Repository: go
Issue: proposal: net: populate source address on OpError when available
Labels: Proposal,Proposal-Hold

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### What version of Go are you using (`go version`)?

<pre>
$ go version
go version go1.16rc1 darwin/amd64
</pre>

### Does this issue reproduce with the latest release?
Yeppers


### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/andybons/Library/Caches/go-build"
GOENV="/Users/andybons/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/andybons/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/andybons/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16rc1"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/4w/rc3rq2s55w5fswy2n8kphy5r0000gn/T/go-build575792972=/tmp/go-build -gno-record-gcc-switches -fno-common"
</pre></details>

### What did you do?

When using `net.Dial` or its variants and a connection fails, it can be extremely helpful to know the source IP/port for diagnostic purposes, but it isn’t populated within the `OpError`. The below code shows the behavior in action:

```
// The Go Failing TCP Connection Challenge!
//
// Print the source IP address and port number of an outgoing TCP connection
// that times out and doesn't connect to the remote host.
//
// Rules:
//
// 1. You can't set the local address or port number.  The local address must
//    be 0.0.0.0 (INADDR_ANY), and the port number must be 0 (so the kernel
//    selects an ephemeral port).
//
// 2. You can't use the "unsafe" package.
//
// 3. Whatever you do needs to return a net.Conn.
package main

import (
	"fmt"
	"net"
	"time"
)

func main() {
	if conn, err := net.DialTimeout("tcp4", "google.com:7357", time.Second); err != nil {
		fmt.Printf("Connection timed out:\n%#v\n%q\n", err, err)
		// &net.OpError{Op:"dial", Net:"tcp4", Source:net.Addr(nil), Addr:(*net.TCPAddr)(0xc000112630), Err:(*net.timeoutError)(0x1209ce0)}
		// "dial tcp4 172.217.10.46:7357: i/o timeout" <-- shows destination but not source
	} else {
		fmt.Printf("Connection succeeded:\n%#v\n%v -> %v\n", conn, conn.LocalAddr(), conn.RemoteAddr())
	}
}
```


### What did you expect to see?

It would be nice if the `OpError` populated the source IP/port when it could, or allowed for clients to get access to it (say, if there were security concerns with exposing the source IP/port in the `OpError` by default if it may be blindly bubbled up to clients).

For reference, this is achieved through the C program below.

```
#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <netdb.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

int main() {
	struct addrinfo hints;
	memset(&hints, 0, sizeof hints);
	hints.ai_family = AF_INET;
	hints.ai_socktype = SOCK_STREAM;

	struct addrinfo *results;
	getaddrinfo("google.com", "7357", &hints, &results);

	int fd;
	struct addrinfo *r = results;

	fd = socket(r->ai_family, r->ai_socktype, r->ai_protocol);
	fcntl(fd, F_SETFL, fcntl(fd, F_GETFL, NULL) | O_NONBLOCK);
	connect(fd, r->ai_addr, r->ai_addrlen);

	struct sockaddr_in sal;
	socklen_t sal_len = sizeof sal;
	getsockname(fd, (struct sockaddr *)&sal, &sal_len);

	char la[256], ra[256];
	strlcpy(la, inet_ntoa(sal.sin_addr), 256);
	strlcpy(ra, inet_ntoa(((struct sockaddr_in *)(r->ai_addr))->sin_addr), 256);

	printf(
	  "%s:%u -> %s:%u\n",
	  la, ntohs(sal.sin_port),
	  ra, ntohs(((struct sockaddr_in *)(r->ai_addr))->sin_port)
	);
	return 0;
}
```


### What did you see instead?

Only a destination IP/port in the error.

/cc @rwg-stripe @bobby-stripe @bradfitz @rsc