diff mercurial/branchmap.py @ 51536:718f28ea3af4

branchcache: add a "pure topological head" fast path

In a narrow but actually quick common case, all topological heads are all on
the same branch and all open. In this case, computing the branch map is very
simple. We can quickly detect situation where this situation will not change.

So we update the V3 format to be able to express this situation and upgrade the
update code to detect we remains in that mode.

The branch cache is populated with the actual value when the branch map is
accessed, but the update_disk method can do the update without needing to
populate it.

--- a/mercurial/branchmap.py	Wed Mar 06 16:18:03 2024 +0100
+++ b/mercurial/branchmap.py	Thu Mar 07 04:15:23 2024 +0100
@@ -62,6 +62,7 @@
     def __getitem__(self, repo):
         self.updatecache(repo)
         bcache = self._per_filter[repo.filtername]
+        bcache._ensure_populated(repo)
         assert bcache._filtername == repo.filtername, (
             bcache._filtername,
             repo.filtername,
@@ -484,6 +485,9 @@
     def _compute_key_hashes(self, repo) -> Tuple[bytes]:
         raise NotImplementedError
 
+    def _ensure_populated(self, repo):
+        """make sure any lazily loaded values are fully populated"""
+
     def validfor(self, repo):
         """check that cache contents are valid for (a subset of) this repo
 
@@ -861,7 +865,18 @@
     _base_filename = b"branch3"
     _default_key_hashes = (None, None)
 
-    def _get_topo_heads(self, repo) -> List[int]:
+    def __init__(self, *args, pure_topo_branch=None, **kwargs):
+        super().__init__(*args, **kwargs)
+        self._pure_topo_branch = pure_topo_branch
+        self._needs_populate = self._pure_topo_branch is not None
+
+    def inherit_for(self, repo):
+        new = super().inherit_for(repo)
+        new._pure_topo_branch = self._pure_topo_branch
+        new._needs_populate = self._needs_populate
+        return new
+
+    def _get_topo_heads(self, repo):
         """returns the topological head of a repoview content up to self.tiprev"""
         cl = repo.changelog
         if self.tiprev == nullrev:
@@ -883,22 +898,33 @@
                 cache_keys[b"filtered-hash"] = hex(self.key_hashes[0])
             if self.key_hashes[1] is not None:
                 cache_keys[b"obsolete-hash"] = hex(self.key_hashes[1])
+        if self._pure_topo_branch is not None:
+            cache_keys[b"topo-mode"] = b"pure"
         pieces = (b"%s=%s" % i for i in sorted(cache_keys.items()))
         fp.write(b" ".join(pieces) + b'\n')
+        if self._pure_topo_branch is not None:
+            label = encoding.fromlocal(self._pure_topo_branch)
+            fp.write(label + b'\n')
 
     def _write_heads(self, repo, fp) -> int:
         """write list of heads to a file
 
         Return the number of heads written."""
         nodecount = 0
-        topo_heads = set(self._get_topo_heads(repo))
+        topo_heads = None
+        if self._pure_topo_branch is None:
+            topo_heads = set(self._get_topo_heads(repo))
         to_rev = repo.changelog.index.rev
         for label, nodes in sorted(self._entries.items()):
+            if label == self._pure_topo_branch:
+                # not need to write anything the header took care of that
+                continue
             label = encoding.fromlocal(label)
             for node in nodes:
-                rev = to_rev(node)
-                if rev in topo_heads:
-                    continue
+                if topo_heads is not None:
+                    rev = to_rev(node)
+                    if rev in topo_heads:
+                        continue
                 if node in self._closednodes:
                     state = b'c'
                 else:
@@ -916,6 +942,7 @@
         args = {}
         filtered_hash = None
         obsolete_hash = None
+        has_pure_topo_heads = False
         for k, v in cache_keys.items():
             if k == b"tip-rev":
                 args["tiprev"] = int(v)
@@ -925,16 +952,28 @@
                 filtered_hash = bin(v)
             elif k == b"obsolete-hash":
                 obsolete_hash = bin(v)
+            elif k == b"topo-mode":
+                if v == b"pure":
+                    has_pure_topo_heads = True
+                else:
+                    msg = b"unknown topo-mode: %r" % v
+                    raise ValueError(msg)
             else:
                 msg = b"unknown cache key: %r" % k
                 raise ValueError(msg)
         args["key_hashes"] = (filtered_hash, obsolete_hash)
+        if has_pure_topo_heads:
+            pure_line = next(lineiter).rstrip(b'\n')
+            args["pure_topo_branch"] = encoding.tolocal(pure_line)
         return args
 
     def _load_heads(self, repo, lineiter):
         """fully loads the branchcache by reading from the file using the line
         iterator passed"""
         super()._load_heads(repo, lineiter)
+        if self._pure_topo_branch is not None:
+            # no need to read the repository heads, we know their value already.
+            return
         cl = repo.changelog
         getbranchinfo = repo.revbranchcache().branchinfo
         obsrevs = obsolete.getrevs(repo, b'obsolete')
@@ -960,6 +999,62 @@
             self.tiprev,
         )
 
+    def _process_new(
+        self,
+        repo,
+        newbranches,
+        new_closed,
+        obs_ignored,
+        max_rev,
+    ) -> None:
+        if (
+            # note: the check about `obs_ignored` is too strict as the
+            # obsolete revision could be non-topological, but lets keep
+            # things simple for now
+            #
+            # The same apply to `new_closed` if the closed changeset are
+            # not a head, we don't care that it is closed, but lets keep
+            # things simple here too.
+            not (obs_ignored or new_closed)
+            and (
+                not newbranches
+                or (
+                    len(newbranches) == 1
+                    and (
+                        self.tiprev == nullrev
+                        or self._pure_topo_branch in newbranches
+                    )
+                )
+            )
+        ):
+            if newbranches:
+                assert len(newbranches) == 1
+                self._pure_topo_branch = list(newbranches.keys())[0]
+                self._needs_populate = True
+                self._entries.pop(self._pure_topo_branch, None)
+            return
+
+        self._ensure_populated(repo)
+        self._pure_topo_branch = None
+        super()._process_new(
+            repo,
+            newbranches,
+            new_closed,
+            obs_ignored,
+            max_rev,
+        )
+
+    def _ensure_populated(self, repo):
+        """make sure any lazily loaded values are fully populated"""
+        if self._needs_populate:
+            assert self._pure_topo_branch is not None
+            cl = repo.changelog
+            to_node = cl.node
+            topo_heads = self._get_topo_heads(repo)
+            heads = [to_node(r) for r in topo_heads]
+            self._entries[self._pure_topo_branch] = heads
+            self._needs_populate = False
+
 
 class remotebranchcache(_BaseBranchCache):
     """Branchmap info for a remote connection, should not write locally"""