[tailscale] net: add SockTrace API

Loosely inspired by nettrace/httptrace, allows functions to be called
when sockets are read from or written to. The hooks are specified via
the context (with a WithSockTrace) function.

Only implemented for network sockets on POSIX systems.

Updates tailscale/corp#9230
Updates #58

api/go1.99999.txt

@@ -1,2 +1,7 @@
 pkg net, func SetDialEnforcer(func(context.Context, []Addr) error) #55
 pkg net, func SetResolveEnforcer(func(context.Context, string, string, string, Addr) error) #55
+pkg net, func WithSockTrace(context.Context, *SockTrace) context.Context #58
+pkg net, func ContextSockTrace(context.Context) *SockTrace #58
+pkg net, type SockTrace struct #58
+pkg net, type SockTrace struct, DidRead func(int) #58
+pkg net, type SockTrace struct, DidWrite func(int) #58

src/net/fd_posix.go

@@ -24,6 +24,11 @@ type netFD struct {
 	net         string
 	laddr       Addr
 	raddr       Addr
+
+	// hooks (if provided) are called after successful reads or writes with the
+	// number of bytes transferred.
+	readHook  func(int)
+	writeHook func(int)
 }
 
 func (fd *netFD) setAddr(laddr, raddr Addr) {
@@ -53,83 +58,125 @@ func (fd *netFD) closeWrite() error {
 
 func (fd *netFD) Read(p []byte) (n int, err error) {
 	n, err = fd.pfd.Read(p)
+	if fd.readHook != nil && err == nil {
+		fd.readHook(n)
+	}
 	runtime.KeepAlive(fd)
 	return n, wrapSyscallError(readSyscallName, err)
 }
 
 func (fd *netFD) readFrom(p []byte) (n int, sa syscall.Sockaddr, err error) {
 	n, sa, err = fd.pfd.ReadFrom(p)
+	if fd.readHook != nil && err == nil {
+		fd.readHook(n)
+	}
 	runtime.KeepAlive(fd)
 	return n, sa, wrapSyscallError(readFromSyscallName, err)
 }
 func (fd *netFD) readFromInet4(p []byte, from *syscall.SockaddrInet4) (n int, err error) {
 	n, err = fd.pfd.ReadFromInet4(p, from)
+	if fd.readHook != nil && err == nil {
+		fd.readHook(n)
+	}
 	runtime.KeepAlive(fd)
 	return n, wrapSyscallError(readFromSyscallName, err)
 }
 
 func (fd *netFD) readFromInet6(p []byte, from *syscall.SockaddrInet6) (n int, err error) {
 	n, err = fd.pfd.ReadFromInet6(p, from)
+	if fd.readHook != nil && err == nil {
+		fd.readHook(n)
+	}
 	runtime.KeepAlive(fd)
 	return n, wrapSyscallError(readFromSyscallName, err)
 }
 
 func (fd *netFD) readMsg(p []byte, oob []byte, flags int) (n, oobn, retflags int, sa syscall.Sockaddr, err error) {
 	n, oobn, retflags, sa, err = fd.pfd.ReadMsg(p, oob, flags)
+	if fd.readHook != nil && err == nil {
+		fd.readHook(n + oobn)
+	}
 	runtime.KeepAlive(fd)
 	return n, oobn, retflags, sa, wrapSyscallError(readMsgSyscallName, err)
 }
 
 func (fd *netFD) readMsgInet4(p []byte, oob []byte, flags int, sa *syscall.SockaddrInet4) (n, oobn, retflags int, err error) {
 	n, oobn, retflags, err = fd.pfd.ReadMsgInet4(p, oob, flags, sa)
+	if fd.readHook != nil && err == nil {
+		fd.readHook(n + oobn)
+	}
 	runtime.KeepAlive(fd)
 	return n, oobn, retflags, wrapSyscallError(readMsgSyscallName, err)
 }
 
 func (fd *netFD) readMsgInet6(p []byte, oob []byte, flags int, sa *syscall.SockaddrInet6) (n, oobn, retflags int, err error) {
 	n, oobn, retflags, err = fd.pfd.ReadMsgInet6(p, oob, flags, sa)
+	if fd.readHook != nil && err == nil {
+		fd.readHook(n + oobn)
+	}
 	runtime.KeepAlive(fd)
 	return n, oobn, retflags, wrapSyscallError(readMsgSyscallName, err)
 }
 
 func (fd *netFD) Write(p []byte) (nn int, err error) {
 	nn, err = fd.pfd.Write(p)
+	if fd.writeHook != nil && err == nil {
+		fd.writeHook(nn)
+	}
 	runtime.KeepAlive(fd)
 	return nn, wrapSyscallError(writeSyscallName, err)
 }
 
 func (fd *netFD) writeTo(p []byte, sa syscall.Sockaddr) (n int, err error) {
 	n, err = fd.pfd.WriteTo(p, sa)
+	if fd.writeHook != nil && err == nil {
+		fd.writeHook(n)
+	}
 	runtime.KeepAlive(fd)
 	return n, wrapSyscallError(writeToSyscallName, err)
 }
 
 func (fd *netFD) writeToInet4(p []byte, sa *syscall.SockaddrInet4) (n int, err error) {
 	n, err = fd.pfd.WriteToInet4(p, sa)
+	if fd.writeHook != nil && err == nil {
+		fd.writeHook(n)
+	}
 	runtime.KeepAlive(fd)
 	return n, wrapSyscallError(writeToSyscallName, err)
 }
 
 func (fd *netFD) writeToInet6(p []byte, sa *syscall.SockaddrInet6) (n int, err error) {
 	n, err = fd.pfd.WriteToInet6(p, sa)
+	if fd.writeHook != nil && err == nil {
+		fd.writeHook(n)
+	}
 	runtime.KeepAlive(fd)
 	return n, wrapSyscallError(writeToSyscallName, err)
 }
 
 func (fd *netFD) writeMsg(p []byte, oob []byte, sa syscall.Sockaddr) (n int, oobn int, err error) {
 	n, oobn, err = fd.pfd.WriteMsg(p, oob, sa)
+	if fd.writeHook != nil && err == nil {
+		fd.writeHook(n + oobn)
+	}
 	runtime.KeepAlive(fd)
 	return n, oobn, wrapSyscallError(writeMsgSyscallName, err)
 }
 
 func (fd *netFD) writeMsgInet4(p []byte, oob []byte, sa *syscall.SockaddrInet4) (n int, oobn int, err error) {
 	n, oobn, err = fd.pfd.WriteMsgInet4(p, oob, sa)
+	if fd.writeHook != nil && err == nil {
+		fd.writeHook(n + oobn)
+	}
 	runtime.KeepAlive(fd)
 	return n, oobn, wrapSyscallError(writeMsgSyscallName, err)
 }
 
 func (fd *netFD) writeMsgInet6(p []byte, oob []byte, sa *syscall.SockaddrInet6) (n int, oobn int, err error) {
 	n, oobn, err = fd.pfd.WriteMsgInet6(p, oob, sa)
+	if fd.writeHook != nil && err == nil {
+		fd.writeHook(n + oobn)
+	}
 	runtime.KeepAlive(fd)
 	return n, oobn, wrapSyscallError(writeMsgSyscallName, err)
 }

src/net/sock_posix.go

@@ -28,6 +28,10 @@ func socket(ctx context.Context, net string, family, sotype, proto int, ipv6only
 		poll.CloseFunc(s)
 		return nil, err
 	}
+	if trace := ContextSockTrace(ctx); trace != nil {
+		fd.readHook = trace.DidRead
+		fd.writeHook = trace.DidWrite
+	}
 
 	// This function makes a network file descriptor for the
 	// following applications:

src/net/socktrace.go

@@ -0,0 +1,43 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package net
+
+import (
+	"context"
+	"log"
+)
+
+// SockTrace is a set of hooks to run at various operations on a network socket.
+// Any particular hook may be nil. Functions may be called concurrently from
+// different goroutines.
+type SockTrace struct {
+	// DidRead is called after a successful read from the socket, where n bytes
+	// were read.
+	DidRead func(n int)
+	// DidWrite is called after a successful write to the socket, where n bytes
+	// were written.
+	DidWrite func(n int)
+}
+
+// WithSockTrace returns a new context based on the provided parent
+// ctx. Socket reads and writes made with the returned context will use
+// the provided trace hooks. Any previous hooks registered with ctx are
+// ovewritten.
+func WithSockTrace(ctx context.Context, trace *SockTrace) context.Context {
+	if trace := ContextSockTrace(ctx); trace != nil {
+		log.Printf("net: context already wrapped WithSockTrace, previous hooks with be overwritten")
+	}
+	return context.WithValue(ctx, sockTraceKey{}, trace)
+}
+
+// ContextSockTrace returns the SockTrace associated with the
+// provided context. If none, it returns nil.
+func ContextSockTrace(ctx context.Context) *SockTrace {
+	trace, _ := ctx.Value(sockTraceKey{}).(*SockTrace)
+	return trace
+}
+
+// unique type to prevent assignment.
+type sockTraceKey struct{}