common/httpserver: listen on an abstract Unix socket

And make healthcheck command use it by default. This makes the
healthcheck command works whatever port the user has configured for the
HTTP service.

common/httpserver/root_test.go

@@ -4,10 +4,15 @@
 package httpserver_test
 
 import (
+	"context"
 	"fmt"
+	"io"
+	"net"
 	"net/http"
+	"runtime"
 	"testing"
 
+	"akvorado/common/daemon"
 	"akvorado/common/helpers"
 	"akvorado/common/httpserver"
 	"akvorado/common/reporter"
@@ -91,3 +96,49 @@ func TestGinRouterPanic(t *testing.T) {
 		},
 	})
 }
+
+func TestUnixSocket(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("unsupported OS")
+	}
+	r := reporter.NewMock(t)
+	config := httpserver.DefaultConfiguration()
+	config.Listen = ""
+	h, err := httpserver.New(r, "mock-unix-test", config, httpserver.Dependencies{Daemon: daemon.NewMock(t)})
+	if err != nil {
+		t.Fatalf("New() error:\n%+v", err)
+	}
+	helpers.StartStop(t, h)
+
+	h.AddHandler("/test",
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			fmt.Fprintf(w, "Hello !")
+		}))
+
+	// We should listen to both @akvorado and @akvorado/mock-unix-test. However,
+	// we may have some parallel tests and @akvorado may not be the handler we
+	// configured. Let's just test the second one.
+	httpc := http.Client{
+		Transport: &http.Transport{
+			DialContext: func(context.Context, string, string) (net.Conn, error) {
+				return net.Dial("unix", "@akvorado/mock-unix-test")
+			},
+		},
+	}
+	response, err := httpc.Get("http://unix/test")
+	if err != nil {
+		t.Fatalf("Get() error:\n%+v", err)
+	}
+	defer response.Body.Close()
+	if response.StatusCode != 200 {
+		t.Errorf("Get() status = %d instead of %d", response.StatusCode, 200)
+	}
+	body, err := io.ReadAll(response.Body)
+	if err != nil {
+		t.Fatalf("ReadAll() error:\n%+v", err)
+	}
+	expected := "Hello !"
+	if diff := helpers.Diff(string(body), expected); diff != "" {
+		t.Fatalf("Get() body (-got, +want):\n%s", diff)
+	}
+}