split: close transaction in the unlikely event of a conflict while rebasing

split: close transaction in the unlikely event of a conflict while rebasing

`hg split` *should* never result in conflicts, but in case there are
bugs, we should at least commit the transaction so they can continue
the rebase. One of our users ran into the regression fixed by
D10120. They fixed the conflict and the tried to continue the rebase,
but it failed with "abort: cannot continue inconsistent rebase"
because the rebase state referred to commits written in a transaction
that was never committed.

Side note: `hg split` should probably turn off copy tracing to reduce
the impact of such bugs, and to speed it up as well. Copies made in
the rebased commits should still be respected because `hg rebase`
calls `copies.graftcopies()`.

Differential Revision: https://phab.mercurial-scm.org/D10164

--- a/hgext/split.py	Mon Mar 15 13:05:00 2021 +0100
+++ b/hgext/split.py	Fri Mar 12 09:15:40 2021 -0800
@@ -27,6 +27,7 @@
     revsetlang,
     rewriteutil,
     scmutil,
+    util,
 )
 
 # allow people to use split without explicitly enabling rebase extension
@@ -69,57 +70,62 @@
     if opts.get(b'rev'):
         revlist.append(opts.get(b'rev'))
     revlist.extend(revs)
-    with repo.wlock(), repo.lock(), repo.transaction(b'split') as tr:
-        revs = scmutil.revrange(repo, revlist or [b'.'])
-        if len(revs) > 1:
-            raise error.InputError(_(b'cannot split multiple revisions'))
+    with repo.wlock(), repo.lock():
+        tr = repo.transaction(b'split')
+        # If the rebase somehow runs into conflicts, make sure
+        # we close the transaction so the user can continue it.
+        with util.acceptintervention(tr):
+            revs = scmutil.revrange(repo, revlist or [b'.'])
+            if len(revs) > 1:
+                raise error.InputError(_(b'cannot split multiple revisions'))
 
-        rev = revs.first()
-        ctx = repo[rev]
-        # Handle nullid specially here (instead of leaving for precheck()
-        # below) so we get a nicer message and error code.
-        if rev is None or ctx.node() == nullid:
-            ui.status(_(b'nothing to split\n'))
-            return 1
-        if ctx.node() is None:
-            raise error.InputError(_(b'cannot split working directory'))
+            rev = revs.first()
+            ctx = repo[rev]
+            # Handle nullid specially here (instead of leaving for precheck()
+            # below) so we get a nicer message and error code.
+            if rev is None or ctx.node() == nullid:
+                ui.status(_(b'nothing to split\n'))
+                return 1
+            if ctx.node() is None:
+                raise error.InputError(_(b'cannot split working directory'))
 
-        if opts.get(b'rebase'):
-            # Skip obsoleted descendants and their descendants so the rebase
-            # won't cause conflicts for sure.
-            descendants = list(repo.revs(b'(%d::) - (%d)', rev, rev))
-            torebase = list(
-                repo.revs(
-                    b'%ld - (%ld & obsolete())::', descendants, descendants
+            if opts.get(b'rebase'):
+                # Skip obsoleted descendants and their descendants so the rebase
+                # won't cause conflicts for sure.
+                descendants = list(repo.revs(b'(%d::) - (%d)', rev, rev))
+                torebase = list(
+                    repo.revs(
+                        b'%ld - (%ld & obsolete())::', descendants, descendants
+                    )
                 )
-            )
-        else:
-            torebase = []
-        rewriteutil.precheck(repo, [rev] + torebase, b'split')
+            else:
+                torebase = []
+            rewriteutil.precheck(repo, [rev] + torebase, b'split')
 
-        if len(ctx.parents()) > 1:
-            raise error.InputError(_(b'cannot split a merge changeset'))
+            if len(ctx.parents()) > 1:
+                raise error.InputError(_(b'cannot split a merge changeset'))
 
-        cmdutil.bailifchanged(repo)
+            cmdutil.bailifchanged(repo)
 
-        # Deactivate bookmark temporarily so it won't get moved unintentionally
-        bname = repo._activebookmark
-        if bname and repo._bookmarks[bname] != ctx.node():
-            bookmarks.deactivate(repo)
+            # Deactivate bookmark temporarily so it won't get moved
+            # unintentionally
+            bname = repo._activebookmark
+            if bname and repo._bookmarks[bname] != ctx.node():
+                bookmarks.deactivate(repo)
 
-        wnode = repo[b'.'].node()
-        top = None
-        try:
-            top = dosplit(ui, repo, tr, ctx, opts)
-        finally:
-            # top is None: split failed, need update --clean recovery.
-            # wnode == ctx.node(): wnode split, no need to update.
-            if top is None or wnode != ctx.node():
-                hg.clean(repo, wnode, show_stats=False)
-            if bname:
-                bookmarks.activate(repo, bname)
-        if torebase and top:
-            dorebase(ui, repo, torebase, top)
+            wnode = repo[b'.'].node()
+            top = None
+            try:
+                top = dosplit(ui, repo, tr, ctx, opts)
+            finally:
+                # top is None: split failed, need update --clean recovery.
+                # wnode == ctx.node(): wnode split, no need to update.
+                if top is None or wnode != ctx.node():
+                    hg.clean(repo, wnode, show_stats=False)
+                if bname:
+                    bookmarks.activate(repo, bname)
+            if torebase and top:
+                dorebase(ui, repo, torebase, top)
 
 
 def dosplit(ui, repo, tr, ctx, opts):