Fix race condition in CopyFrom

In case of an error it was possible for the goroutine that builds the
copy stream to still be running after CopyFrom returned. Since that
goroutine uses the connections ConnInfo data types to encode the copy
data it was possible for those types to be concurrently used in an
unsafe fashion.

CopyFrom will no longer return until that goroutine has completed.

copy_from.go

@@ -75,8 +75,11 @@ func (ct *copyFrom) run(ctx context.Context) (int64, error) {
 	}
 
 	r, w := io.Pipe()
+	doneChan := make(chan struct{})
 
 	go func() {
+		defer close(doneChan)
+
 		// Purposely NOT using defer w.Close(). See https://github.com/golang/go/issues/24283.
 		buf := ct.conn.wbuf
 
@@ -114,6 +117,9 @@ func (ct *copyFrom) run(ctx context.Context) (int64, error) {
 
 	commandTag, err := ct.conn.pgConn.CopyFrom(ctx, r, fmt.Sprintf("copy %s ( %s ) from stdin binary;", quotedTableName, quotedColumnNames))
 
+	r.Close()
+	<-doneChan
+
 	return commandTag.RowsAffected(), err
 }