Unix transport used when HTTP host is specified

[jamesmoessis]

Here is a demo repo that demonstrates the bug. You can toggle on/off the replace in the go.mod in order to switch the bug on and off. The example uses the docker client. This problem doesn't exist in 0.4.0.

There's a problem in master currently where it overrides the proto and doesn't reset it when a new scheme is applied. This had the effect of making request to a unix socket even though an http endpoint was specified for the docker engine.

A debugging session indicated the problem was from a removal of piece of code in sockets.go, which was changed here: https://github.com/docker/go-connections/pull/61/files#r923998601

The issue seems possibly related to the following which also had trouble making TCP connections:

• Bump go-connections for TLS 1.3 support  moby/moby#41084
• Overriding NextProtos can interfere with Application-Layer Protocol Negotiation or ALPN #95

[nicks]

I can repro this as well -

Repro Steps

1. Create the following test case:

package main

import (
	"context"
	"log"

	"github.com/docker/docker/client"
)

func main() {
	cli, _ := client.NewClientWithOpts(
		client.WithHost("tcp://localhost:5000"))

	info, err := cli.Info(context.Background())
	log.Printf("INFO %s %v\n", info.ServerVersion, err)
}

2. Use the following go.mod

module github.com/docker/bug

go 1.21.3

require github.com/docker/docker v24.0.7+incompatible

require (
	github.com/Microsoft/go-winio v0.6.1 // indirect
	github.com/distribution/reference v0.5.0 // indirect
	github.com/docker/distribution v2.8.3+incompatible // indirect
	github.com/docker/go-connections v0.4.1-0.20210727194412-58542c764a11 // indirect
	github.com/docker/go-units v0.5.0 // indirect
	github.com/gogo/protobuf v1.3.2 // indirect
	github.com/moby/term v0.5.0 // indirect
	github.com/morikuni/aec v1.0.0 // indirect
	github.com/opencontainers/go-digest v1.0.0 // indirect
	github.com/opencontainers/image-spec v1.0.2 // indirect
	github.com/pkg/errors v0.9.1 // indirect
	golang.org/x/mod v0.8.0 // indirect
	golang.org/x/sys v0.5.0 // indirect
	golang.org/x/time v0.3.0 // indirect
	golang.org/x/tools v0.6.0 // indirect
	gotest.tools/v3 v3.5.1 // indirect
)

3. Run go run main.go

Expected results:
The Docker API client will send a request to localhost:5000, or fail if docker isn't running on that port.

Actual results:
The Docker API client will send a request to unix://var/run/docker.sock

the problem goes away if you change the go.mod to go-connections v0.4.0

[nicks]

I think the fix is to update the TCP codepath to set DialContext to nil and DisableCompression to false -- effectively to "undo" http.Transport changes made by the Unix socket and npipe codepaths.

But I also don't 100% understand the contract of this function or how it's used