Create Snapshot From Stream

Summary:
Adding an instrumentation endpoint that allows us to write a snapshot out to an
arbitrary output stream.  This is in addition to `createSnapshotFromFile`.  I
reserve the right to replace the latter with the former in a later diff.

This is necessary to allow snapshots to be requested over the chrome debugger
protocol.  The protocol sends the snapshot over the wire in chunks so we need
to be able to use an output stream that can do the chunking.

Because LLVM types are not available at the JSI layer, we accept a
`std::ostream` reference which we wrap with `llvm::raw_os_ostream` within
Hermes.  We should not incur the static initializer cost (or other code bloat)
of using ostreams as a result.

Reviewed By: cwdick

Differential Revision: D16016319

fbshipit-source-id: 2d0b4848fd5cbe9ddee371d856cd8eb19dd80396

ReactCommon/jsi/jsi/decorator.h

@@ -337,6 +337,10 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
     return plain().instrumentation().createSnapshotToFile(path, compact);
   }
 
+  bool createSnapshotToStream(std::ostream& os, bool compact) override {
+    return plain().instrumentation().createSnapshotToStream(os, compact);
+  }
+
   void writeBridgeTrafficTraceToFile(
       const std::string& fileName) const override {
     const_cast<Plain&>(plain()).instrumentation().writeBridgeTrafficTraceToFile(

ReactCommon/jsi/jsi/instrumentation.h

@@ -6,6 +6,7 @@
  */
 #pragma once
 
+#include <ostream>
 #include <string>
 
 #include <jsi/jsi.h>
@@ -55,6 +56,15 @@ class Instrumentation {
   /// \return true iff the heap capture succeeded
   virtual bool createSnapshotToFile(const std::string& path, bool compact) = 0;
 
+  /// Captures the heap to an output stream
+  ///
+  /// \param os output stream to write to.
+  ///
+  /// \param compact Whether the JSON should be compact or pretty
+  ///
+  /// \return true iff the heap capture succeeded.
+  virtual bool createSnapshotToStream(std::ostream& os, bool compact) = 0;
+
   /// Write a trace of bridge traffic to the given file name.
   virtual void writeBridgeTrafficTraceToFile(
       const std::string& fileName) const = 0;

ReactCommon/jsi/jsi/jsi.cpp

@@ -81,6 +81,10 @@ Instrumentation& Runtime::instrumentation() {
       return false;
     }
 
+    bool createSnapshotToStream(std::ostream&, bool) override {
+      return false;
+    }
+
     void writeBridgeTrafficTraceToFile(const std::string&) const override {
       std::abort();
     }