Support experimental re-mmap wrappers

Summary: Add experimental support for reordering the pages of a file that is mmap:ed by JSBigFileString. The wrapper is auto-detected (by checking file size and magic header) and transparently reorders the pages.

Reviewed By: ridiculousfish

Differential Revision: D14721397

fbshipit-source-id: 34e095350a9eeb9b07105bed6f3379f2fe472ae6

ReactCommon/cxxreact/tests/jsbigstring.cpp

@@ -60,3 +60,46 @@ TEST(JSBigFileString, MapPartTest) {
     ASSERT_EQ(needle[i], bigStr.c_str()[i]);
   }
 }
+
+TEST(JSBigFileString, RemapTest) {
+  static const uint8_t kRemapMagic[] = {
+    0xc6, 0x1f, 0xbc, 0x03, 0xc1, 0x03, 0x19, 0x1f, 0xa1, 0xd0, 0xeb, 0x73
+  };
+  std::string data(std::begin(kRemapMagic), std::end(kRemapMagic));
+  auto app = [&data](uint16_t v) {
+    data.append(reinterpret_cast<char *>(&v), sizeof(v));
+  };
+  size_t pageSizeLog2 = 16;
+  app(pageSizeLog2);
+  size_t pageSize = 1 << pageSizeLog2;
+  app(1);  // header pages
+  app(2);  // num mappings
+  // file page 0 -> memory page 1
+  app(1);  // memory page
+  app(1);  // num pages
+  // file page 1 -> memory page 0
+  app(0);  // memory page
+  app(1);  // num pages
+  while (data.size() < pageSize) {
+    app(0);
+  }
+  while (data.size() < pageSize * 2) {
+    app(0x1111);
+  }
+  while (data.size() < pageSize * 3) {
+    app(0x2222);
+  }
+
+  int fd = tempFileFromString(data);
+  JSBigFileString bigStr {fd, data.size()};
+
+  ASSERT_EQ(pageSize * 2, bigStr.size());
+  auto remapped = bigStr.c_str();
+  size_t i = 0;
+  for (; i < pageSize; ++i) {
+    ASSERT_EQ(0x22, remapped[i]);
+  }
+  for (; i < pageSize * 2; ++i) {
+    ASSERT_EQ(0x11, remapped[i]);
+  }
+}