Update rebaseUrl with latest upstream changes

rebaseUrl was extracted and repurposed in January 2016. It has since
been updated since to handle raw paths.

Add tests for trailing slashes where appropriate.

Relates to https://gitlab.com/gitlab-org/gitlab-workhorse/-/issues/359

workhorse/internal/api/api.go

@@ -151,7 +151,7 @@ type Response struct {
 	MaximumSize int64
 }
 
-// singleJoiningSlash is taken from reverseproxy.go:NewSingleHostReverseProxy
+// singleJoiningSlash is taken from reverseproxy.go:singleJoiningSlash
 func singleJoiningSlash(a, b string) string {
 	aslash := strings.HasSuffix(a, "/")
 	bslash := strings.HasPrefix(b, "/")
@@ -164,21 +164,36 @@ func singleJoiningSlash(a, b string) string {
 	return a + b
 }
 
+// joinURLPath is taken from reverseproxy.go:joinURLPath
+func joinURLPath(a *url.URL, b string) (path string, rawpath string) {
+	if a.RawPath == "" && b == "" {
+		return singleJoiningSlash(a.Path, b), ""
+	}
+
+	// Same as singleJoiningSlash, but uses EscapedPath to determine
+	// whether a slash should be added
+	apath := a.EscapedPath()
+	bpath := b
+
+	aslash := strings.HasSuffix(apath, "/")
+	bslash := strings.HasPrefix(bpath, "/")
+
+	switch {
+	case aslash && bslash:
+		return a.Path + bpath[1:], apath + bpath[1:]
+	case !aslash && !bslash:
+		return a.Path + "/" + bpath, apath + "/" + bpath
+	}
+	return a.Path + bpath, apath + bpath
+}
+
 // rebaseUrl is taken from reverseproxy.go:NewSingleHostReverseProxy
 func rebaseUrl(url *url.URL, onto *url.URL, suffix string) *url.URL {
 	newUrl := *url
 	newUrl.Scheme = onto.Scheme
 	newUrl.Host = onto.Host
-	if suffix != "" {
-		if newUrl.RawPath != "" {
-			// Since the GitLab API cares about encoded slashes (%2F),
-			// we need to adjust both RawPath and Path for Go to use the
-			// original encoded form. See https://go-review.googlesource.com/c/go/+/11302/.
-			newUrl.RawPath = singleJoiningSlash(url.RawPath, suffix)
-		}
+	newUrl.Path, newUrl.RawPath = joinURLPath(url, suffix)
 
-		newUrl.Path = singleJoiningSlash(url.Path, suffix)
-	}
 	if onto.RawQuery == "" || newUrl.RawQuery == "" {
 		newUrl.RawQuery = onto.RawQuery + newUrl.RawQuery
 	} else {

workhorse/upload_test.go

@@ -124,9 +124,13 @@ func TestAcceleratedUpload(t *testing.T) {
 		{"PUT", "/api/v4/projects/group%2Fproject/packages/nuget/v1/files", true},
 		{"PUT", "/api/v4/projects/group%2Fsubgroup%2Fproject/packages/nuget/v1/files", true},
 		{"POST", `/api/v4/groups/import`, true},
+		{"POST", `/api/v4/groups/import/`, true},
 		{"POST", `/api/v4/projects/import`, true},
+		{"POST", `/api/v4/projects/import/`, true},
 		{"POST", `/import/gitlab_project`, true},
+		{"POST", `/import/gitlab_project/`, true},
 		{"POST", `/import/gitlab_group`, true},
+		{"POST", `/import/gitlab_group/`, true},
 		{"POST", `/api/v4/projects/9001/packages/pypi`, true},
 		{"POST", `/api/v4/projects/group%2Fproject/packages/pypi`, true},
 		{"POST", `/api/v4/projects/group%2Fsubgroup%2Fproject/packages/pypi`, true},
@@ -134,14 +138,16 @@ func TestAcceleratedUpload(t *testing.T) {
 		{"POST", `/api/v4/projects/group%2Fproject/issues/30/metric_images`, true},
 		{"POST", `/api/v4/projects/group%2Fsubgroup%2Fproject/issues/30/metric_images`, true},
 		{"POST", `/my/project/-/requirements_management/requirements/import_csv`, true},
+		{"POST", `/my/project/-/requirements_management/requirements/import_csv/`, true},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.resource, func(t *testing.T) {
 			ts := uploadTestServer(t,
 				func(r *http.Request) {
+					resource := strings.TrimRight(tt.resource, "/")
 					// Validate %2F characters haven't been unescaped
-					require.Equal(t, tt.resource+"/authorize", r.URL.String())
+					require.Equal(t, resource+"/authorize", r.URL.String())
 				},
 				func(r *http.Request) {
 					if tt.signedFinalization {