fix: failed to migrate

vendor/github.com/jackc/pgx/v5/copy_from.go

@@ -85,6 +85,7 @@ type copyFrom struct {
 	columnNames   []string
 	rowSrc        CopyFromSource
 	readerErrChan chan error
+	mode          QueryExecMode
 }
 
 func (ct *copyFrom) run(ctx context.Context) (int64, error) {
@@ -105,9 +106,29 @@ func (ct *copyFrom) run(ctx context.Context) (int64, error) {
 	}
 	quotedColumnNames := cbuf.String()
 
-	sd, err := ct.conn.Prepare(ctx, "", fmt.Sprintf("select %s from %s", quotedColumnNames, quotedTableName))
-	if err != nil {
-		return 0, err
+	var sd *pgconn.StatementDescription
+	switch ct.mode {
+	case QueryExecModeExec, QueryExecModeSimpleProtocol:
+		// These modes don't support the binary format. Before the inclusion of the
+		// QueryExecModes, Conn.Prepare was called on every COPY operation to get
+		// the OIDs. These prepared statements were not cached.
+		//
+		// Since that's the same behavior provided by QueryExecModeDescribeExec,
+		// we'll default to that mode.
+		ct.mode = QueryExecModeDescribeExec
+		fallthrough
+	case QueryExecModeCacheStatement, QueryExecModeCacheDescribe, QueryExecModeDescribeExec:
+		var err error
+		sd, err = ct.conn.getStatementDescription(
+			ctx,
+			ct.mode,
+			fmt.Sprintf("select %s from %s", quotedColumnNames, quotedTableName),
+		)
+		if err != nil {
+			return 0, fmt.Errorf("statement description failed: %w", err)
+		}
+	default:
+		return 0, fmt.Errorf("unknown QueryExecMode: %v", ct.mode)
 	}
 
 	r, w := io.Pipe()
@@ -167,8 +188,13 @@ func (ct *copyFrom) run(ctx context.Context) (int64, error) {
 }
 
 func (ct *copyFrom) buildCopyBuf(buf []byte, sd *pgconn.StatementDescription) (bool, []byte, error) {
+	const sendBufSize = 65536 - 5 // The packet has a 5-byte header
+	lastBufLen := 0
+	largestRowLen := 0
 
 	for ct.rowSrc.Next() {
+		lastBufLen = len(buf)
+
 		values, err := ct.rowSrc.Values()
 		if err != nil {
 			return false, nil, err
@@ -185,7 +211,15 @@ func (ct *copyFrom) buildCopyBuf(buf []byte, sd *pgconn.StatementDescription) (b
 			}
 		}
 
-		if len(buf) > 65536 {
+		rowLen := len(buf) - lastBufLen
+		if rowLen > largestRowLen {
+			largestRowLen = rowLen
+		}
+
+		// Try not to overflow size of the buffer PgConn.CopyFrom will be reading into. If that happens then the nature of
+		// io.Pipe means that the next Read will be short. This can lead to pathological send sizes such as 65531, 13, 65531
+		// 13, 65531, 13, 65531, 13.
+		if len(buf) > sendBufSize-largestRowLen {
 			return true, buf, nil
 		}
 	}
@@ -208,6 +242,7 @@ func (c *Conn) CopyFrom(ctx context.Context, tableName Identifier, columnNames [
 		columnNames:   columnNames,
 		rowSrc:        rowSrc,
 		readerErrChan: make(chan error),
+		mode:          c.config.DefaultQueryExecMode,
 	}
 
 	return ct.run(ctx)