setdiscovery: include tip in localheads if it's empty

Summary:
Previously we include tip in sample. But that is problematic for fast paths
like:

    if set(commonsample).issuperset(set(localheads) - {nullid}):
        ui.note(_("all local heads known remotely\n"))
        return localheads, True, remoteheads

If `localheads` is empty, then the returned "common heads" are empty, causing
downloading the entire repo inefficiently.

Fix the issue by moving `tip` from `sample` to `localheads` so the common
heads will include tip in the fast path.

Reviewed By: DurhamG

Differential Revision: D26374303

fbshipit-source-id: 45a2a44e4db4c4ec2a341522a257d46a62b058d5

eden/scm/edenscm/mercurial/setdiscovery.py

@@ -372,6 +372,13 @@ def _findcommonheadsnew(
     else:
         localheads = ancestorsof
 
+    # localheads can be empty in special case: after initial streamclone,
+    # because both remotenames and visible heads are empty. Ensure 'tip' is
+    # part of 'localheads' so we don't pull the entire repo.
+    # TODO: Improve clone protocol so streamclone transfers remote names.
+    if not localheads:
+        localheads = [local["tip"].node()]
+
     # Filter out 'nullid' immediately.
     localheads = sorted(h for h in localheads if h != nullid)
     unknown = set()
@@ -416,14 +423,15 @@ def _findcommonheadsnew(
     remotename = remotenameforurl(ui, remote.url())  # ex. 'default' or 'remote'
     selected = list(selectivepullbookmarknames(local, remotename))
 
-    # Include names that the server might have.  'tip' is useful during clone
-    # code path where remote names are not known yet.
-    # TODO: Improve clone protocol so streamclone transfers remote names.
-    for name in selected + ["tip"]:
+    # Include names (public heads) that the server might have in sample.
+    # This can efficiently extend the "common" set, if the server does
+    # have them.
+    for name in selected:
         if name in local:
             node = local[name].node()
             if node not in sample:
                 sample.add(node)
+
     # Drop nullid special case.
     sample.discard(nullid)
     sample = list(sample)

eden/scm/tests/test-setdiscovery-t84623377.t

@@ -0,0 +1,52 @@
+#chg-compatible
+
+  $ configure modern
+
+  $ newserver server
+
+Populate commits
+
+  $ clone server client1
+  $ cd client1
+  $ drawdag << 'EOS'
+  > B
+  > |
+  > A
+  > EOS
+  $ hg push -r $B --to master --create -q
+
+Check stream clone "pull"
+
+  $ hg clone --shallow -U "ssh://user@dummy/server" client2 --debug
+  running /home/quark/bin/python3 /data/users/quark/fbsource/fbcode/eden/scm/tests/dummyssh 'user@dummy' 'hg -R server serve --stdio'
+  sending hello command
+  sending between command
+  remote: 658
+  remote: capabilities: treeonly unbundle=HG10GZ,HG10BZ,HG10UN unbundlereplay streamreqs=generaldelta,lz4revlog,revlogv1 knownnodes pushkey batch listkeyspatterns branchmap bundle2=HG20%0Ab2x%253Ainfinitepush%0Ab2x%253Ainfinitepushmutation%0Ab2x%253Ainfinitepushscratchbookmarks%0Abookmarks%0Achangegroup%3D01%2C02%2C03%0Adigests%3Dmd5%2Csha1%2Csha512%0Aerror%3Dabort%2Cunsupportedcontent%2Cpushraced%2Cpushkey%0Alistkeys%0Aphases%3Dheads%0Apushkey%0Aremote-changegroup%3Dhttp%2Chttps%0Atreemanifest%3DTrue%0Atreemanifestserver%3DTrue%0Atreeonly%3DTrue known unbundlehash lookup getbundle changegroupsubset stream_option gettreepack remotefilelog getflogheads getfile
+  remote: 1
+  fetching changelog
+  running /home/quark/bin/python3 /data/users/quark/fbsource/fbcode/eden/scm/tests/dummyssh 'user@dummy' 'hg -R server serve --stdio'
+  sending hello command
+  sending between command
+  remote: 658
+  remote: capabilities: treeonly unbundle=HG10GZ,HG10BZ,HG10UN unbundlereplay streamreqs=generaldelta,lz4revlog,revlogv1 knownnodes pushkey batch listkeyspatterns branchmap bundle2=HG20%0Ab2x%253Ainfinitepush%0Ab2x%253Ainfinitepushmutation%0Ab2x%253Ainfinitepushscratchbookmarks%0Abookmarks%0Achangegroup%3D01%2C02%2C03%0Adigests%3Dmd5%2Csha1%2Csha512%0Aerror%3Dabort%2Cunsupportedcontent%2Cpushraced%2Cpushkey%0Alistkeys%0Aphases%3Dheads%0Apushkey%0Aremote-changegroup%3Dhttp%2Chttps%0Atreemanifest%3DTrue%0Atreemanifestserver%3DTrue%0Atreeonly%3DTrue known unbundlehash lookup getbundle changegroupsubset stream_option gettreepack remotefilelog getflogheads getfile
+  remote: 1
+  sending stream_out_shallow command
+  3 files to transfer, 465 bytes of data
+  adding 00manifesttree.i (227 bytes)
+  adding 00changelog.i (128 bytes)
+  adding 00changelog.d (110 bytes)
+  transferred 465 bytes in 0.0 seconds (454 KB/sec)
+  fetching selected remote bookmarks
+  reusing connection from pool
+  preparing listkeys for "bookmarks" with pattern "['master']"
+  sending listkeyspatterns command
+  received listkey for "bookmarks": 47 bytes
+  no changes found
+  sending getbundle command
+  bundle2-input-bundle: with-transaction
+  bundle2-input-part: "bookmarks" supported
+  bundle2-input-part: total payload size 28
+  bundle2-input-part: "phase-heads" supported
+  bundle2-input-part: total payload size 24
+  bundle2-input-bundle: 1 parts total