e2e: add a cuke feature to test compose errors with port conflicts

Signed-off-by: Nick Sieger <nick@nicksieger.com>

e2e/cucumber-features/port-conflict.feature

@@ -0,0 +1,28 @@
+Feature: Report port conflicts
+
+Background:
+    Given a compose file
+        """
+        services:
+          web:
+            image: nginx
+            ports:
+            - 31415:80
+        """
+    And I run "docker rm -f nginx-pi-31415"
+
+Scenario: Reports a port allocation conflict with another container
+    Given I run "docker run -d -p 31415:80 --name nginx-pi-31415 nginx"
+    When I run "compose up -d"
+    Then the output contains "port is already allocated"
+    And the exit code is 1
+
+Scenario: Reports a port conflict with some other process
+    Given a process listening on port 31415
+    When I run "compose up -d"
+    Then the output contains "address already in use"
+    And the exit code is 1
+
+Scenario: Cleanup
+    Given I run "docker rm -f nginx-pi-31415"
+

e2e/cucumber_test.go

@@ -19,6 +19,7 @@ package cucumber
 import (
 	"context"
 	"fmt"
+	"net"
 	"os"
 	"path/filepath"
 	"regexp"
@@ -87,6 +88,7 @@ func setup(s *godog.ScenarioContext) {
 	s.Step(`output contains "(.*)"$`, th.outputContains(true))
 	s.Step(`output does not contain "(.*)"$`, th.outputContains(false))
 	s.Step(`exit code is (\d+)$`, th.exitCodeIs)
+	s.Step(`a process listening on port (\d+)$`, th.listenerOnPort)
 }
 
 type testHelper struct {
@@ -174,3 +176,16 @@ func (th *testHelper) setDockerfile(dockerfileString string) error {
 	}
 	return nil
 }
+
+func (th *testHelper) listenerOnPort(port int) error {
+	l, err := net.Listen("tcp", fmt.Sprintf(":%d", port))
+	if err != nil {
+		return err
+	}
+
+	th.T.Cleanup(func() {
+		_ = l.Close()
+	})
+
+	return nil
+}