Fix memory leak in prism when syntax error in iseq compilation

If there's a syntax error during iseq compilation then prism would leak
memory because it would not free the pm_parse_result_t.

This commit changes pm_iseq_new_with_opt to have a rb_protect to catch
when an error is raised, and return NULL and set error_state to a value
that can be raised by calling rb_jump_tag after memory has been freed.

For example:

    10.times do
      10_000.times do
        eval("/[/=~s")
      rescue SyntaxError
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    39280
    68736
    99232
    128864
    158896
    188208
    217344
    246304
    275376
    304592

After:

    12192
    13200
    14256
    14848
    16000
    16000
    16000
    16064
    17232
    17952

1 files changed, 15 insertions, 2 deletions

diff --git a/prism_compile.c b/prism_compile.c
index 53938ebb0c..f4a35fa429 100644
--- a/prism_compile.c
+++ b/prism_compile.c
@@ -1268,11 +1268,17 @@ pm_new_child_iseq(rb_iseq_t *iseq, pm_scope_node_t *node, VALUE name, const rb_i
 {
     debugs("[new_child_iseq]> ---------------------------------------\n");
     int isolated_depth = ISEQ_COMPILE_DATA(iseq)->isolated_depth;
+    int error_state;
     rb_iseq_t *ret_iseq = pm_iseq_new_with_opt(node, name,
             rb_iseq_path(iseq), rb_iseq_realpath(iseq),
             line_no, parent,
             isolated_depth ? isolated_depth + 1 : 0,
-            type, ISEQ_COMPILE_DATA(iseq)->option);
+            type, ISEQ_COMPILE_DATA(iseq)->option, &error_state);
+
+    if (error_state) {
+        RUBY_ASSERT(ret_iseq == NULL);
+        rb_jump_tag(error_state);
+    }
     debugs("[new_child_iseq]< ---------------------------------------\n");
     return ret_iseq;
 }
@@ -3479,6 +3485,7 @@ pm_compile_builtin_mandatory_only_method(rb_iseq_t *iseq, pm_scope_node_t *scope
     pm_scope_node_t next_scope_node;
     pm_scope_node_init(&def.base, &next_scope_node, scope_node);
 
+    int error_state;
     ISEQ_BODY(iseq)->mandatory_only_iseq = pm_iseq_new_with_opt(
         &next_scope_node,
         rb_iseq_base_label(iseq),
@@ -3488,9 +3495,15 @@ pm_compile_builtin_mandatory_only_method(rb_iseq_t *iseq, pm_scope_node_t *scope
         NULL,
         0,
         ISEQ_TYPE_METHOD,
-        ISEQ_COMPILE_DATA(iseq)->option
+        ISEQ_COMPILE_DATA(iseq)->option,
+        &error_state
     );
 
+    if (error_state) {
+        RUBY_ASSERT(ISEQ_BODY(iseq)->mandatory_only_iseq == NULL);
+        rb_jump_tag(error_state);
+    }
+
     pm_scope_node_destroy(&next_scope_node);
     return COMPILE_OK;
 }