Fix behavior for PathPrefixStrip

When pushing data to downstream proxies; malformed requests were being
sent.

The corrected behavior is as follows:

| Route Stripped    |     URL                |  Passed to Backend |
| ----------------- | ---------------------- | ------------------ |
| /                 |     /                  |  /                 |

| Route Stripped    |     URL                |  Passed to Backend |
| ----------------- | ---------------------- | ------------------ |
| /stat             |     /stat              |  /                 |
| /stat             |     /stat/             |  /                 |
| /stat             |     /status            |  /status           |
| /stat             |     /stat/us           |  /us               |

| Route Stripped    |     URL                |  Passed to Backend |
| ----------------- | ---------------------- | ------------------ |
| /stat/            |     /stat              |  /stat             |
| /stat/            |     /stat/             |  /                 |
| /stat/            |     /status            |  /status           |
| /stat/            |     /stat/us           |  /us               |

Prior, we could strip the prefixing `/`, and we'd also ignore the case
where you want to serve something like `/api` as both the index and as a
subpath.

Additionally, this should resolve a myriad of issues relating to
kubernetes ingress `PathPrefixStrip`.

middlewares/stripPrefix.go

@@ -17,17 +17,29 @@ type StripPrefix struct {
 
 func (s *StripPrefix) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	for _, prefix := range s.Prefixes {
-		if p := strings.TrimPrefix(r.URL.Path, strings.TrimSpace(prefix)); len(p) < len(r.URL.Path) {
-			r.URL.Path = p
-			r.Header[forwardedPrefixHeader] = []string{prefix}
-			r.RequestURI = r.URL.RequestURI()
-			s.Handler.ServeHTTP(w, r)
+		origPrefix := strings.TrimSpace(prefix)
+		if origPrefix == r.URL.Path {
+			r.URL.Path = "/"
+			s.serveRequest(w, r, origPrefix)
+			return
+		}
+
+		prefix = strings.TrimSuffix(origPrefix, "/") + "/"
+		if p := strings.TrimPrefix(r.URL.Path, prefix); len(p) < len(r.URL.Path) {
+			r.URL.Path = "/" + strings.TrimPrefix(p, "/")
+			s.serveRequest(w, r, origPrefix)
 			return
 		}
 	}
 	http.NotFound(w, r)
 }
 
+func (s *StripPrefix) serveRequest(w http.ResponseWriter, r *http.Request, prefix string) {
+	r.Header[forwardedPrefixHeader] = []string{prefix}
+	r.RequestURI = r.URL.RequestURI()
+	s.Handler.ServeHTTP(w, r)
+}
+
 // SetHandler sets handler
 func (s *StripPrefix) SetHandler(Handler http.Handler) {
 	s.Handler = Handler

middlewares/stripPrefix_test.go

@@ -0,0 +1,103 @@
+package middlewares
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStripPrefix(t *testing.T) {
+	tests := []struct {
+		desc               string
+		prefixes           []string
+		path               string
+		expectedStatusCode int
+		expectedPath       string
+	}{
+		{
+			desc:               "no prefixes configured",
+			prefixes:           []string{},
+			path:               "/noprefixes",
+			expectedStatusCode: http.StatusNotFound,
+		},
+		{
+			desc:               "wildcard (.*) requests",
+			prefixes:           []string{"/"},
+			path:               "/",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/",
+		},
+		{
+			desc:               "prefix and path matching",
+			prefixes:           []string{"/stat"},
+			path:               "/stat",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/",
+		},
+		{
+			desc:               "path prefix on exactly matching path",
+			prefixes:           []string{"/stat/"},
+			path:               "/stat/",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/",
+		},
+		{
+			desc:               "path prefix on matching longer path",
+			prefixes:           []string{"/stat/"},
+			path:               "/stat/us",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/us",
+		},
+		{
+			desc:               "path prefix on mismatching path",
+			prefixes:           []string{"/stat/"},
+			path:               "/status",
+			expectedStatusCode: http.StatusNotFound,
+		},
+		{
+			desc:               "general prefix on matching path",
+			prefixes:           []string{"/stat"},
+			path:               "/stat/",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/",
+		},
+		{
+			desc:               "earlier prefix matching",
+			prefixes:           []string{"/stat", "/stat/us"},
+			path:               "/stat/us",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/us",
+		},
+		{
+			desc:               "later prefix matching",
+			prefixes:           []string{"/mismatch", "/stat"},
+			path:               "/stat",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/",
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.desc, func(t *testing.T) {
+			t.Parallel()
+			var gotPath string
+			server := httptest.NewServer(&StripPrefix{
+				Prefixes: test.prefixes,
+				Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+					gotPath = r.URL.Path
+				}),
+			})
+			defer server.Close()
+
+			resp, err := http.Get(server.URL + test.path)
+			require.NoError(t, err, "Failed to send GET request")
+			assert.Equal(t, test.expectedStatusCode, resp.StatusCode, "Unexpected status code")
+
+			assert.Equal(t, test.expectedPath, gotPath, "Unexpected path")
+		})
+	}
+}