Add ErrTxClosed error.

Commit/Rollback and mutable calls on Tx and Bucket now return ErrTxClosed
if the transaction has already been committed or rolled back. Non-mutable
calls have added an assertion to check if the transaction is closed which
will cause a panic. I don't want to introduce an error return for accessor
methods that are being used improperly so I think the panic is appropriate.

1 files changed, 4 insertions, 0 deletions

diff --git a/error.go b/error.go
index 7f867b2..f88f47e 100644
--- a/error.go
+++ b/error.go
@@ -20,6 +20,10 @@ var (
 	// read-only transaction.
 	ErrTxNotWritable = &Error{"tx not writable", nil}
 
+	// ErrTxClosed is returned when committing or rolling back a transaction
+	// that has already been committed or rolled back.
+	ErrTxClosed = &Error{"tx closed", nil}
+
 	// ErrBucketNotFound is returned when trying to access a bucket that has
 	// not been created yet.
 	ErrBucketNotFound = &Error{"bucket not found", nil}