Fix sl pull --rebase when default branch is "master"

Summary:
When `selectivepulldefault=main,master` and a repo's default branch is actually `"master"` (no `"main"` exists), `sl pull --rebase` would abort with "missing rebase destination" because `bookmarks.mainbookmark()` always returned the first config entry (`"main"`).

The fix is in `tweakdefaults.py`: the `pull --rebase` code path now tries each configured `selectivepulldefault` name when the first one doesn't exist in the repo. `mainbookmark()` itself is left unchanged (always returns `names[0]`) since many callers depend on it being a stable, side-effect-free config lookup — changing it to do `name in repo` caused regressions in mutation/visibility behavior (e.g., `test-pushrebase-remotenames.t`, `test-remotenames-pushto.t`).

Resolves #1116

Differential Revision: D99509206

fbshipit-source-id: 2bff968e3aee2502ef386616dbd5a73af9145b3f

Author: bherila

eden/scm/sapling/ext/tweakdefaults.py

@@ -275,6 +275,14 @@ def pull(orig, ui, repo, *args, **opts):
     if isrebase and not dest:
         # Not using main bookmark for non-rebase pull. See D38066104 and D38066103.
         dest = bookmarks.mainbookmark(repo)
+        if dest not in repo:
+            # mainbookmark() returns the first selectivepulldefault entry, which
+            # may not exist (e.g. config lists "main,master" but only "master"
+            # exists). Try each configured name before giving up.
+            for name in repo.ui.configlist("remotenames", "selectivepulldefault"):
+                if name in repo:
+                    dest = name
+                    break
         if dest not in repo:
             raise error.Abort(_("missing rebase destination - supply --dest / -d"))
 

eden/scm/tests/test-pull-rebase-master.t

@@ -0,0 +1,64 @@
+
+#require no-eden
+
+# coding=utf-8
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2.
+
+# Test that `sl pull --rebase` works when the default branch is "master"
+# and selectivepulldefault lists "main" first.
+# Regression test for https://github.com/facebook/sapling/issues/1116
+
+  $ export HGIDENTITY=sl
+  $ cat >> $HGRCPATH << 'EOF'
+  > [extensions]
+  > rebase=
+  > tweakdefaults=
+  > EOF
+
+# Set up server repo with "master" as the only bookmark (no "main").
+# Configure selectivepulldefault=main,master to simulate git-mode default.
+
+  $ newclientrepo repo
+  $ cd ..
+  $ echo a > repo/a
+  $ sl -R repo commit -qAm a
+  $ sl -R repo push -q -r . --to master --create
+
+# Clone with selectivepulldefault=main,master (main listed first, but only master exists)
+
+  $ newclientrepo clone repo_server master
+  $ setconfig remotenames.selectivepulldefault=main,master
+
+# Pull --rebase with no local changes should auto-detect "master" as destination
+
+  $ echo b > ../repo/b
+  $ sl -R ../repo commit -qAm b
+  $ sl -R ../repo push -q -r . --to master
+  $ sl pull --rebase
+  pulling from test:repo_server
+  searching for changes
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  nothing to rebase - fast-forwarded to master
+
+# Make a local commit and check pull --rebase still works without -d
+
+  $ echo x > x
+  $ sl commit -qAm x
+  $ echo c > ../repo/c
+  $ sl -R ../repo commit -qAm c
+  $ sl -R ../repo push -q -r . --to master
+  $ sl pull --rebase
+  pulling from test:repo_server
+  searching for changes
+  rebasing * "x" (glob)
+  $ sl log -G -T '{desc}'
+  @  x
+  │
+  o  c
+  │
+  o  b
+  │
+  o  a