feat(go): advance validation sidecar toward production

go-agent/internal/eventlog/read.go

@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package eventlog
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"os"
+)
+
+const maxRecordBytes = 1 << 20
+
+// ReadRecent returns at most limit records in chronological order, reading the
+// older rotated segment before the active file. Every returned row is valid
+// JSON; corruption is reported instead of passed to an operator as evidence.
+func ReadRecent(path string, limit int) ([][]byte, error) {
+	if path == "" {
+		return nil, fmt.Errorf("event log path is required")
+	}
+	if limit <= 0 {
+		return nil, fmt.Errorf("event limit must be positive")
+	}
+	buffer := recentBuffer{limit: limit}
+	if _, err := os.Stat(path + ".1"); err == nil {
+		if err := scanRecords(path+".1", buffer.add); err != nil {
+			return nil, err
+		}
+	} else if !os.IsNotExist(err) {
+		return nil, fmt.Errorf("stat rotated event log: %w", err)
+	}
+	if err := scanRecords(path, buffer.add); err != nil {
+		return nil, err
+	}
+	return buffer.values(), nil
+}
+
+func scanRecords(path string, consume func([]byte)) error {
+	file, err := os.Open(path)
+	if err != nil {
+		return fmt.Errorf("open event log %s: %w", path, err)
+	}
+	defer file.Close()
+	scanner := bufio.NewScanner(file)
+	scanner.Buffer(make([]byte, 64*1024), maxRecordBytes)
+	line := 0
+	for scanner.Scan() {
+		line++
+		raw := scanner.Bytes()
+		if len(raw) == 0 {
+			continue
+		}
+		if !json.Valid(raw) {
+			return fmt.Errorf("invalid JSON in %s at line %d", path, line)
+		}
+		consume(append([]byte(nil), raw...))
+	}
+	if err := scanner.Err(); err != nil {
+		return fmt.Errorf("read event log %s: %w", path, err)
+	}
+	return nil
+}
+
+type recentBuffer struct {
+	limit int
+	rows  [][]byte
+	next  int
+}
+
+func (b *recentBuffer) add(raw []byte) {
+	if len(b.rows) < b.limit {
+		b.rows = append(b.rows, raw)
+		return
+	}
+	b.rows[b.next] = raw
+	b.next = (b.next + 1) % b.limit
+}
+
+func (b *recentBuffer) values() [][]byte {
+	if len(b.rows) < b.limit || b.next == 0 {
+		return b.rows
+	}
+	result := make([][]byte, 0, len(b.rows))
+	result = append(result, b.rows[b.next:]...)
+	result = append(result, b.rows[:b.next]...)
+	return result
+}