[core] finish must-revalidate support

.gitignore

@@ -19,8 +19,8 @@ xcuserdata
 /test/fixtures/offline_database/offline.db-*
 /test/fixtures/offline_database/invalid.db
 /test/fixtures/offline_database/invalid.db-*
-/test/fixtures/offline_database/v*.db
-/test/fixtures/offline_database/v*.db-*
+/test/fixtures/offline_database/migrated.db
+/test/fixtures/offline_database/migrated.db-*
 /test/fixtures/**/actual.png
 /test/fixtures/**/diff.png
 /test/output

include/mbgl/storage/resource.hpp

@@ -68,6 +68,7 @@ class Resource {
     optional<Timestamp> priorModified = {};
     optional<Timestamp> priorExpires = {};
     optional<std::string> priorEtag = {};
+    std::shared_ptr<const std::string> priorData;
 };
 
 } // namespace mbgl

include/mbgl/storage/response.hpp

@@ -27,6 +27,10 @@ class Response {
     // This is set to true for 304 Not Modified responses.
     bool notModified = false;
 
+    // This is set to true when the server requested that no expired resources be used by
+    // specifying "Cache-Control: must-revalidate".
+    bool mustRevalidate = false;
+
     // The actual data of the response. Present only for non-error, non-notModified responses.
     std::shared_ptr<const std::string> data;
 
@@ -37,6 +41,12 @@ class Response {
     bool isFresh() const {
         return expires ? *expires > util::now() : !error;
     }
+
+    // Indicates whether we are allowed to use this response according to HTTP caching rules.
+    // It may or may not be stale.
+    bool isUsable() const {
+        return !mustRevalidate || (expires && *expires > util::now());
+    }
 };
 
 class Response::Error {

platform/android/src/http_file_source.cpp

@@ -117,7 +117,9 @@ void HTTPRequest::onResponse(jni::JNIEnv& env, int code,
     }
 
     if (cacheControl) {
-        response.expires = http::CacheControl::parse(jni::Make<std::string>(env, cacheControl).c_str()).toTimePoint();
+        const auto cc = http::CacheControl::parse(jni::Make<std::string>(env, cacheControl).c_str());
+        response.expires = cc.toTimePoint();
+        response.mustRevalidate = cc.mustRevalidate;
     }
 
     if (expires) {

platform/darwin/src/http_file_source.mm

@@ -266,7 +266,9 @@ void cancel() {
                     NSDictionary *headers = [(NSHTTPURLResponse *)res allHeaderFields];
                     NSString *cache_control = [headers objectForKey:@"Cache-Control"];
                     if (cache_control) {
-                        response.expires = http::CacheControl::parse([cache_control UTF8String]).toTimePoint();
+                        const auto cc = http::CacheControl::parse([cache_control UTF8String]);
+                        response.expires = cc.toTimePoint();
+                        response.mustRevalidate = cc.mustRevalidate;
                     }
 
                     NSString *expires = [headers objectForKey:@"Expires"];

platform/default/default_file_source.cpp

@@ -140,7 +140,17 @@ class DefaultFileSource::Impl {
                     revalidation.priorModified = offlineResponse->modified;
                     revalidation.priorExpires = offlineResponse->expires;
                     revalidation.priorEtag = offlineResponse->etag;
-                    callback(*offlineResponse);
+
+                    // Don't return resources the server requested not to show when they're stale.
+                    // Even if we can't directly use the response, we may still use it to send a
+                    // conditional HTTP request.
+                    if (offlineResponse->isUsable()) {
+                        callback(*offlineResponse);
+                    } else {
+                        // Since we can't return the data immediately, we'll have to hold on so that
+                        // we can return it later in case we get a 304 Not Modified response.
+                        revalidation.priorData = offlineResponse->data;
+                    }
                 }
             }
 

platform/default/http_file_source.cpp

@@ -325,7 +325,9 @@ size_t HTTPRequest::headerCallback(char *const buffer, const size_t size, const
         baton->response->etag = std::string(buffer + begin, length - begin - 2); // remove \r\n
     } else if ((begin = headerMatches("cache-control: ", buffer, length)) != std::string::npos) {
         const std::string value { buffer + begin, length - begin - 2 }; // remove \r\n
-        baton->response->expires = http::CacheControl::parse(value.c_str()).toTimePoint();
+        const auto cc = http::CacheControl::parse(value.c_str());
+        baton->response->expires = cc.toTimePoint();
+        baton->response->mustRevalidate = cc.mustRevalidate;
     } else if ((begin = headerMatches("expires: ", buffer, length)) != std::string::npos) {
         const std::string value { buffer + begin, length - begin - 2 }; // remove \r\n
         baton->response->expires = Timestamp{ Seconds(curl_getdate(value.c_str(), nullptr)) };