Add new FileSourceProvider

Summary: Add a new interface to JSC that allows loading a file lazily from disk, i.e. using mmap, instead of loading the whole file upfront and copying into the VM.

Reviewed By: michalgr

Differential Revision: D3534042

fbshipit-source-id: 98b193cc7b7e33248073e2556ea94ce3391507c7

ReactCommon/cxxreact/Executor.h

@@ -7,6 +7,8 @@
 #include <string>
 #include <vector>
 
+#include <sys/mman.h>
+
 #include <folly/dynamic.h>
 
 #include "JSModulesUnbundle.h"
@@ -134,6 +136,68 @@ private:
   size_t m_size;
 };
 
+class JSBigMmapString : public JSBigString  {
+public:
+  enum class Encoding {
+    Unknown,
+    Ascii,
+    Utf8,
+    Utf16,
+  };
+
+
+  JSBigMmapString(int fd, size_t size, const uint8_t sha1[20], Encoding encoding) :
+    m_fd(fd),
+    m_size(size),
+    m_encoding(encoding),
+    m_str(nullptr)
+  {
+    memcpy(m_hash, sha1, 20);
+  }
+
+  ~JSBigMmapString() {
+    if (m_str) {
+      CHECK(munmap((void *)m_str, m_size) != -1);
+    }
+    close(m_fd);
+  }
+
+  bool isAscii() const override {
+    return m_encoding == Encoding::Ascii;
+  }
+
+  const char* c_str() const override {
+    if (!m_str) {
+      m_str = (const char *)mmap(0, m_size, PROT_READ, MAP_SHARED, m_fd, 0);
+      CHECK(m_str != MAP_FAILED);
+    }
+    return m_str;
+  }
+
+  size_t size() const override {
+    return m_size;
+  }
+
+  int fd() const {
+    return m_fd;
+  }
+
+  const uint8_t* hash() const {
+    return m_hash;
+  }
+
+  Encoding encoding() const {
+    return m_encoding;
+  }
+
+private:
+  int m_fd;
+  size_t m_size;
+  uint8_t m_hash[20];
+  Encoding m_encoding;
+  mutable const char *m_str;
+};
+
 class JSExecutor {
 public:
   /**