net: fix parsing of interfaces on plan9 without associated devices

Fixes #72060
Updates #39908

Change-Id: I7d5bda1654753acebc8aa9937d010b41c5722b36
Reviewed-on: https://go-review.googlesource.com/c/go/+/654055
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>

src/net/interface_plan9.go

@@ -57,6 +57,17 @@ func readInterface(i int) (*Interface, error) {
 	}
 
 	fields := getFields(line)
+
+	// If the interface has no device file then we see two spaces between "device" and
+	// "maxtu" and and getFields treats the two spaces as one delimiter.
+	// Insert a gap for the missing device name.
+	// See https://go.dev/issue/72060.
+	if stringslite.HasPrefix(line, "device  maxtu ") {
+		fields = append(fields, "")
+		copy(fields[2:], fields[1:])
+		fields[1] = ""
+	}
+
 	if len(fields) < 4 {
 		return nil, errors.New("invalid interface status file: " + ifcstat)
 	}
@@ -163,7 +174,7 @@ func interfaceAddrTable(ifi *Interface) ([]Addr, error) {
 		for line, ok := statusf.readLine(); ok; line, ok = statusf.readLine() {
 			fields := getFields(line)
 			if len(fields) < 1 {
-				return nil, errors.New("cannot parse IP address for interface: " + status)
+				continue
 			}
 			addr := fields[0]
 			ip := ParseIP(addr)