socket: try removing if socket-location is a directory on Linux

Due to race-conditions between containers starting and the Docker
remote API being up, containers bind-mounting the docker-socket may
cause the socket-path to be created as a directory.

This patch will attempt to remove the directory in such situations.
Removing will fail if the directory is not empty.

MacOS does not allow us to detect that the path is a directory, and we'll
return immediately instead of retrying.

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

sockets/unix_socket.go

@@ -79,8 +79,18 @@ func WithChmod(mask os.FileMode) SockOption {
 
 // NewUnixSocketWithOpts creates a unix socket with the specified options
 func NewUnixSocketWithOpts(path string, opts ...SockOption) (net.Listener, error) {
+	// Using syscall.Unlink(), not os.Remove() to prevent deleting the socket if it's in use
 	if err := syscall.Unlink(path); err != nil && !os.IsNotExist(err) {
-		return nil, err
+		if err != syscall.EISDIR {
+			// On Linux, attempting to remove a directory returns syscall.EISDIR,
+			// in which case we try to remove the directory. MacOS does not return
+			// this error, so we'll return immediately, see:
+			// https://github.com/golang/go/blob/6b420169d798c7ebe733487b56ea5c3fa4aab5ce/src/os/file_unix.go#L300-L311
+			return nil, err
+		}
+		if err := syscall.Rmdir(path); err != nil {
+			return nil, err
+		}
 	}
 	mask := syscall.Umask(0777)
 	defer syscall.Umask(mask)

sockets/unix_socket_test.go

@@ -2,8 +2,11 @@ package sockets
 
 import (
 	"fmt"
+	"io/ioutil"
 	"net"
 	"os"
+	"path/filepath"
+	"runtime"
 	"testing"
 )
 
@@ -56,3 +59,48 @@ func TestUnixSocketWithOpts(t *testing.T) {
 	defer l.Close()
 	runTest(t, path, l, echoStr)
 }
+
+func TestUnixSocketConflictDirectory(t *testing.T) {
+	if runtime.GOOS == "darwin" {
+		t.Skip("not supported on macOS")
+	}
+	tmpDir, err := ioutil.TempDir("", t.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	t.Run("conflicting directory", func(t *testing.T){
+		path := filepath.Join(tmpDir, "test.sock")
+
+		// Create a conflicting directory at the socket location
+		err = os.MkdirAll(path, 0700)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		l, err := NewUnixSocketWithOpts(path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer l.Close()
+		runTest(t, path, l, "hello")
+	})
+
+	t.Run("conflicting file", func(t *testing.T){
+		// Create a conflicting file at the socket location
+		path := filepath.Join(tmpDir, "test2.sock")
+		f, err := os.Create(path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		f.Close()
+
+		l, err := NewUnixSocketWithOpts(path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer l.Close()
+		runTest(t, path, l, "hello")
+	})
+}