fix: while connected to MySQL if internet was lost no qso would be logged anymore

internal/db/connloss.go

@@ -0,0 +1,66 @@
+package db
+
+import (
+	"database/sql"
+	"database/sql/driver"
+	"errors"
+	"net"
+	"strings"
+
+	"github.com/go-sql-driver/mysql"
+)
+
+// IsConnLost reports whether err means the database was UNREACHABLE (network
+// down, server gone, dead pooled connection) as opposed to a legitimate
+// server-side SQL error (constraint violation, bad data, syntax).
+//
+// This distinction is the linchpin of the offline safety net: a QSO is parked in
+// the local ADIF outbox ONLY on a connection loss. If we queued on any error, a
+// genuine data bug would silently vanish into the file instead of being
+// reported — the worst possible outcome.
+//
+// A *mysql.MySQLError means the SERVER answered and rejected us, so it is never
+// a connection loss, whatever its code.
+func IsConnLost(err error) bool {
+	if err == nil {
+		return false
+	}
+	// The server responded → a real SQL error, not a lost link.
+	var me *mysql.MySQLError
+	if errors.As(err, &me) {
+		return false
+	}
+	if errors.Is(err, driver.ErrBadConn) || errors.Is(err, sql.ErrConnDone) || errors.Is(err, sql.ErrTxDone) {
+		return true
+	}
+	var ne net.Error
+	if errors.As(err, &ne) {
+		return true
+	}
+	var oe *net.OpError
+	if errors.As(err, &oe) {
+		return true
+	}
+	// The MySQL driver reports a few of these as plain strings.
+	s := strings.ToLower(err.Error())
+	for _, p := range []string{
+		"invalid connection",
+		"bad connection",
+		"connection refused",
+		"connection reset",
+		"broken pipe",
+		"no such host",
+		"i/o timeout",
+		"dial tcp",
+		"unexpected eof",
+		"driver: bad connection",
+		"can't connect",
+		"network is unreachable",
+		"host is unreachable",
+	} {
+		if strings.Contains(s, p) {
+			return true
+		}
+	}
+	return false
+}