net: never use backlog > 65535

The system call takes an int, but the kernel stores it in a uint16.
At least one Linux system sets /proc/sys/net/core/somaxconn
to 262144, which ends up being 0 in the uint16. Avoid being tricked.

FreeBSD sources also store the backlog in a uint16.
Assume the problem is systemic and fix it everywhere.

Fixes #5030.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7480046

src/pkg/net/sock_bsd.go

@@ -27,5 +27,11 @@ func maxListenerBacklog() int {
 	if n == 0 || err != nil {
 		return syscall.SOMAXCONN
 	}
+	// FreeBSD stores the backlog in a uint16, as does Linux.
+	// Assume the other BSDs do too. Truncate number to avoid wrapping.
+	// See issue 5030.
+	if n > 1<<16-1 {
+		n = 1<<16 - 1
+	}
 	return int(n)
 }

src/pkg/net/sock_linux.go

@@ -21,5 +21,11 @@ func maxListenerBacklog() int {
 	if n == 0 || !ok {
 		return syscall.SOMAXCONN
 	}
+	// Linux stores the backlog in a uint16.
+	// Truncate number to avoid wrapping.
+	// See issue 5030.
+	if n > 1<<16-1 {
+		n = 1<<16 - 1
+	}
 	return n
 }

src/pkg/net/sock_windows.go

@@ -8,6 +8,7 @@ import "syscall"
 
 func maxListenerBacklog() int {
 	// TODO: Implement this
+	// NOTE: Never return a number bigger than 1<<16 - 1. See issue 5030.
 	return syscall.SOMAXCONN
 }