ovsdb-idl: Simplify transaction retry.

Originally the IDL transaction state machine had a return value
TXN_TRY_AGAIN to signal the client to wait for a change in the database and
then retry its transaction.  However, this logic was incomplete, because
it was possible for the database to change before the reply to the
transaction RPC was received, in which case the client would wait for a
further change.  Commit 4fdfe5ccf8 (ovsdb-idl: Prevent occasional hang
when multiple database clients race.) fixed the problem by breaking
TXN_TRY_AGAIN into two status codes, TXN_AGAIN_WAIT that meant to wait for
a further change and TXN_AGAIN_NOW that meant that a change had already
occurred so try again immediately.

This is correct enough, but it is more complicated than necessary.  It is
simpler and just as correct to use a single "try again" status that
requires the client to wait for a change relative to the database contents
*before* the transaction was committed.  This commit makes that change.
It also changes ovsdb_idl_run()'s return type from bool to void because
its return type is hardly useful anymore.

Signed-off-by: Ben Pfaff <blp@nicira.com>

python/ovs/db/idl.py

@@ -447,7 +447,7 @@ class Idl:
     def __txn_abort_all(self):
         while self._outstanding_txns:
             txn = self._outstanding_txns.popitem()[1]
-            txn._status = Transaction.AGAIN_WAIT
+            txn._status = Transaction.TRY_AGAIN
 
     def __txn_process_reply(self, msg):
         txn = self._outstanding_txns.pop(msg.id, None)
@@ -567,9 +567,7 @@ class Row(object):
         if 'column_name' changed in this row (or if this row was deleted)
         between the time that the IDL originally read its contents and the time
         that the transaction commits, then the transaction aborts and
-        Transaction.commit() returns Transaction.AGAIN_WAIT or
-        Transaction.AGAIN_NOW (depending on whether the database change has
-        already been received).
+        Transaction.commit() returns Transaction.TRY_AGAIN.
 
         The intention is that, to ensure that no transaction commits based on
         dirty reads, an application should call Row.verify() on each data item
@@ -628,12 +626,10 @@ class Transaction(object):
     INCOMPLETE = "incomplete"    # Commit in progress, please wait.
     ABORTED = "aborted"          # ovsdb_idl_txn_abort() called.
     SUCCESS = "success"          # Commit successful.
-    AGAIN_WAIT = "wait then try again"
-                                 # Commit failed because a "verify" operation
+    TRY_AGAIN = "try again"      # Commit failed because a "verify" operation
                                  # reported an inconsistency, due to a network
                                  # problem, or other transient failure.  Wait
                                  # for a change, then try again.
-    AGAIN_NOW = "try again now"  # Same as AGAIN_WAIT but try again right away.
     NOT_LOCKED = "not locked"    # Server hasn't given us the lock yet.
     ERROR = "error"              # Commit failed due to a hard error.
 
@@ -839,7 +835,7 @@ class Transaction(object):
                 self.idl._outstanding_txns[self._request_id] = self
                 self._status = Transaction.INCOMPLETE
             else:
-                self._status = Transaction.AGAIN_WAIT
+                self._status = Transaction.TRY_AGAIN
 
         self.__disassemble()
         return self._status
@@ -994,10 +990,7 @@ class Transaction(object):
             elif lock_errors:
                 self._status = Transaction.NOT_LOCKED
             elif soft_errors:
-                if self._commit_seqno == self.idl.change_seqno:
-                    self._status = Transaction.AGAIN_WAIT
-                else:
-                    self._status = Transaction.AGAIN_NOW
+                self._status = Transaction.TRY_AGAIN
             else:
                 self._status = Transaction.SUCCESS