scionutil: use "sciondFromIA" logic if no default.sock

Seems simple enough, no need to specify this flag explicitly. Note that
the fallback could be either way (i.e. prefer default.sock or prefer
ia-specific name), as these cases are typically exclusive (either
running single AS or local topology with many ASes).

This change allows to run e.g. lib/shttp/examples/minimal on a local
topology.

Author: matzf

lib/scionutil/initialization.go

@@ -15,14 +15,16 @@
 package scionutil
 
 import (
+	"github.com/scionproto/scion/go/lib/addr"
 	"github.com/scionproto/scion/go/lib/sciond"
 	"github.com/scionproto/scion/go/lib/snet"
+	"os"
 )
 
 // InitSCION initializes the default SCION networking context with the provided SCION address
 // and the default SCIOND/SCION dispatcher
 func InitSCION(localAddr *snet.Addr) error {
-	err := snet.Init(localAddr.IA, GetDefaultSCIOND(), GetDefaultDispatcher())
+	err := snet.Init(localAddr.IA, GetSCIONDPath(&localAddr.IA), GetDefaultDispatcher())
 	if err != nil {
 		return err
 	}
@@ -37,17 +39,23 @@ func InitSCIONString(localAddr string) (*snet.Addr, error) {
 		return nil, err
 	}
 
-	err = snet.Init(addr.IA, GetDefaultSCIOND(), GetDefaultDispatcher())
+	err = snet.Init(addr.IA, GetSCIONDPath(&addr.IA), GetDefaultDispatcher())
 	if err != nil {
 		return nil, err
 	}
 
 	return addr, nil
 }
 
-// GetDefaultSCIOND returns the path to the default SCION socket
-func GetDefaultSCIOND() string {
-	return sciond.GetDefaultSCIONDPath(nil)
+// GetSCIONDPath returns the path to the SCION socket.
+func GetSCIONDPath(ia *addr.IA) string {
+
+	// Use default.sock if exists:
+	if _, err := os.Stat(sciond.DefaultSCIONDPath); err == nil {
+		return sciond.DefaultSCIONDPath
+	}
+	// otherwise, use socket with ia name:
+	return sciond.GetDefaultSCIONDPath(ia)
 }
 
 // GetDefaultDispatcher returns the path to the default SCION dispatcher