phases: rework the logic of _pushdiscoveryphase to bound complexity

This rework the various graph traversal in _pushdiscoveryphase to keep the
complexity in check.

This is done though a couple of things:

- first, limiting the space we have to explore, for example, if we are not in
  publishing push, we don't need to consider remote draft roots that are also
  draft locally, as there is nothing to be moved there.

- avoid unbounded descendant computation, and use the faster "rev between"
  computation.

This provide a massive boost to performance when exchanging with repository with
a massive amount of draft, like mozilla-try:

### data-env-vars.name                             = mozilla-try-2023-03-22-zstd-sparse-revlog
  # benchmark.name                                 = hg.command.push
  # bin-env-vars.hg.flavor                         = default
  # bin-env-vars.hg.py-re2-module                  = default
  # benchmark.variants.explicit-rev                = all-out-heads
  # benchmark.variants.issue6528                   = disabled
  # benchmark.variants.protocol                    = ssh
  # benchmark.variants.reuse-external-delta-parent = default

 ## benchmark.variants.revs                        = any-1-extra-rev
before: 20.346590 seconds
after:  11.232059 seconds (-38.15%, -7.48 seconds)
 ## benchmark.variants.revs                        = any-100-extra-rev
before: 24.752051 seconds
after:  15.367412 seconds (-37.91%, -9.38 seconds)


After this changes, the push operation is still quite too slow. Some of this
can be attributed to general phases slowness (reading all the roots from disk
for example) and other know slowness (not using persistent-nodemap, branchmap,
tags, etc. We are also working on them, but with this series, phase discovery
during push no longer showing up in profile and this is a pretty nice and bit
low-hanging fruit out of the way.

### (same case as the above)
  # benchmark.variants.revs                        = any-1-extra-rev
pre-%ln-change: 44.235070
this-changeset: 11.232059 seconds (-74.61%, -33.00 seconds)
  # benchmark.variants.revs                        = any-100-extra-rev
pre-%ln-change: 49.234697
this-changeset: 15.367412 seconds (-68.79%, -33.87 seconds)


Note that with this change, the `hg push` performance is now much closer to the
`hg pull` performance, even it still lagging behind a bit. (and the overall
performance are still too slow).

### data-env-vars.name                           = mozilla-try-2023-03-22-ds2-pnm
  # benchmark.variants.explicit-rev              = all-out-heads
  # benchmark.variants.issue6528                 = disabled
  # benchmark.variants.protocol                  = ssh
  # benchmark.variants.pulled-delta-reuse-policy = default
  # bin-env-vars.hg.flavor                       = rust
 ## benchmark.variants.revs                      = any-1-extra-rev
hg.command.pull: 6.517450
hg.command.push: 11.219888
 ## benchmark.variants.revs                      = any-100-extra-rev
hg.command.pull: 10.160991
hg.command.push: 14.251107

### data-env-vars.name                           = mozilla-try-2023-03-22-zstd-sparse-revlog
  # bin-env-vars.hg.py-re2-module                = default
  # benchmark.variants.explicit-rev              = all-out-heads
  # benchmark.variants.issue6528                 = disabled
  # benchmark.variants.protocol                  = ssh
  # benchmark.variants.pulled-delta-reuse-policy = default
 ## bin-env-vars.hg.flavor                       = default
 ## benchmark.variants.revs                      = any-1-extra-rev
hg.command.pull: 8.577772
hg.command.push: 11.232059
 ## bin-env-vars.hg.flavor                       = default
 ## benchmark.variants.revs                      = any-100-extra-rev
hg.command.pull: 13.152976
hg.command.push: 15.367412
 ## bin-env-vars.hg.flavor                       = rust
 ## benchmark.variants.revs                      = any-1-extra-rev
hg.command.pull: 8.731982
hg.command.push: 11.178751
 ## bin-env-vars.hg.flavor                       = rust
 ## benchmark.variants.revs                      = any-100-extra-rev
hg.command.pull: 13.184236
hg.command.push: 15.620843

mercurial/exchange.py

@@ -621,8 +621,10 b' def _pushdiscoveryphase(pushop):'
 
     (computed for both success and failure case for changesets push)"""
     outgoing = pushop.outgoing
-    unfi = pushop.repo.unfiltered()
-    to_rev = unfi.changelog.index.rev
+    repo = pushop.repo
+    unfi = repo.unfiltered()
+    cl = unfi.changelog
+    to_rev = cl.index.rev
     remotephases = listkeys(pushop.remote, b'phases')
 
     if (
@@ -644,42 +646,43 b' def _pushdiscoveryphase(pushop):'
         pushop.fallbackoutdatedphases = []
         return
 
-    fallbackheads_rev = [to_rev(n) for n in pushop.fallbackheads]
-    futureheads_rev = [to_rev(n) for n in pushop.futureheads]
-
+    fallbackheads_rev = {to_rev(n) for n in pushop.fallbackheads}
     pushop.remotephases = phases.RemotePhasesSummary(
         pushop.repo,
         fallbackheads_rev,
         remotephases,
     )
-    droots = pushop.remotephases.draft_roots
-
-    extracond = b''
-    if not pushop.remotephases.publishing:
-        extracond = b' and public()'
-    revset = b'heads((%%ld::%%ld) %s)' % extracond
-    # Get the list of all revs draft on remote by public here.
-    # XXX Beware that revset break if droots is not strictly
-    # XXX root we may want to ensure it is but it is costly
-    fallback = list(unfi.set(revset, droots, fallbackheads_rev))
-    if not pushop.remotephases.publishing and pushop.publish:
-        future = list(
-            unfi.set(
-                b'%ld and (not public() or %ld::)', futureheads_rev, droots
-            )
-        )
-    elif not outgoing.missing:
-        future = fallback
+    droots = set(pushop.remotephases.draft_roots)
+
+    fallback_publishing = pushop.remotephases.publishing
+    push_publishing = pushop.remotephases.publishing or pushop.publish
+    missing_revs = {to_rev(n) for n in outgoing.missing}
+    drafts = unfi._phasecache.get_raw_set(unfi, phases.draft)
+
+    if fallback_publishing:
+        fallback_roots = droots - missing_revs
+        revset = b'heads(%ld::%ld)'
     else:
-        # adds changeset we are going to push as draft
-        #
-        # should not be necessary for publishing server, but because of an
-        # issue fixed in xxxxx we have to do it anyway.
-        missing_rev = [to_rev(n) for n in outgoing.missing]
-        fdroots = list(unfi.set(b'roots(%ld  + %ld::)', missing_rev, droots))
-        fdroots = [f.rev() for f in fdroots]
-        future = list(unfi.set(revset, fdroots, futureheads_rev))
-    pushop.outdatedphases = future
+        fallback_roots = droots - drafts
+        fallback_roots -= missing_revs
+        # Get the list of all revs draft on remote but public here.
+        revset = b'heads((%ld::%ld) and public())'
+    if not fallback_roots:
+        fallback = fallback_rev = []
+    else:
+        fallback_rev = unfi.revs(revset, fallback_roots, fallbackheads_rev)
+        fallback = [repo[r] for r in fallback_rev]
+
+    if push_publishing:
+        published = missing_revs.copy()
+    else:
+        published = missing_revs - drafts
+    if pushop.publish:
+        published.update(fallbackheads_rev & drafts)
+    elif fallback:
+        published.update(fallback_rev)
+
+    pushop.outdatedphases = [repo[r] for r in cl.headrevs(published)]
     pushop.fallbackoutdatedphases = fallback