Fix URL construction problems when no trailing slash path is present

Fixes #75

internal/api.go

@@ -251,13 +251,11 @@ func (c *APIClient) ping(handle, rid, typePath string, body io.Reader) (*Instanc
 		return nil, err
 	}
 
-	path, err := url.JoinPath(handle, typePath)
+	u.Path, err = url.JoinPath(u.Path, handle, typePath)
 	if err != nil {
 		return nil, err
 	}
 
-	u.Path += path
-
 	if len(rid) > 0 {
 		q := url.Values{}
 		q.Add("rid", rid)

internal/api_test.go

@@ -5,6 +5,7 @@ package internal_test
 import (
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"strings"
 	"sync/atomic"
 	"testing"
@@ -144,6 +145,47 @@ func TestPostNonRetriable(t *testing.T) {
 	}
 }
 
+// Tests if POST URI is constructed correctly
+func TestPostURIConstruction(t *testing.T) {
+	t.Parallel()
+
+	type ping func() (*InstanceConfig, error)
+
+	c := &APIClient{}
+
+	testCases := map[string]string{
+		"suffix=":     "/",
+		"suffix=/":    "/",
+		"suffix=//":   "/",
+		"suffix=/foo": "/foo",
+	}
+
+	ts := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		testCase := r.Header.Get("test-case")
+		expectedPathPrefix, ok := testCases[testCase]
+		if !ok {
+			t.Fatalf("Unexpected test case %s", testCase)
+		}
+
+		expectedPath, _ := url.JoinPath(expectedPathPrefix, TestHandle, "start")
+		if r.URL.Path != expectedPath {
+			t.Errorf("For test case %s expected to get path %s, got %s\n", testCase, expectedPath, r.URL.Path)
+		}
+	}))
+
+	defer ts.Close()
+
+	c.Client = ts.Client()
+	for testCase := range testCases {
+		reqPath := strings.TrimPrefix(testCase, "suffix=")
+		c.BaseURL = ts.URL + reqPath
+		c.ReqHeaders = map[string]string{"test-case": testCase}
+		if _, err := c.PingStart(TestHandle, TestRunId); err != nil {
+			t.Fatalf("Request for test case %s failed: %v", testCase, err)
+		}
+	}
+}
+
 // Tests if Ping{Start,Log,Status} functions hit the correct URI paths.
 func TestPostURIs(t *testing.T) {
 	t.Parallel()