From 49df7b4895fe4cf4a491d3c81b88a2e105c549b0 Mon Sep 17 00:00:00 2001
From: Josh Yelon <jyelon@gmail.com>
Date: Mon, 1 Mar 2021 18:10:40 -0500
Subject: [PATCH] LuaSnap is now working based on eris (at least
 superficially).

---
 luprex/core/cpp/luasnap.cpp  | 68 +++++++++++++++++++++++++++++++++---
 luprex/core/cpp/luasnap.hpp  | 13 ++-----
 luprex/core/cpp/luastack.hpp |  1 +
 3 files changed, 67 insertions(+), 15 deletions(-)

diff --git a/luprex/core/cpp/luasnap.cpp b/luprex/core/cpp/luasnap.cpp
index 1ceba283..6c9b81a3 100644
--- a/luprex/core/cpp/luasnap.cpp
+++ b/luprex/core/cpp/luasnap.cpp
@@ -9,6 +9,15 @@
 LuaSnap::LuaSnap() {
     state_ = luaL_newstate();
     LuaStack LS(state_);
+
+    // Create the persist table and the unpersist table.
+    //
+    // These tables need to contain all C functions.  Whenever
+    // the source module inserts a C function into the lua environment,
+    // it also needs to register the C function in these tables.
+    //
+    // I don't think anything else needs to go in these tables.
+    //
     LS.rawset(LuaRegistry, "persist", LuaNewTable);
     LS.rawset(LuaRegistry, "unpersist", LuaNewTable);
 }
@@ -21,11 +30,32 @@ bool LuaSnap::have_snapshot() const {
     return !snapshot_.empty();
 }
 
-static void oss_writer(lua_State *L, const void *p, size_t sz, void *ud) {
+static int oss_writer(lua_State *L, const void *p, size_t sz, void *ud) {
     std::ostringstream *oss = (std::ostringstream *)ud;
     oss->write((const char *)p, sz);
+    return 0;
 }
 
+// Convert a C++ string into a lua_Reader.
+class StringReader {
+public:
+    const std::string &str_;
+    bool done_;
+    StringReader(const std::string &s) : str_(s), done_(false) {}
+    
+    static const char *fn(lua_State *L, void *data, size_t *size) {
+        StringReader *sr = (StringReader*)data;
+        if (sr->done_) {
+            *size = 0;
+            return nullptr;
+        } else {
+            sr->done_ = true;
+            *size = sr->str_.size();
+            return sr->str_.c_str();
+        }
+    }
+};
+
 void LuaSnap::snapshot() {
     // Snapshot and the lua stack should both be empty.
     assert(snapshot_.empty());
@@ -43,16 +73,20 @@ void LuaSnap::snapshot() {
         LS.rawset(regcopy, key, value);
     }
 
-    // Remove certain things from the copy.
+    // Remove certain things from the copy.  These items
+    // don't get included in the snapshot.
+    //
+    // From a modularity perspective, this is bad.
+    //
     LS.rawset(regcopy, LUA_RIDX_MAINTHREAD, LuaNil);
     LS.rawset(regcopy, "persist", LuaNil);
     LS.rawset(regcopy, "unpersist", LuaNil);
     LS.rawset(regcopy, "world", LuaNil);
     LS.rawset(regcopy, "gui", LuaNil);
-    LS.result();
 
-    // Get the permanents table from the registry.
+    // Get the eris permanents table from the registry.
     LS.rawget(permstable, LuaRegistry, "persist");
+    assert(LS.istable(permstable));
 
     // When we call 'LS.result', this should leave the
     // permstable and the regcopy on the stack.
@@ -64,9 +98,35 @@ void LuaSnap::snapshot() {
     std::ostringstream oss;
     eris_dump(state_, oss_writer, (void *)&oss);
     snapshot_ = oss.str();
+    std::cerr << "Eris dump is " << snapshot_.size() << " bytes." << std::endl;
+    lua_settop(state_, 0);
 }
 
 void LuaSnap::rollback() {
+    // Snapshot should have data, lua stack should be empty.
     assert(!snapshot_.empty());
+    assert(lua_gettop(state_) == 0);
+
+    // Call eris with the permanents table and passing the snapshot as a lua_Reader.
+    lua_getfield(state_, LUA_REGISTRYINDEX, "unpersist");
+    StringReader sr(snapshot_);
+    eris_undump(state_, StringReader::fn, &sr);
+    assert(lua_gettop(state_) == 2);
+
+    // Set up a stack frame.
+    LuaArg permstable, regcopy;
+    LuaVar key, value;
+    LuaStack LS(state_, permstable, regcopy, key, value);
+
+    assert(LS.istable(regcopy));
+
+    // Copy the contents of the snapshot over to the registry.
+    LS.set(key, LuaNil);
+    while (LS.next(regcopy, key, value) != 0) {
+        LS.rawset(LuaRegistry, key, value);
+    }
+    LS.result();
+    assert(lua_gettop(state_) == 0);
+    snapshot_.clear();
 }
 
diff --git a/luprex/core/cpp/luasnap.hpp b/luprex/core/cpp/luasnap.hpp
index c8ce019e..d8794c34 100644
--- a/luprex/core/cpp/luasnap.hpp
+++ b/luprex/core/cpp/luasnap.hpp
@@ -6,17 +6,8 @@
 // possible to roll the entire interpreter back to a previously-snapshotted
 // state.
 //
-// To accomplish this, we take advantage of the 'allocf' parameter to
-// lua_newstate.  This lets us hook the lua allocator, which in turn lets us
-// find every block of memory currently in use by lua.  To snapshot, we 'memcpy'
-// every block of memory that lua is using into a buffer.  To rollback, we just
-// 'memcpy' the data back.
-//
-// The current implementation is a proof-of-concept.  It's quite wasteful of
-// memory, roughly doubling the amount of RAM that LUA uses.  It's also not
-// super-fast, since it allocates tons of tiny blocks.  But it totally
-// works, so it demonstrate that this method of snapshot and rollback is
-// feasible.
+// To accomplish this, we use eris serialization.  This is messy and not
+// very modular, but it does work.
 //
 /////////////////////////////////////////////////////////////////////////////
 
diff --git a/luprex/core/cpp/luastack.hpp b/luprex/core/cpp/luastack.hpp
index e5ab639f..b3edcdab 100644
--- a/luprex/core/cpp/luastack.hpp
+++ b/luprex/core/cpp/luastack.hpp
@@ -158,6 +158,7 @@ extern "C" {
 #include "lua.h"
 #include "lauxlib.h"
 #include "lualib.h"
+#include "eris.h"
 }
 
 #include <string>