Add docs regarding Batch.Send deadlock potential

refs jackc#374

Author: jackc

batch.go

@@ -59,6 +59,24 @@ func (b *Batch) Queue(query string, arguments []interface{}, parameterOIDs []pgt
 // If the batch is created from a conn Object then All queries are wrapped
 // in a transaction. The transaction can optionally be configured with
 // txOptions. The context is in effect until the Batch is closed.
+//
+// Warning: Send writes all queued queries before reading any results. This can
+// cause a deadlock if an excessive number of queries are queued. It is highly
+// advisable to use a timeout context to protect against this possibility.
+// Unfortunately, this excessive number can vary based on operating system,
+// connection type (TCP or Unix domain socket), and type of query. Unix domain
+// sockets seem to be much more susceptible to this issue than TCP connections.
+// However, it usually is at least several thousand.
+//
+// The deadlock occurs when the batched queries to be sent are so large that the
+// PostgreSQL server cannot receive it all at once. PostgreSQL received some of
+// the queued queries and starts executing them. As PostgreSQL executes the
+// queries it sends responses back. pgx will not read any of these responses
+// until it has finished sending. Therefore, if all network buffers are full pgx
+// will not be able to finish sending the queries and PostgreSQL will not be
+// able to finish sending the responses.
+//
+// See https://github.com/jackc/pgx/issues/374.
 func (b *Batch) Send(ctx context.Context, txOptions *TxOptions) error {
 	if b.err != nil {
 		return b.err