add etag support and if-none-match/if-modified-since to cocoa http handling

6 files changed, 155 insertions, 473 deletions

diff --git a/common/curl_request.cpp b/common/curl_request.cpp
deleted file mode 100644
index 456bbfd6d1..0000000000
--- a/common/curl_request.cpp
+++ /dev/null
@@ -1,414 +0,0 @@
-//
-//#include <mbgl/platform/platform.hpp>
-//#include <mbgl/platform/request.hpp>
-//#include <mbgl/util/uv_detail.hpp>
-//#include <mbgl/util/std.hpp>
-//
-//#include <queue>
-//#include <boost/lockfree/queue.hpp>
-//
-//#include <curl/curl.h>
-//
-//// This file contains code from http://curl.haxx.se/libcurl/c/multi-uv.html:
-//
-///***************************************************************************
-// *                                  _   _ ____  _
-// *  Project                     ___| | | |  _ \| |
-// *                             / __| | | | |_) | |
-// *                            | (__| |_| |  _ <| |___
-// *                             \___|\___/|_| \_\_____|
-// *
-// * Copyright (C) 1998 - 2013, Daniel Stenberg, <daniel@haxx.se>, et al.
-// *
-// * This software is licensed as described in the file COPYING, which
-// * you should have received as part of this distribution. The terms
-// * are also available at http://curl.haxx.se/docs/copyright.html.
-// *
-// * You may opt to use, copy, modify, merge, publish, distribute and/or sell
-// * copies of the Software, and permit persons to whom the Software is
-// * furnished to do so, under the terms of the COPYING file.
-// *
-// * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
-// * KIND, either express or implied.
-// *
-// ***************************************************************************/
-//
-///* Example application code using the multi socket interface to download
-//   multiple files at once, but instead of using curl_multi_perform and
-//   curl_multi_wait, which uses select(), we use libuv.
-//   It supports epoll, kqueue, etc. on unixes and fast IO completion ports on
-//   Windows, which means, it should be very fast on all platforms..
-//
-//   Written by Clemens Gruber, based on an outdated example from uvbook and
-//   some tests from libuv.
-//
-//   Requires libuv and (of course) libcurl.
-//
-//   See http://nikhilm.github.com/uvbook/ for more information on libuv.
-//*/
-//
-//namespace mbgl {
-//namespace platform {
-//namespace request {
-//
-//struct curl_context {
-//    uv_poll_t poll_handle;
-//    curl_socket_t sockfd;
-//};
-//
-//// Handles the request thread + messaging to the thread.
-//static uv_thread_t thread;
-//static uv::once init_thread_once;
-//static uv_loop_t *loop = nullptr;
-//static uv_async_t async_add;
-//static uv_async_t async_cancel;
-//
-//// Stores pointers (!) to shared_ptrs. We use shared_ptrs so that request objects don't get
-//// auto-destructed while they're in progress. The TileData object retains a weak_ptr to this
-//// request, so we have to use a shared_ptr here to ensure that this object stays alive.
-//static boost::lockfree::queue<std::shared_ptr<mbgl::platform::Request> *> add_queue(8);
-//static boost::lockfree::queue<std::shared_ptr<mbgl::platform::Request> *> cancel_queue(8);
-//
-//// Used as the CURL timer function to periodically check for socket updates.
-//static uv_timer_t timeout;
-//
-//// CURL multi handle that we use to request multiple URLs at the same time, without having to block
-//// and spawn threads.
-//static CURLM *curl_multi = nullptr;
-//
-//// CURL share handles are used for sharing session state (e.g.)
-//static uv::mutex curl_share_mutex;
-//static CURLSH *curl_share = nullptr;
-//
-//// A queue that we use for storing resuable CURL easy handles to avoid creating and destroying them
-//// all the time.
-//static std::queue<CURL *> curl_handle_cache;
-//
-//
-//class CURLRequest : public mbgl::platform::Request {
-//public:
-//    CURLRequest(const std::string &url,
-//                std::function<void(mbgl::platform::Response *)> callback,
-//                std::shared_ptr<uv::loop> loop)
-//        : Request(url, callback, loop) {}
-//
-//    CURL *curl = nullptr;
-//};
-//
-//
-//// Implementation starts here.
-//
-//// Locks the CURL share handle
-//void curl_share_lock(CURL *, curl_lock_data, curl_lock_access, void *) { curl_share_mutex.lock(); }
-//
-//// Unlocks the CURL share handle
-//void curl_share_unlock(CURL *, curl_lock_data, void *) { curl_share_mutex.unlock(); }
-//
-//curl_context *create_curl_context(curl_socket_t sockfd) {
-//    curl_context *context = new curl_context;
-//    context->sockfd = sockfd;
-//
-//    uv_poll_init_socket(loop, &context->poll_handle, sockfd);
-//    context->poll_handle.data = context;
-//
-//    return context;
-//}
-//
-//void curl_close_cb(uv_handle_t *handle) {
-//    curl_context *context = (curl_context *)handle->data;
-//    free(context);
-//}
-//
-//void destroy_curl_context(curl_context *context) {
-//    uv_close((uv_handle_t *)&context->poll_handle, curl_close_cb);
-//}
-//
-//void remove_curl_handle(CURL *handle) {
-//    CURLMcode error = curl_multi_remove_handle(curl_multi, handle);
-//    if (error != CURLM_OK) {
-//        throw std::runtime_error(std::string("CURL multi error: ") + curl_multi_strerror(error));
-//    }
-//
-//    curl_easy_reset(handle);
-//    curl_handle_cache.push(handle);
-//}
-//
-//void curl_perform(uv_poll_t *req, int /*status*/, int events) {
-//    int running_handles;
-//    int flags = 0;
-//    curl_context *context;
-//    CURLMsg *message;
-//    int pending;
-//
-//    uv_timer_stop(&timeout);
-//
-//    if (events & UV_READABLE)
-//        flags |= CURL_CSELECT_IN;
-//    if (events & UV_WRITABLE)
-//        flags |= CURL_CSELECT_OUT;
-//
-//    context = (curl_context *)req;
-//
-//    curl_multi_socket_action(curl_multi, context->sockfd, flags, &running_handles);
-//
-//    while ((message = curl_multi_info_read(curl_multi, &pending))) {
-//        switch (message->msg) {
-//        case CURLMSG_DONE: {
-//            std::shared_ptr<Request> *req = nullptr;
-//            curl_easy_getinfo(message->easy_handle, CURLINFO_PRIVATE, (char *)&req);
-//
-//            // Add human-readable error code
-//            if (message->data.result != CURLE_OK) {
-//                (*req)->res->error_message = curl_easy_strerror(message->data.result);
-//                (*req)->res->code = -1;
-//            } else {
-//                curl_easy_getinfo(message->easy_handle, CURLINFO_RESPONSE_CODE, &(*req)->res->code);
-//            }
-//
-//            // We're currently in the CURL request thread. We're going to schedule a uv_work request
-//            // that executes the background function in a threadpool, and tell it to call the
-//            // after callback back in the main uv loop.
-//            (*req)->complete();
-//
-//            CURL *handle = message->easy_handle;
-//            remove_curl_handle(handle);
-//
-//            // We're setting this to NULL because there might still be shared_ptrs around that could
-//            // be cancelled.
-//            ((CURLRequest *)req->get())->curl = nullptr;
-//
-//            // Delete the shared_ptr pointer we created earlier.
-//            delete req;
-//            break;
-//        }
-//
-//        default:
-//            // This should never happen, because there are no other message types.
-//            throw std::runtime_error("CURLMSG returned unknown message type");
-//        }
-//    }
-//}
-//
-//int handle_socket(CURL * /*easy*/, curl_socket_t s, int action, void * /*userp*/, void *socketp) {
-//    curl_context *context = nullptr;
-//
-//    if (socketp) {
-//        context = (curl_context *)socketp;
-//    } else if (action != CURL_POLL_REMOVE) {
-//        context = create_curl_context(s);
-//    }
-//
-//    if (context) {
-//        curl_multi_assign(curl_multi, s, (void *)context);
-//        if (action == CURL_POLL_IN || action == CURL_POLL_INOUT) {
-//            uv_poll_start(&context->poll_handle, UV_READABLE, curl_perform);
-//        }
-//        if (action == CURL_POLL_OUT || action == CURL_POLL_INOUT) {
-//            uv_poll_start(&context->poll_handle, UV_WRITABLE, curl_perform);
-//        }
-//        if (action == CURL_POLL_REMOVE && socketp) {
-//            uv_poll_stop(&context->poll_handle);
-//            destroy_curl_context(context);
-//            curl_multi_assign(curl_multi, s, NULL);
-//        }
-//    }
-//
-//    return 0;
-//}
-//
-//void on_timeout(uv_timer_t * /*req*/) {
-//    int running_handles;
-//    CURLMcode error =
-//        curl_multi_socket_action(curl_multi, CURL_SOCKET_TIMEOUT, 0, &running_handles);
-//    if (error != CURLM_OK) {
-//        throw std::runtime_error(std::string("CURL multi error: ") + curl_multi_strerror(error));
-//    }
-//}
-//
-//void start_timeout(CURLM * /*multi*/, long timeout_ms, void * /*userp*/) {
-//    if (timeout_ms <= 0) {
-//        on_timeout(&timeout);
-//    } else {
-//        uv_timer_start(&timeout, on_timeout, timeout_ms, 0);
-//    }
-//}
-//
-//// This function is the first function called in the request thread. It sets up the CURL share/multi
-//// handles and runs the thread loop.
-//void thread_init(void * /*ptr*/) {
-//    uv_timer_init(loop, &timeout);
-//
-//    CURLSHcode share_error;
-//    curl_share = curl_share_init();
-//
-//    share_error = curl_share_setopt(curl_share, CURLSHOPT_LOCKFUNC, curl_share_lock);
-//    if (share_error != CURLSHE_OK) {
-//        throw std::runtime_error(std::string("CURL share error: ") + curl_share_strerror(share_error));
-//    }
-//
-//    share_error = curl_share_setopt(curl_share, CURLSHOPT_UNLOCKFUNC, curl_share_unlock);
-//    if (share_error != CURLSHE_OK) {
-//        throw std::runtime_error(std::string("CURL share error: ") + curl_share_strerror(share_error));
-//    }
-//
-//    CURLMcode multi_error;
-//    curl_multi = curl_multi_init();
-//
-//    multi_error = curl_multi_setopt(curl_multi, CURLMOPT_SOCKETFUNCTION, handle_socket);
-//    if (multi_error != CURLM_OK) {
-//        throw std::runtime_error(std::string("CURL multi error: ") + curl_multi_strerror(multi_error));
-//    }
-//    multi_error = curl_multi_setopt(curl_multi, CURLMOPT_TIMERFUNCTION, start_timeout);
-//    if (multi_error != CURLM_OK) {
-//        throw std::runtime_error(std::string("CURL multi error: ") + curl_multi_strerror(multi_error));
-//
-//    }
-//
-//    // Main event loop. This will not return until the request loop is terminated.
-//    uv_run(loop, UV_RUN_DEFAULT);
-//
-//    curl_multi_cleanup(curl_multi);
-//    curl_multi = nullptr;
-//    curl_share_cleanup(curl_share);
-//    curl_share = nullptr;
-//
-//    // Clean up all the CURL easy handles that we kept around for potential future reuse.
-//    while (!curl_handle_cache.empty()) {
-//        curl_easy_cleanup(curl_handle_cache.front());
-//        curl_handle_cache.pop();
-//    }
-//}
-//
-//// This function is called when we have new data for a request. We just append it to the string
-//// containing the previous data.
-//size_t curl_write_cb(void *contents, size_t size, size_t nmemb, void *userp) {
-//    ((std::string *)userp)->append((char *)contents, size * nmemb);
-//    return size * nmemb;
-//}
-//
-//// Compares the beginning of the (non-zero-terminated!) data buffer with the (zero-terminated!)
-//// header string. If the data buffer contains the header string at the beginning, it returns
-//// the length of the header string == begin of the value, otherwise it returns npos.
-//// The comparison of the header is ASCII-case-insensitive.
-//size_t header_matches(const char *header, const char *buffer, size_t length) {
-//    const size_t header_length = strlen(header);
-//    if (length < header_length) return std::string::npos;
-//    size_t i = 0;
-//    while (i < length && i < header_length && std::tolower(buffer[i]) == header[i]) {
-//        i++;
-//    }
-//    return i == header_length ? i : std::string::npos;
-//}
-//
-//size_t curl_header_cb(char *buffer, size_t size, size_t nmemb, void *userp) {
-//    const size_t length = size * nmemb;
-//
-//    size_t begin = std::string::npos;
-//    if ((begin = header_matches("last-modified: ", buffer, length)) != std::string::npos) {
-//        const std::string value { buffer + begin, length - begin - 2 /* remove \r\n */ };
-//        static_cast<Response *>(userp)->setLastModified(value.c_str());
-//    } else if ((begin = header_matches("cache-control: ", buffer, length)) != std::string::npos) {
-//        const std::string value { buffer + begin, length - begin - 2 /* remove \r\n */ };
-//        static_cast<Response *>(userp)->setCacheControl(value.c_str());
-//    }
-//
-//    return length;
-//}
-//
-//// This callback is called in the request event loop (on the request thread).
-//// It initializes newly queued up download requests and adds them to the CURL
-//// multi handle.
-//void async_add_cb(uv_async_t * /*async*/) {
-//    std::shared_ptr<Request> *req = nullptr;
-//    while (add_queue.pop(req)) {
-//        // Make sure that we're not starting requests that have been cancelled
-//        // already by async_cancel_cb.
-//        if ((*req)->cancelled) {
-//            delete req;
-//            continue;
-//        }
-//
-//        // Obtain a curl handle (and try to reuse existing handles before creating new ones).
-//        CURL *handle = nullptr;
-//        if (!curl_handle_cache.empty()) {
-//            handle = curl_handle_cache.front();
-//            curl_handle_cache.pop();
-//        } else {
-//            handle = curl_easy_init();
-//        }
-//
-//        ((CURLRequest *)req->get())->curl = handle;
-//
-//        curl_easy_setopt(handle, CURLOPT_PRIVATE, req);
-//        curl_easy_setopt(handle, CURLOPT_CAINFO, "ca-bundle.crt");
-//        curl_easy_setopt(handle, CURLOPT_URL, (*req)->url.c_str());
-//        curl_easy_setopt(handle, CURLOPT_WRITEFUNCTION, curl_write_cb);
-//        curl_easy_setopt(handle, CURLOPT_WRITEDATA, &(*req)->res->body);
-//        curl_easy_setopt(handle, CURLOPT_HEADERFUNCTION, curl_header_cb);
-//        curl_easy_setopt(handle, CURLOPT_HEADERDATA, (*req)->res.get());
-//        curl_easy_setopt(handle, CURLOPT_ACCEPT_ENCODING, "gzip, deflate");
-//        curl_easy_setopt(handle, CURLOPT_SHARE, curl_share);
-//        curl_multi_add_handle(curl_multi, handle);
-//    }
-//}
-//
-//void async_cancel_cb(uv_async_t * /*async*/) {
-//    std::shared_ptr<Request> *req = nullptr;
-//    while (cancel_queue.pop(req)) {
-//        // It is possible that the request has not yet been started, but that it already has been
-//        // added to the queue for scheduling new requests. In this case, the CURL handle is invalid
-//        // and we manually mark the Request as cancelled.
-//        CURL *handle = ((CURLRequest *)req->get())->curl;
-//        if (handle && !(*req)->cancelled) {
-//            remove_curl_handle(handle);
-//            ((CURLRequest *)req->get())->curl = nullptr;
-//        }
-//        (*req)->cancelled = true;
-//
-//        delete req;
-//        req = nullptr;
-//    }
-//}
-//
-//void thread_init_cb() {
-//    curl_global_init(CURL_GLOBAL_ALL);
-//
-//    loop = uv_loop_new();
-//    uv_async_init(loop, &async_add, async_add_cb);
-//    uv_async_init(loop, &async_cancel, async_cancel_cb);
-//    uv_thread_create(&thread, thread_init, nullptr);
-//}
-//} // end namespace request
-//} // end namespace platform
-//
-//
-//std::shared_ptr<platform::Request>
-//platform::request_http(const std::string &url,
-//                       std::function<void(Response *)> callback,
-//                       std::shared_ptr<uv::loop> loop) {
-//    using namespace request;
-//    init_thread_once(thread_init_cb);
-//    std::shared_ptr<CURLRequest> req = std::make_shared<CURLRequest>(url, callback, loop);
-//
-//    // Note that we are creating a new shared_ptr pointer(!) because the lockless queue can't store
-//    // objects with nontrivial destructors. We have to make absolutely sure that we manually delete
-//    // the shared_ptr when we pop it from the queue.
-//    add_queue.push(new std::shared_ptr<Request>(req));
-//    uv_async_send(&async_add);
-//
-//    return req;
-//}
-//
-//// Cancels an HTTP request.
-//void platform::cancel_request_http(const std::shared_ptr<Request> &req) {
-//    if (req) {
-//        using namespace request;
-//
-//        // Note that we are creating a new shared_ptr pointer(!) because the lockless queue can't
-//        // store objects with nontrivial destructors. We have to make absolutely shure that we
-//        // manually delete the shared_ptr when we pop it from the queue.
-//        cancel_queue.push(new std::shared_ptr<Request>(req));
-//        uv_async_send(&async_cancel);
-//    }
-//}
-//} // end namespace mbgl
diff --git a/common/http_request_baton_cocoa.mm b/common/http_request_baton_cocoa.mm
index 497d55653a..2ebbec481c 100644
--- a/common/http_request_baton_cocoa.mm
+++ b/common/http_request_baton_cocoa.mm
@@ -35,13 +35,16 @@ void HTTPRequestBaton::start(const util::ptr<HTTPRequestBaton> &ptr) {
     });
 
     NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:@(baton->path.c_str())]];
-    if (baton->response && baton->response->modified) {
-        struct tm *timeinfo;
-        char buffer[32];
-        const time_t modified = baton->response->modified;
-        timeinfo = std::gmtime(&modified);
-        strftime_l(buffer, 32, "%a, %d %b %Y %H:%M:%S GMT", timeinfo, locale);
-        [request addValue:@(buffer) forHTTPHeaderField:@"If-Modified-Since"];
+    if (baton->response) {
+        if (!baton->response->etag.empty()) {
+            [request addValue:@(baton->response->etag.c_str()) forHTTPHeaderField:@"If-None-Match"];
+        } else if (baton->response->modified) {
+            const time_t modified = baton->response->modified;
+            struct tm *timeinfo = std::gmtime(&modified);
+            char buffer[32];
+            strftime_l(buffer, 32, "%a, %d %b %Y %H:%M:%S GMT", timeinfo, locale);
+            [request addValue:@(buffer) forHTTPHeaderField:@"If-Modified-Since"];
+        }
     }
 
     NSURLSessionDataTask *task = [session dataTaskWithRequest:request
@@ -113,6 +116,11 @@ void HTTPRequestBaton::start(const util::ptr<HTTPRequestBaton> &ptr) {
             if (last_modified) {
                 baton->response->modified = parse_date([last_modified UTF8String]);
             }
+
+            NSString *etag = [headers objectForKey:@"ETag"];
+            if (etag) {
+                baton->response->etag = [etag UTF8String];
+            }
         } else {
             // This should never happen.
             baton->type = HTTPResponseType::PermanentError;
diff --git a/common/http_request_baton_curl.cpp b/common/http_request_baton_curl.cpp
index 55147fbfc7..8cf6a903fe 100644
--- a/common/http_request_baton_curl.cpp
+++ b/common/http_request_baton_curl.cpp
@@ -8,6 +8,8 @@
 
 #include <queue>
 #include <cassert>
+#include <ctime>
+#include <xlocale.h>
 
 // This file contains code from http://curl.haxx.se/libcurl/c/multi-uv.html:
 
@@ -72,9 +74,25 @@ static std::queue<CURL *> handles;
 
 namespace mbgl {
 
-struct curl_context {
-    uv_poll_t poll_handle;
-    curl_socket_t sockfd;
+struct CURLContext {
+    util::ptr<HTTPRequestBaton> baton;
+    uv_poll_t *poll_handle = nullptr;
+    curl_socket_t sockfd = 0;
+    curl_slist *headers = nullptr;
+
+    CURLContext(const util::ptr<HTTPRequestBaton> &baton_) : baton(baton_) {
+    }
+
+    ~CURLContext() {
+        // We are destructing the poll handle in a CURL callback already.
+        assert(!poll_handle);
+
+        // Once the CURLContext gets destroyed, CURL doesn't need any headers anymore.
+        if (headers) {
+            curl_slist_free_all(headers);
+            headers = nullptr;
+        }
+    }
 };
 
 // Locks the CURL share handle
@@ -87,22 +105,6 @@ void curl_share_unlock(CURL *, curl_lock_data, void *) {
     uv_mutex_unlock(&share_mutex);
 }
 
-curl_context *create_curl_context(curl_socket_t sockfd) {
-    curl_context *context = new curl_context;
-    context->sockfd = sockfd;
-
-    uv_poll_init_socket(&loop, &context->poll_handle, sockfd);
-    context->poll_handle.data = context;
-
-    return context;
-}
-
-void destroy_curl_context(curl_context *context) {
-    uv_close((uv_handle_t *)&context->poll_handle, [](uv_handle_t *handle) {
-        delete (curl_context *)handle->data;
-    });
-}
-
 // This function must run in the CURL thread.
 // It is either called when the request is completed, or when we try to cancel the request.
 void finish_request(const util::ptr<HTTPRequestBaton> &baton) {
@@ -117,10 +119,12 @@ void finish_request(const util::ptr<HTTPRequestBaton> &baton) {
         }
 
         // Destroy the shared pointer. We still have one pointing to it
-        util::ptr<HTTPRequestBaton> *baton_ptr = nullptr;
-        curl_easy_getinfo(handle, CURLINFO_PRIVATE, (char *)&baton_ptr);
+        CURLContext *context = nullptr;
+        curl_easy_getinfo(handle, CURLINFO_PRIVATE, (char *)&context);
         curl_easy_setopt(handle, CURLOPT_PRIVATE, nullptr);
-        delete baton_ptr;
+        delete context;
+
+        // TODO: delete the headers object again.
 
         curl_easy_reset(handle);
         handles.push(handle);
@@ -128,10 +132,10 @@ void finish_request(const util::ptr<HTTPRequestBaton> &baton) {
     }
 }
 
-void curl_perform(uv_poll_t *req, int status, int events) {
+void curl_perform(uv_poll_t *req, int, int events) {
     int running_handles;
     int flags = 0;
-    curl_context *context = (curl_context *)req;
+    CURLContext *context = (CURLContext *)req->data;
     CURLMsg *message;
     int pending;
 
@@ -149,9 +153,11 @@ void curl_perform(uv_poll_t *req, int status, int events) {
     while ((message = curl_multi_info_read(multi, &pending))) {
         switch (message->msg) {
         case CURLMSG_DONE: {
-            util::ptr<HTTPRequestBaton> *baton_ptr = nullptr;
-            curl_easy_getinfo(message->easy_handle, CURLINFO_PRIVATE, (char *)&baton_ptr);
-            util::ptr<HTTPRequestBaton> baton = *baton_ptr;
+            CURLContext *context = nullptr;
+            curl_easy_getinfo(message->easy_handle, CURLINFO_PRIVATE, (char *)&context);
+
+            // Make a copy so that the Baton stays around even after we are calling finish_request
+            util::ptr<HTTPRequestBaton> baton = context->baton;
 
             // Add human-readable error code
             if (message->data.result != CURLE_OK) {
@@ -211,34 +217,42 @@ void curl_perform(uv_poll_t *req, int status, int events) {
     }
 }
 
-int handle_socket(CURL *easy, curl_socket_t s, int action, void * userp, void *socketp) {
-    curl_context *context = nullptr;
-
-    if (socketp) {
-        context = (curl_context *)socketp;
-    } else if (action != CURL_POLL_REMOVE) {
-        context = create_curl_context(s);
+int handle_socket(CURL *handle, curl_socket_t sockfd, int action, void *, void *socketp) {
+    CURLContext *context = nullptr;
+    curl_easy_getinfo(handle, CURLINFO_PRIVATE, (char *)&context);
+
+    if (!socketp && action != CURL_POLL_REMOVE) {
+        // We haven't initialized the socket yet, so we need to do this now.
+        assert(context->sockfd == 0);
+        context->sockfd = sockfd;
+        assert(!context->poll_handle);
+        context->poll_handle = new uv_poll_t;
+        uv_poll_init_socket(&loop, context->poll_handle, sockfd);
+        context->poll_handle->data = context;
+        curl_multi_assign(multi, sockfd, context);
     }
 
     if (context) {
-        curl_multi_assign(multi, s, (void *)context);
         if (action == CURL_POLL_IN || action == CURL_POLL_INOUT) {
-            uv_poll_start(&context->poll_handle, UV_READABLE, curl_perform);
+            uv_poll_start(context->poll_handle, UV_READABLE, curl_perform);
         }
         if (action == CURL_POLL_OUT || action == CURL_POLL_INOUT) {
-            uv_poll_start(&context->poll_handle, UV_WRITABLE, curl_perform);
+            uv_poll_start(context->poll_handle, UV_WRITABLE, curl_perform);
         }
         if (action == CURL_POLL_REMOVE && socketp) {
-            uv_poll_stop(&context->poll_handle);
-            destroy_curl_context(context);
-            curl_multi_assign(multi, s, NULL);
+            uv_poll_stop(context->poll_handle);
+            uv_close((uv_handle_t *)context->poll_handle, [](uv_handle_t *handle) {
+                delete (uv_poll_t *)handle;
+            });
+            context->poll_handle = nullptr;
+            curl_multi_assign(multi, sockfd, NULL);
         }
     }
 
     return 0;
 }
 
-void on_timeout(uv_timer_t *req) {
+void on_timeout(uv_timer_t *) {
     int running_handles;
     CURLMcode error = curl_multi_socket_action(multi, CURL_SOCKET_TIMEOUT, 0, &running_handles);
     if (error != CURLM_OK) {
@@ -246,7 +260,7 @@ void on_timeout(uv_timer_t *req) {
     }
 }
 
-void start_timeout(CURLM * multi, long timeout_ms, void * userp) {
+void start_timeout(CURLM *, long timeout_ms, void *) {
     if (timeout_ms <= 0) {
         on_timeout(&timeout);
     } else {
@@ -325,13 +339,19 @@ size_t header_matches(const char *const header, const char *const buffer, const
 size_t curl_header_cb(char * const buffer, const size_t size, const size_t nmemb, void *const userp) {
     const size_t length = size * nmemb;
 
+    Response *response = static_cast<Response *>(userp);
+
     size_t begin = std::string::npos;
     if ((begin = header_matches("last-modified: ", buffer, length)) != std::string::npos) {
+        // Always overwrite the modification date; We might already have a value here from the
+        // Date header, but this one is more accurate.
         const std::string value { buffer + begin, length - begin - 2 }; // remove \r\n
-        static_cast<Response *>(userp)->modified = curl_getdate(value.c_str(), nullptr);
+        response->modified = curl_getdate(value.c_str(), nullptr);
+    } else if ((begin = header_matches("etag: ", buffer, length)) != std::string::npos) {
+        response->etag = { buffer + begin, length - begin - 2 }; // remove \r\n
     } else if ((begin = header_matches("cache-control: ", buffer, length)) != std::string::npos) {
         const std::string value { buffer + begin, length - begin - 2 }; // remove \r\n
-        static_cast<Response *>(userp)->expires = Response::parseCacheControl(value.c_str());
+        response->expires = Response::parseCacheControl(value.c_str());
     }
 
     return length;
@@ -340,9 +360,13 @@ size_t curl_header_cb(char * const buffer, const size_t size, const size_t nmemb
 // This function must run in the CURL thread.
 void start_request(void *const ptr) {
     assert(uv_thread_self() == thread_id);
-    util::ptr<HTTPRequestBaton> &baton = *(util::ptr<HTTPRequestBaton> *)ptr;
+    std::unique_ptr<util::ptr<HTTPRequestBaton>> baton_guard { (util::ptr<HTTPRequestBaton> *)ptr };
+    util::ptr<HTTPRequestBaton> &baton = *baton_guard.get();
     assert(baton);
 
+    // Create a C locale
+    static locale_t locale = newlocale(LC_ALL_MASK, nullptr, nullptr);
+
     CURL *handle = nullptr;
     if (!handles.empty()) {
         handle = handles.front();
@@ -353,12 +377,32 @@ void start_request(void *const ptr) {
 
     baton->ptr = handle;
 
+    // Wrap this in a unique_ptr for now so that it destructs until we assign it the the CURL handle.
+    std::unique_ptr<CURLContext> context = std::make_unique<CURLContext>(baton);
+
+    if (baton->response) {
+        if (!baton->response->etag.empty()) {
+            const std::string header = std::string("If-None-Match: ") + baton->response->etag;
+            context->headers = curl_slist_append(context->headers, header.c_str());
+        } else if (baton->response->modified) {
+            const time_t modified = baton->response->modified;
+            struct tm *timeinfo = std::gmtime(&modified);
+            char buffer[64];
+            strftime_l(buffer, 64, "If-Modified-Since: %a, %d %b %Y %H:%M:%S GMT", timeinfo, locale);
+            context->headers = curl_slist_append(context->headers, buffer);
+        }
+    }
+
+    if (context->headers) {
+        curl_easy_setopt(handle, CURLOPT_HTTPHEADER, context->headers);
+    }
+
     if (!baton->response) {
         baton->response = std::make_unique<Response>();
     }
 
     // Carry on the shared pointer in the private information of the CURL handle.
-    curl_easy_setopt(handle, CURLOPT_PRIVATE, ptr);
+    curl_easy_setopt(handle, CURLOPT_PRIVATE, context.release());
     curl_easy_setopt(handle, CURLOPT_CAINFO, "ca-bundle.crt");
     curl_easy_setopt(handle, CURLOPT_URL, baton->path.c_str());
     curl_easy_setopt(handle, CURLOPT_WRITEFUNCTION, curl_write_cb);
@@ -375,7 +419,8 @@ void start_request(void *const ptr) {
 // This function must run in the CURL thread.
 void stop_request(void *const ptr) {
     assert(uv_thread_self() == thread_id);
-    util::ptr<HTTPRequestBaton> &baton = *(util::ptr<HTTPRequestBaton> *)ptr;
+    std::unique_ptr<util::ptr<HTTPRequestBaton>> baton_guard { (util::ptr<HTTPRequestBaton> *)ptr };
+    util::ptr<HTTPRequestBaton> &baton = *baton_guard.get();
     assert(baton);
 
     if (baton->async) {
@@ -392,8 +437,6 @@ void stop_request(void *const ptr) {
         // the pointer below is the last lifeline of the HTTPRequestBaton. This means we're going
         // to delete the HTTPRequestBaton in the current (CURL) thread.
     }
-
-    delete (util::ptr<HTTPRequestBaton> *)ptr;
 }
 
 void create_thread() {
diff --git a/common/ios.mm b/common/ios.mm
index 1076343e24..7989e73a4e 100644
--- a/common/ios.mm
+++ b/common/ios.mm
@@ -8,12 +8,14 @@ namespace platform {
 // Returns the path to the default cache database on this system.
 std::string defaultCacheDatabase() {
     NSArray *paths = NSSearchPathForDirectoriesInDomains(NSLibraryDirectory, NSUserDomainMask, YES);
+    if ([paths count] == 0) {
+        // Disable the cache if we don't have a location to write.
+        return "";
+    }
+
     NSString *libraryDirectory = [paths objectAtIndex:0];
     return [[libraryDirectory stringByAppendingPathComponent:@"cache.db"] UTF8String];
 }
 
 }
 }
-
-
-// Returns the path to the default cache database on this system.
diff --git a/common/linux.cpp b/common/linux.cpp
new file mode 100644
index 0000000000..6132ace692
--- /dev/null
+++ b/common/linux.cpp
@@ -0,0 +1,12 @@
+#include <mbgl/platform/platform.hpp>
+
+namespace mbgl {
+namespace platform {
+
+// Returns the path to the default cache database on this system.
+std::string defaultCacheDatabase() {
+    return "/tmp/mbgl-cache.db";
+}
+
+}
+}
diff --git a/common/osx.mm b/common/osx.mm
new file mode 100644
index 0000000000..974ea537cb
--- /dev/null
+++ b/common/osx.mm
@@ -0,0 +1,31 @@
+#import <Foundation/Foundation.h>
+
+#include <mbgl/platform/platform.hpp>
+
+namespace mbgl {
+namespace platform {
+
+// Returns the path to the default cache database on this system.
+std::string defaultCacheDatabase() {
+    NSArray *paths =
+        NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, YES);
+    if ([paths count] == 0) {
+        // Disable the cache if we don't have a location to write.
+        return "";
+    }
+
+    NSString *path = [[paths objectAtIndex:0] stringByAppendingPathComponent:@"Mapbox GL"];
+
+    if (![[NSFileManager defaultManager] createDirectoryAtPath:path
+                                   withIntermediateDirectories:YES
+                                                    attributes:nil
+                                                         error:nil]) {
+        // Disable the cache if we couldn't create the directory.
+        return "";
+    }
+
+    return [[path stringByAppendingPathComponent:@"cache.db"] UTF8String];
+}
+
+}
+}