transaction: automatically clean up empty abandoned txs

Summary:
When creating a transaction we now automatically clean up an existing (abandoned) transaction if it is empty. This seems safe since recover() should be a no-op (other than cleaning up the tx files).

I've seen multiple cases of empty transaction files due to commands crashing/being killed in a transaction (but before anything has been written).

Reviewed By: quark-zju

Differential Revision: D33482320

fbshipit-source-id: a6ef74a30de96c600385a701ab2ab61bb149afb9

eden/scm/edenscm/mercurial/localrepo.py

@@ -1665,12 +1665,21 @@ class localrepository(object):
         if tr is not None:
             return tr.nest()
 
-        # abort here if the journal already exists
-        if self.svfs.exists("journal"):
-            raise errormod.AbandonedTransactionFoundError(
-                _("abandoned transaction found"),
-                hint=_("run 'hg recover' to clean up transaction"),
-            )
+        try:
+            stat = self.svfs.stat("journal")
+        except FileNotFoundError:
+            # No existing transaction - this is the normal case.
+            pass
+        else:
+            if stat.st_size > 0:
+                # Non-empty transaction already exists - bail.
+                raise errormod.AbandonedTransactionFoundError(
+                    _("abandoned transaction found"),
+                    hint=_("run 'hg recover' to clean up transaction"),
+                )
+            else:
+                self.ui.status(_("cleaning up empty abandoned transaction\n"))
+                self.recover()
 
         idbase = b"%.40f#%f" % (random.random(), time.time())
         ha = hex(hashlib.sha1(idbase).digest())

eden/scm/tests/test-journal-exists.t

@@ -9,7 +9,7 @@
   $ newclientrepo foo test:repo_server book
   $ cd ../repo
 
-  $ touch .hg/store/journal
+  $ echo something > .hg/store/journal
 
   $ echo foo > a
   $ hg ci -Am0
@@ -19,3 +19,10 @@
 
   $ hg recover
   rolling back interrupted transaction
+  couldn't read journal entry 'something\n'!
+
+Empty journal is cleaned up automatically.
+  $ touch .hg/store/journal
+  $ hg ci -Am0
+  cleaning up empty abandoned transaction
+  rolling back interrupted transaction

eden/scm/tests/test-visibility.t

@@ -567,7 +567,7 @@ Migration up
   $ setconfig visibility.enabled=true
 
  (Test if the repo contains an abandoned transaction, the auto migration does not crash)
-  $ touch .hg/store/journal
+  $ echo something > .hg/store/journal
   $ hg debugedenimporthelper --get-manifest-node df4f53cec30af1e4f669102135076fd4f9673fcc
   switching to explicit tracking of visible commits
   4e7eb8574ed56675aa89d2b5abbced12d5688cef