Implement UDP Preflight

Add a preflight method that validates a set of provided write options against
the current stack state, possibly binding the endpoint if a `To` address is
provided as a write option. In Fuchsia, this method will be used along with a
cache to amortize the cost of validating send-path arguments across multiple
sends.

Fuchsia Bug: https://fxbug.dev/99501

PiperOrigin-RevId: 456777866

pkg/tcpip/tcpip.go

@@ -687,6 +687,16 @@ type Endpoint interface {
 	SocketOptions() *SocketOptions
 }
 
+// EndpointWithPreflight is the interface implemented by endpoints that need
+// to expose the `Preflight` method for preparing the endpoint prior to
+// calling `Write`.
+type EndpointWithPreflight interface {
+	// Prepares the endpoint for writes using the provided WriteOptions,
+	// returning an error if the options were incompatible with the endpoint's
+	// current state.
+	Preflight(WriteOptions) Error
+}
+
 // LinkPacketInfo holds Link layer information for a received packet.
 //
 // +stateify savable

pkg/tcpip/transport/udp/BUILD

@@ -59,6 +59,7 @@ go_test(
         "//pkg/tcpip/network/ipv6",
         "//pkg/tcpip/stack",
         "//pkg/tcpip/testutil",
+        "//pkg/tcpip/transport",
         "//pkg/tcpip/transport/icmp",
         "//pkg/tcpip/transport/testing/context",
         "//pkg/waiter",

pkg/tcpip/transport/udp/endpoint.go

@@ -15,6 +15,7 @@
 package udp
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"math"
@@ -344,6 +345,17 @@ func (e *endpoint) prepareForWriteInner(to *tcpip.FullAddress) (retry bool, err
 	return true, nil
 }
 
+var _ tcpip.EndpointWithPreflight = (*endpoint)(nil)
+
+// Validates the passed WriteOptions and prepares the endpoint for writes
+// using those options. If the endpoint is unbound and the `To` address
+// is specified, binds the endpoint to that address.
+func (e *endpoint) Preflight(opts tcpip.WriteOptions) tcpip.Error {
+	var r bytes.Reader
+	_, err := e.prepareForWrite(&r, opts)
+	return err
+}
+
 // Write writes data to the endpoint's peer. This method does not block
 // if the data cannot be written.
 func (e *endpoint) Write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, tcpip.Error) {