use the default cache location on ios to avoid writing to unwritable file system locations

common/ios.mm

@@ -0,0 +1,19 @@
+#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(NSLibraryDirectory, NSUserDomainMask, YES);
+    NSString *libraryDirectory = [paths objectAtIndex:0];
+    return [[libraryDirectory stringByAppendingPathComponent:@"cache.db"] UTF8String];
+}
+
+}
+}
+
+
+// Returns the path to the default cache database on this system.

include/mbgl/platform/platform.hpp

@@ -17,6 +17,9 @@ std::string uppercase(const std::string &string);
 // Lowercase a string, potentially using platform-specific routines.
 std::string lowercase(const std::string &string);
 
+// Returns the path to the default cache database on this system.
+std::string defaultCacheDatabase();
+
 // Shows an alpha image with the specified dimensions in a named window.
 void show_debug_image(std::string name, const char *data, size_t width, size_t height);
 

include/mbgl/storage/file_source.hpp

@@ -24,7 +24,7 @@ private:
     FileSource& operator=(FileSource &&) = delete;
 
 public:
-    FileSource(uv_loop_t *loop);
+    FileSource(uv_loop_t *loop, const std::string &path);
     ~FileSource();
 
 public:

src/map/map.cpp

@@ -571,7 +571,7 @@ void Map::prepare() {
     view.make_active();
 
     if (!fileSource) {
-        fileSource = std::make_shared<FileSource>(**loop);
+        fileSource = std::make_shared<FileSource>(**loop, platform::defaultCacheDatabase());
         glyphStore = std::make_shared<GlyphStore>(fileSource);
     }
 

src/storage/file_source.cpp

@@ -8,9 +8,9 @@
 
 namespace mbgl {
 
-FileSource::FileSource(uv_loop_t *loop_)
+FileSource::FileSource(uv_loop_t *loop_, const std::string &path)
     : thread_id(uv_thread_self()),
-      store(util::ptr<SQLiteStore>(new SQLiteStore(loop_, "cache.db"))),
+      store(util::ptr<SQLiteStore>(new SQLiteStore(loop_, path))),
       loop(loop_),
       queue(new uv_messenger_t) {