Add option to set file buffer flags via URI parameter when writing files

Summary: Until recently, it was difficult to inject file handling customization into file handlers, but we now can. This diff uses that new path to make it possible to customize the android specific default, without having to touch the VRS code.

Reviewed By: jtbraun

Differential Revision: D60793634

fbshipit-source-id: 4b373aa5f83732abaed83abf833c027efb545b2b

vrs/DiskFileChunk.hpp

@@ -20,6 +20,7 @@
 #include <cstdio>
 
 #include <vrs/FileHandler.h>
+#include <vrs/helpers/Strings.h>
 #include <vrs/os/CompilerAttributes.h>
 #include <vrs/os/Utils.h>
 
@@ -52,6 +53,10 @@ class DiskFileChunk {
     path_ = newPath;
     offset_ = 0;
     size_ = 0;
+    uint64_t bufferSize = 0;
+    if (helpers::getUInt64(options, "io_buffer_size", bufferSize)) {
+      return setvbuf(file_, nullptr, bufferSize == 0 ? _IONBF : _IOFBF, bufferSize);
+    }
 #if IS_ANDROID_PLATFORM()
     const size_t kBufferingSize = 128 * 1024;
     return setvbuf(file_, nullptr, _IOFBF, kBufferingSize);