rpc: avoid use of cgo by hard-coding maxPathSize (ethereum#27447)

Package rpc uses cgo to find the maximum UNIX domain socket path 
length. If exceeded, a warning is printed. This is the only use of cgo in this
package. It seems excessive to depend on cgo just for this warning, so
we now hard-code the usual limit for Linux instead.

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

Author: 2 people

rpc/constants_unix.go

@@ -29,11 +29,17 @@ import (
 	"github.com/ethereum/go-ethereum/log"
 )
 
+const (
+	// On Linux, sun_path is 108 bytes in size
+	// see http://man7.org/linux/man-pages/man7/unix.7.html
+	maxPathSize = int(108)
+)
+
 // ipcListen will create a Unix socket on the given endpoint.
 func ipcListen(endpoint string) (net.Listener, error) {
 	// account for null-terminator too
-	if len(endpoint)+1 > int(max_path_size) {
-		log.Warn(fmt.Sprintf("The ipc endpoint is longer than %d characters. ", max_path_size-1),
+	if len(endpoint)+1 > maxPathSize {
+		log.Warn(fmt.Sprintf("The ipc endpoint is longer than %d characters. ", maxPathSize-1),
 			"endpoint", endpoint)
 	}