Implement WebDAV detection via OPTIONS request with redirect support

Author: sunkup

src/main/kotlin/at/bitfire/dav4jvm/DavResource.kt

@@ -68,6 +68,8 @@ open class DavResource @JvmOverloads constructor(
     companion object {
         const val MAX_REDIRECTS = 5
 
+        const val WEBDAV_VERSIONS = "1, 2, 3" // valid WebDAV versions
+
         const val HTTP_MULTISTATUS = 207
         val MIME_XML = "application/xml; charset=utf-8".toMediaType()
 
@@ -175,6 +177,39 @@ open class DavResource @JvmOverloads constructor(
         }
     }
 
+    /**
+     * Finds WebDAV capable location by sending OPTIONS request to this resource without HTTP
+     * compression (because some servers have broken compression for OPTIONS). Follows up
+     * to [MAX_REDIRECTS] redirects.
+     *
+     * @return the location of the WebDAV resource if it supports WebDAV, `null` otherwise
+     *
+     * @throws IOException on I/O error
+     * @throws HttpException on HTTP error
+     * @throws DavException on HTTPS -> HTTP redirect
+     */
+    @Throws(IOException::class, HttpException::class, DavException::class)
+    fun detectWebDav(): HttpUrl? = followRedirects {
+        httpClient.newCall(
+            Request.Builder()
+                .method("OPTIONS", null)
+                .header("Content-Length", "0")
+                .url(location)
+                .header("Accept-Encoding", "identity")      // disable compression
+                .build()
+        ).execute()
+    }.use { response ->
+        checkStatus(response)
+
+        // check whether WebDAV is supported
+        val davCapabilities = HttpUtils.listHeader(response, "DAV").map { it.trim() }.toSet()
+        val supportsWebDav = davCapabilities.any { it in WEBDAV_VERSIONS }
+        return if (supportsWebDav)
+            location
+        else
+            null
+    }
+
     /**
      * Sends a MOVE request to this resource. Follows up to [MAX_REDIRECTS] redirects.
      * Updates [location] on success.

src/test/kotlin/at/bitfire/dav4jvm/DavResourceTest.kt

@@ -484,6 +484,51 @@ class DavResourceTest {
         assertTrue(called)
     }
 
+    @Test
+    fun testDetectWebDav() {
+        val url = sampleUrl()
+        val dav = DavResource(httpClient, url)
+
+        // Without WebDAV
+        mockServer.enqueue(MockResponse()
+            .setResponseCode(HttpURLConnection.HTTP_OK)
+            .setHeader("DAV", "  0 ,, 4,5,6, hyperactive-access")
+        )
+        assertNull(dav.detectWebDav())
+
+        // With WebDAV
+        mockServer.enqueue(MockResponse()
+            .setResponseCode(HttpURLConnection.HTTP_OK)
+            .setHeader("DAV", " 1 ") // Class 1
+        )
+        assertEquals(url.encodedPath, dav.detectWebDav()?.encodedPath)
+        mockServer.enqueue(MockResponse()
+            .setResponseCode(HttpURLConnection.HTTP_OK)
+            .setHeader("DAV", "1, 2") // Class 2
+        )
+        assertEquals(url.encodedPath, dav.detectWebDav()?.encodedPath)
+        mockServer.enqueue(MockResponse()
+            .setResponseCode(HttpURLConnection.HTTP_OK)
+            .setHeader("DAV", " 1, 3 ") // Class 3
+        )
+        assertEquals(url.encodedPath, dav.detectWebDav()?.encodedPath)
+
+        // Follows redirects
+        mockServer.enqueue(MockResponse()
+            .setResponseCode(HttpURLConnection.HTTP_MOVED_TEMP)
+            .setHeader("Location", "/moved")
+        )
+        mockServer.enqueue(MockResponse()
+            .setResponseCode(HttpURLConnection.HTTP_MOVED_TEMP)
+            .setHeader("Location", "/redirected")
+        )
+        mockServer.enqueue(MockResponse()
+            .setResponseCode(HttpURLConnection.HTTP_OK)
+            .setHeader("DAV", "2")
+        )
+        assertEquals("/redirected", dav.detectWebDav()?.encodedPath)
+    }
+
     @Test
     fun testPropfindAndMultiStatus() {
         val url = sampleUrl()