Improve mtree build message on an empty table

internal/consistency/mtree/merkle.go

@@ -1502,6 +1502,7 @@ func (m *MerkleTreeTask) BuildMtree() (err error) {
 	logger.Info("Getting row estimates from all nodes...")
 	var maxRows int64
 	var refNode map[string]any
+	successfulEstimates := 0
 	for _, nodeInfo := range m.ClusterNodes {
 		pool, err := auth.GetSizedClusterNodeConnection(nodeInfo, auth.ConnectionOptions{PoolSize: poolSize})
 		if err != nil {
@@ -1517,16 +1518,30 @@ func (m *MerkleTreeTask) BuildMtree() (err error) {
 			logger.Warn("Warning: Could not get row estimate from node %s: %v", nodeInfo["Name"], err)
 			continue
 		}
+		successfulEstimates++
 
-		if count > maxRows {
+		// Seed refNode on the first successful estimate so an all-empty table
+		// (every count == 0) still resolves a reference node instead of being
+		// mistaken for a total estimate failure below.
+		if refNode == nil || count > maxRows {
 			maxRows = count
 			refNode = nodeInfo
 		}
 	}
 
-	if refNode == nil {
+	if successfulEstimates == 0 {
+		for _, p := range pools {
+			p.Close()
+		}
 		return fmt.Errorf("could not determine a reference node; failed to get row estimates from all nodes")
 	}
+
+	if maxRows == 0 {
+		for _, p := range pools {
+			p.Close()
+		}
+		return fmt.Errorf("table %s has 0 rows on all nodes; add data before building a Merkle tree", m.QualifiedTableName)
+	}
 	logger.Info("Using node %s as the reference for defining block ranges.", refNode["Name"])
 
 	if len(blockRanges) == 0 {

tests/integration/mtree_empty_table_test.go

@@ -0,0 +1,59 @@
+// ///////////////////////////////////////////////////////////////////////////
+//
+// # ACE - Active Consistency Engine
+//
+// Copyright (C) 2023 - 2026, pgEdge (https://www.pgedge.com/)
+//
+// This software is released under the PostgreSQL License:
+// https://opensource.org/license/postgresql
+//
+// ///////////////////////////////////////////////////////////////////////////
+
+package integration
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+	"github.com/stretchr/testify/require"
+)
+
+// mtree build on a table that is empty on every node must fail with a clear,
+// actionable "0 rows on all nodes" message -- not the internal "could not
+// determine a reference node" error that leaves the user unsure whether the
+// table is missing, the nodes are unreachable, or something is broken.
+func TestBuildMtreeFailsClearlyOnEmptyTable(t *testing.T) {
+	ctx := context.Background()
+	tableName := "mtree_empty_table_test"
+	qualifiedTable := fmt.Sprintf("%s.%s", testSchema, tableName)
+	safeTable := pgx.Identifier{testSchema, tableName}.Sanitize()
+	nodes := []string{serviceN1, serviceN2}
+
+	// Create the table on both nodes but insert no rows.
+	for _, pool := range []*pgxpool.Pool{pgCluster.Node1Pool, pgCluster.Node2Pool} {
+		_, err := pool.Exec(ctx, "CREATE TABLE IF NOT EXISTS "+safeTable+" (id INT PRIMARY KEY, payload TEXT)") // nosemgrep
+		require.NoError(t, err)
+	}
+	t.Cleanup(func() {
+		for _, pool := range []*pgxpool.Pool{pgCluster.Node1Pool, pgCluster.Node2Pool} {
+			_, _ = pool.Exec(ctx, "DROP TABLE IF EXISTS "+safeTable) // nosemgrep
+		}
+	})
+
+	task := newTestMerkleTreeTask(t, qualifiedTable, nodes)
+	require.NoError(t, task.RunChecks(false))
+	require.NoError(t, task.MtreeInit())
+	t.Cleanup(func() { _ = task.MtreeTeardown() })
+
+	err := task.BuildMtree()
+	require.Error(t, err, "build on an empty table must fail")
+	require.Contains(t, err.Error(), "has 0 rows on all nodes",
+		"error must name the real cause: the table is empty")
+	require.Contains(t, err.Error(), qualifiedTable,
+		"error must name the offending table")
+	require.NotContains(t, err.Error(), "could not determine a reference node",
+		"the internal reference-node error must no longer leak for empty tables")
+}