \
                "tag cache returned bogus head %s" % short(head)
 
         fnode = tagfnode.get(head)
         if fnode and fnode not in seen:
             seen.add(fnode)
             if not fctx:
                 fctx = repo.filectx('.hgtags', fileid=fnode)
             else:
                 fctx = fctx.filectx(fnode)
 
             filetags = _readtags(ui, repo, fctx.data().splitlines(), fctx)
             _updatetags(filetags, 'global', alltags, tagtypes)
 
     # and update the cache (if necessary)
     if shouldwrite:
         _writetagcache(ui, repo, heads, tagfnode, alltags)
 
 def readlocaltags(ui, repo, alltags, tagtypes):
-    '''Read local tags in repo.  Update alltags and tagtypes.'''
+    '''Read local tags in repo. Update alltags and tagtypes.'''
     try:
         data = repo.vfs.read("localtags")
     except IOError, inst:
         if inst.errno != errno.ENOENT:
             raise
         return
 
     # localtags is in the local encoding; re-encode to UTF-8 on
     # input for consistency with the rest of this module.
     filetags = _readtags(
         ui, repo, data.splitlines(), "localtags",
         recode=encoding.fromlocal)
 
     # remove tags pointing to invalid nodes
     cl = repo.changelog
     for t in filetags.keys():
         try:
             cl.rev(filetags[t][0])
         except (LookupError, ValueError):
             del filetags[t]
 
     _updatetags(filetags, "local", alltags, tagtypes)
 
 def _readtaghist(ui, repo, lines, fn, recode=None, calcnodelines=False):
     '''Read tag definitions from a file (or any source of lines).
+
     This function returns two sortdicts with similar information:
+
     - the first dict, bintaghist, contains the tag information as expected by
       the _readtags function, i.e. a mapping from tag name to (node, hist):
         - node is the node id from the last line read for that name,
         - hist is the list of node ids previously associated with it (in file
-          order).  All node ids are binary, not hex.
+          order). All node ids are binary, not hex.
+
     - the second dict, hextaglines, is a mapping from tag name to a list of
       [hexnode, line number] pairs, ordered from the oldest to the newest node.
+
     When calcnodelines is False the hextaglines dict is not calculated (an
     empty dict is returned). This is done to improve this function's
     performance in cases where the line numbers are not needed.
     '''
 
     bintaghist = util.sortdict()
     hextaglines = util.sortdict()
     count = 0
 
     def warn(msg):
         ui.warn(_("%s, line %s: %s\n") % (fn, count, msg))
 
     for nline, line in enumerate(lines):
         count += 1
         if not line:
             continue
         try:
             (nodehex, name) = line.split(" ", 1)
         except ValueError:
             warn(_("cannot parse entry"))
             continue
         name = name.strip()
         if recode:
             name = recode(name)
         try:
             nodebin = bin(nodehex)
         except TypeError:
             warn(_("node '%s' is not well formed") % nodehex)
             continue
 
         # update filetags
         if calcnodelines:
             # map tag name to a list of line numbers
             if name not in hextaglines:
                 hextaglines[name] = []
             hextaglines[name].append([nodehex, nline])
             continue
         # map tag name to (node, hist)
         if name not in bintaghist:
             bintaghist[name] = []
         bintaghist[name].append(nodebin)
     return bintaghist, hextaglines
 
 def _readtags(ui, repo, lines, fn, recode=None, calcnodelines=False):
     '''Read tag definitions from a file (or any source of lines).
-    Return a mapping from tag name to (node, hist): node is the node id
-    from the last line read for that name, and hist is the list of node
-    ids previously associated with it (in file order).  All node ids are
-    binary, not hex.'''
+
+    Returns a mapping from tag name to (node, hist).
+
+    "node" is the node id from the last line read for that name. "hist"
+    is the list of node ids previously associated with it (in file order).
+    All node ids are binary, not hex.
+    '''
     filetags, nodelines = _readtaghist(ui, repo, lines, fn, recode=recode,
                                        calcnodelines=calcnodelines)
     for tag, taghist in filetags.items():
         filetags[tag] = (taghist[-1], taghist[:-1])
     return filetags
 
 def _updatetags(filetags, tagtype, alltags, tagtypes):
     '''Incorporate the tag info read from one file into the two
     dictionaries, alltags and tagtypes, that contain all tag
     info (global across all heads plus local).'''
 
     for name, nodehist in filetags.iteritems():
         if name not in alltags:
             alltags[name] = nodehist
             tagtypes[name] = tagtype
             continue
 
         # we prefer alltags[name] if:
         #  it supersedes us OR
         #  mutual supersedes and it has a higher rank
         # otherwise we win because we're tip-most
         anode, ahist = nodehist
         bnode, bhist = alltags[name]
         if (bnode != anode and anode in bhist and
             (bnode not in ahist or len(bhist) > len(ahist))):
             anode = bnode
         else:
             tagtypes[name] = tagtype
         ahist.extend([n for n in bhist if n not in ahist])
         alltags[name] = anode, ahist
 
+def _readtagcache(ui, repo):
+    '''Read the tag cache.
 
-# The tag cache only stores info about heads, not the tag contents
-# from each head.  I.e. it doesn't try to squeeze out the maximum
-# performance, but is simpler has a better chance of actually
-# working correctly.  And this gives the biggest performance win: it
-# avoids looking up .hgtags in the manifest for every head, and it
-# can avoid calling heads() at all if there have been no changes to
-# the repo.
+    Returns a tuple (heads, fnodes, cachetags, shouldwrite).
+
+    If the cache is completely up-to-date, "cachetags" is a dict of the
+    form returned by _readtags() and "heads" and "fnodes" are None and
+    "shouldwrite" is False.
 
-def _readtagcache(ui, repo):
-    '''Read the tag cache and return a tuple (heads, fnodes, cachetags,
-    shouldwrite).  If the cache is completely up-to-date, cachetags is a
-    dict of the form returned by _readtags(); otherwise, it is None and
-    heads and fnodes are set.  In that case, heads is the list of all
-    heads currently in the repository (ordered from tip to oldest) and
-    fnodes is a mapping from head to .hgtags filenode.  If those two are
-    set, caller is responsible for reading tag info from each head.'''
+    If the cache is not up to date, "cachetags" is None. "heads" is a list
+    of all heads currently in the repository, ordered from tip to oldest.
+    "fnodes" is a mapping from head to .hgtags filenode. "shouldwrite" is
+    True.
+
+    If the cache is not up to date, the caller is responsible for reading tag
+    info from each returned head. (See findglobaltags().)
+    '''
 
     try:
         cachefile = repo.vfs('cache/tags', 'r')
         # force reading the file for static-http
         cachelines = iter(cachefile)
     except IOError:
         cachefile = None
 
-    # The cache file consists of lines like
-    #   <headrev> <headnode> [<tagnode>]
-    # where <headrev> and <headnode> redundantly identify a repository
-    # head from the time the cache was written, and <tagnode> is the
-    # filenode of .hgtags on that head.  Heads with no .hgtags file will
-    # have no <tagnode>.  The cache is ordered from tip to oldest (which
-    # is part of why <headrev> is there: a quick visual check is all
-    # that's required to ensure correct order).
-    #
-    # This information is enough to let us avoid the most expensive part
-    # of finding global tags, which is looking up <tagnode> in the
-    # manifest for each head.
-    cacherevs = []                      # list of headrev
-    cacheheads = []                     # list of headnode
-    cachefnode = {}                     # map headnode to filenode
+    cacherevs = []  # list of headrev
+    cacheheads = [] # list of headnode
+    cachefnode = {} # map headnode to filenode
     if cachefile:
         try:
             for line in cachelines:
                 if line == "\n":
                     break
                 line = line.split()
                 cacherevs.append(int(line[0]))
                 headnode = bin(line[1])
                 cacheheads.append(headnode)
                 if len(line) == 3:
                     fnode = bin(line[2])
                     cachefnode[headnode] = fnode
         except Exception:
             # corruption of the tags cache, just recompute it
             ui.warn(_('.hg/cache/tags is corrupt, rebuilding it\n'))
             cacheheads = []
             cacherevs = []
             cachefnode = {}
 
     tipnode = repo.changelog.tip()
     tiprev = len(repo.changelog) - 1
 
     # Case 1 (common): tip is the same, so nothing has changed.
     # (Unchanged tip trivially means no changesets have been added.
     # But, thanks to localrepository.destroyed(), it also means none
     # have been destroyed by strip or rollback.)
     if cacheheads and cacheheads[0] == tipnode and cacherevs[0] == tiprev:
         tags = _readtags(ui, repo, cachelines, cachefile.name)
         cachefile.close()
         return (None, None, tags, False)
     if cachefile:
         cachefile.close()               # ignore rest of file
 
     repoheads = repo.heads()
     # Case 2 (uncommon): empty repo; get out quickly and don't bother
     # writing an empty cache.
     if repoheads == [nullid]:
         return ([], {}, {}, False)
 
     # Case 3 (uncommon): cache file missing or empty.
 
     # Case 4 (uncommon): tip rev decreased.  This should only happen
     # when we're called from localrepository.destroyed().  Refresh the
     # cache so future invocations will not see disappeared heads in the
     # cache.
 
     # Case 5 (common): tip has changed, so we've added/replaced heads.
 
     # As it happens, the code to handle cases 3, 4, 5 is the same.
 
     # N.B. in case 4 (nodes destroyed), "new head" really means "newly
     # exposed".
     if not len(repo.file('.hgtags')):
         # No tags have ever been committed, so we can avoid a
         # potentially expensive search.
         return (repoheads, cachefnode, None, True)
 
     starttime = time.time()
 
     newheads = [head
                 for head in repoheads
                 if head not in set(cacheheads)]
 
     # Now we have to lookup the .hgtags filenode for every new head.
     # This is the most expensive part of finding tags, so performance
     # depends primarily on the size of newheads.  Worst case: no cache
     # file, so newheads == repoheads.
     for head in reversed(newheads):
         cctx = repo[head]
         try:
             fnode = cctx.filenode('.hgtags')
             cachefnode[head] = fnode
         except error.LookupError:
             # no .hgtags file on this head
             pass
 
     duration = time.time() - starttime
     ui.log('tagscache',
            'resolved %d tags cache entries from %d manifests in %0.4f '
            'seconds\n',
            len(cachefnode), len(newheads), duration)
 
     # Caller has to iterate over all heads, but can use the filenodes in
     # cachefnode to get to each .hgtags revision quickly.
     return (repoheads, cachefnode, None, True)
 
 def _writetagcache(ui, repo, heads, tagfnode, cachetags):
-
     try:
         cachefile = repo.vfs('cache/tags', 'w', atomictemp=True)
     except (OSError, IOError):
         return
 
     ui.log('tagscache', 'writing tags cache file with %d heads and %d tags\n',
             len(heads), len(cachetags))
 
     realheads = repo.heads()            # for sanity checks below
     for head in heads:
         # temporary sanity checks; these can probably be removed
         # once this code has been in crew for a few weeks
         assert head in repo.changelog.nodemap, \
                'trying to write non-existent node %s to tag cache' % short(head)
         assert head in realheads, \
                'trying to write non-head %s to tag cache' % short(head)
         assert head != nullid, \
                'trying to write nullid to tag cache'
 
         # This can't fail because of the first assert above.  When/if we
         # remove that assert, we might want to catch LookupError here
         # and downgrade it to a warning.
         rev = repo.changelog.rev(head)
 
         fnode = tagfnode.get(head)
         if fnode:
             cachefile.write('%d %s %s\n' % (rev, hex(head), hex(fnode)))
         else:
             cachefile.write('%d %s\n' % (rev, hex(head)))
 
     # Tag names in the cache are in UTF-8 -- which is the whole reason
     # we keep them in UTF-8 throughout this module.  If we converted
     # them local encoding on input, we would lose info writing them to
     # the cache.
     cachefile.write('\n')
     for (name, (node, hist)) in sorted(cachetags.iteritems()):
         for n in hist:
             cachefile.write("%s %s\n" % (hex(n), name))
         cachefile.write("%s %s\n" % (hex(node), name))
 
     try:
         cachefile.close()
     except (OSError, IOError):
         pass