fix: modify COPY prepare handling to use internal utility

conn.go

@@ -22,6 +22,7 @@ import (
 	"time"
 	"unicode"
 
+	"github.com/lib/pq/internal"
 	"github.com/lib/pq/oid"
 	"github.com/lib/pq/scram"
 )
@@ -857,7 +858,7 @@ func (cn *conn) Prepare(q string) (_ driver.Stmt, err error) {
 	}
 	defer cn.errRecover(&err)
 
-	if len(q) >= 4 && strings.EqualFold(q[:4], "COPY") {
+	if len(q) >= 4 && internal.StartsWithCOPY(q[:4]) {
 		s, err := cn.prepareCopyIn(q)
 		if err == nil {
 			cn.inCopy = true

internal/copy.go

@@ -0,0 +1,102 @@
+package internal
+
+func consumeWhitespace(input string, i int, n int) int {
+	for i < n && (input[i] == ' ' || input[i] == '\t' || input[i] == '\n' || input[i] == '\r') {
+		i++
+	}
+	return i
+}
+
+func consumeLineComment(input string, i int, n int) int {
+	i += 2 // skip '--'
+	for i < n && input[i] != '\n' {
+		i++
+	}
+	return i
+}
+
+func consumeBlockComment(input string, i int, n int) int {
+	i += 2 // skip '/*'
+	for i < n-1 {
+		if input[i] == '*' && input[i+1] == '/' {
+			i += 2
+			return i
+		}
+		i++
+	}
+	// Unterminated comment? Consider as done consuming.
+	return i
+}
+
+func StartsWithCOPY(input string) bool {
+	const (
+		Start = iota
+		WhitespaceOrComment
+		C
+		O
+		P
+		Y
+		Done
+		Fail
+	)
+
+	state := Start
+	i := 0
+	n := len(input)
+
+	for state != Done && state != Fail {
+		if i >= n {
+			if state == Y {
+				state = Done
+			} else {
+				state = Fail
+			}
+			break
+		}
+
+		switch state {
+		case Start, WhitespaceOrComment:
+			i = consumeWhitespace(input, i, n)
+			if i+1 < n && input[i] == '-' && input[i+1] == '-' {
+				i = consumeLineComment(input, i, n)
+			} else if i+1 < n && input[i] == '/' && input[i+1] == '*' {
+				i = consumeBlockComment(input, i, n)
+			} else if i < n {
+				switch input[i] {
+				case 'C', 'c':
+					state = C
+					i++
+				case ' ', '\t', '\n', '\r':
+					// handled in consumeWhitespace
+				default:
+					state = Fail
+				}
+			}
+		case C:
+			if i < n && (input[i] == 'O' || input[i] == 'o') {
+				state = O
+				i++
+			} else {
+				state = Fail
+			}
+		case O:
+			if i < n && (input[i] == 'P' || input[i] == 'p') {
+				state = P
+				i++
+			} else {
+				state = Fail
+			}
+		case P:
+			if i < n && (input[i] == 'Y' || input[i] == 'y') {
+				state = Y
+				i++
+			} else {
+				state = Fail
+			}
+		case Y:
+			state = Done
+		}
+	}
+
+	return state == Done
+}

internal/copy_test.go

@@ -0,0 +1,69 @@
+package internal
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestStartsWithCOPY(t *testing.T) {
+	tests := []struct {
+		input string
+		valid bool
+	}{
+		{
+			input: "COPY data;",
+			valid: true,
+		},
+		{
+			input: "   COPY",
+			valid: true,
+		},
+		{
+			input: "SELECT * FROM users;",
+			valid: false,
+		},
+		{
+			input: "-- comment only\n/* and another */COPY table",
+			valid: true,
+		},
+		{
+			input: "\n\n/* header */  COPY my_table FROM stdin;",
+			valid: true,
+		},
+		{
+			input: "  -- some comment\n /* block */  COPY table FROM stdin;",
+			valid: true,
+		},
+		{
+			input: "-- some comment not terminated on purpose (or not) COPY table FROM stdin;",
+			valid: false,
+		},
+		{
+			input: "-- COPY table FROM stdin;\nSELECT * FROM users;",
+			valid: false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.input, func(t *testing.T) {
+			valid := StartsWithCOPY(test.input)
+			if valid != test.valid {
+				t.Errorf("Expected %q to be %v, got %v", test.input, test.valid, valid)
+			}
+		})
+	}
+}
+
+func BenchmarkStartsWithCOPY(b *testing.B) {
+	sql := "  -- comment\n /* block */ COPY table FROM stdin;"
+	for i := 0; i < b.N; i++ {
+		_ = StartsWithCOPY(sql)
+	}
+}
+
+func BenchmarkEqualFold(b *testing.B) {
+	sql := "COPY table FROM stdin;"
+	for i := 0; i < b.N; i++ {
+		_ = strings.EqualFold(sql, "COPY")
+	}
+}