Fix pool header block problem

Summary:
There seems to be a bug in the pool allocation regarding the header block in the scenario where no words are left (besides the header word).

Added a panic in the scenario I believe is broken and where the null terminating word is actually overwriting the header word.

In OCaml, any heap block has to be at least 2 words (1 header + one block word). The ocamlpool code needs to change so that when we cannot allocate the next block, there is one word left in the pool (in addition to the reserved header word).

That way, the zero terminator can be properly written in the String_tagged header block.

Reviewed By: rjbailey

Differential Revision: D52574962

fbshipit-source-id: 41795b3900551627e444cf627ad8642745a18434

Author: Manuel Fahndrich

ocamlrep_ocamlpool/ocamlpool.c

@@ -118,7 +118,8 @@ static void assert_out_of_section(void) {
  */
 
 static void init_cursor(void) {
-  ocamlpool_limit = (value*)ocamlpool_root;
+  ocamlpool_limit =
+      (value*)ocamlpool_root + 1; // Need one word left for header block
   ocamlpool_bound = (value*)ocamlpool_root + Wosize_val(ocamlpool_root);
   ocamlpool_cursor = ocamlpool_bound;
 }
@@ -135,6 +136,7 @@ __attribute__((no_sanitize("undefined"))) static void ocamlpool_chunk_truncate(
 
   OCAMLPOOL_SET_HEADER(ocamlpool_root, word_size, String_tag, ocamlpool_color);
   value* first_word = (value*)ocamlpool_root;
+  abort_unless(word_size > 0, "ocamlpool_truncate 0 words left for terminator");
   first_word[word_size - 1] = 0;
 }
 
@@ -253,8 +255,8 @@ value ocamlpool_reserve_block(int tag, size_t words) {
   if (pointer < ocamlpool_limit || pointer >= ocamlpool_bound) {
     size_t old_ocamlpool_next_chunk_size = ocamlpool_next_chunk_size;
     if (size >= ocamlpool_next_chunk_size) {
-      // Add 1 word for ocaml's header
-      ocamlpool_next_chunk_size = size + 1;
+      // Add 2 words for ocaml's header + block
+      ocamlpool_next_chunk_size = size + 2;
     }
     ocamlpool_chunk_truncate();
     ocamlpool_chunk_alloc();
@@ -276,7 +278,7 @@ value ocamlpool_reserve_string(size_t bytes) {
   size_t words =
       ((bytes + 1 /*null-ending*/ + (WORD_SIZE - 1) /*rounding*/) / WORD_SIZE);
   size_t length = (words * WORD_SIZE);
-
+  abort_unless(words > 0, "ocamlpool_reserve_string 0 length");
   value result = ocamlpool_reserve_block(String_tag, words);
 
   ((value*)result)[words - 1] = 0;