From ccda2ada577477d13fa14c132d8c5f578c881235 Mon Sep 17 00:00:00 2001
From: Julien Gilli <julien.gilli@joyent.com>
Date: Tue, 8 Sep 2015 16:27:44 -0700
Subject: [PATCH] joyent/mdb_v8#23: fix scopeinfo for v8 > 3.28.x

---
 src/mdb_v8.c         | 12 ++++++----
 src/mdb_v8_context.c | 53 +++++++++++++++++++++++++++++++++++++++++---
 src/mdb_v8_impl.h    |  1 +
 3 files changed, 59 insertions(+), 7 deletions(-)

diff --git a/src/mdb_v8.c b/src/mdb_v8.c
index c3d55db..f4ca854 100644
--- a/src/mdb_v8.c
+++ b/src/mdb_v8.c
@@ -169,6 +169,7 @@ intptr_t V8_CONTEXT_IDX_GLOBAL;
 
 intptr_t V8_SCOPEINFO_IDX_NPARAMS;
 intptr_t V8_SCOPEINFO_IDX_NSTACKLOCALS;
+intptr_t V8_SCOPEINFO_OFFSET_STACK_LOCALS;
 intptr_t V8_SCOPEINFO_IDX_NCONTEXTLOCALS;
 intptr_t V8_SCOPEINFO_IDX_FIRST_VARS;
 
@@ -355,14 +356,17 @@ static v8_constant_t v8_constants[] = {
 	    V8_CONSTANT_FALLBACK(0, 0), 3 },
 
 	{ &V8_SCOPEINFO_IDX_NPARAMS, "v8dbg_scopeinfo_idx_nparams",
-	    V8_CONSTANT_FALLBACK(0, 0), 1 },
+	    V8_CONSTANT_FALLBACK(3, 7), 1 },
 	{ &V8_SCOPEINFO_IDX_NSTACKLOCALS, "v8dbg_scopeinfo_idx_nstacklocals",
-	    V8_CONSTANT_FALLBACK(0, 0), 2 },
+	    V8_CONSTANT_FALLBACK(3, 7), 2 },
+	{ &V8_SCOPEINFO_OFFSET_STACK_LOCALS,
+		"v8dbg_scopeinfo_offset_stack_locals",
+	    V8_CONSTANT_ADDED_SINCE(4, 4) },
 	{ &V8_SCOPEINFO_IDX_NCONTEXTLOCALS,
 	    "v8dbg_scopeinfo_idx_ncontextlocals",
-	    V8_CONSTANT_FALLBACK(0, 0), 3 },
+	    V8_CONSTANT_FALLBACK(3, 7), 3 },
 	{ &V8_SCOPEINFO_IDX_FIRST_VARS, "v8dbg_scopeinfo_idx_first_vars",
-	    V8_CONSTANT_FALLBACK(0, 0), 4 },
+	    V8_CONSTANT_FALLBACK(3, 7), 4 },
 };
 
 static int v8_nconstants = sizeof (v8_constants) / sizeof (v8_constants[0]);
diff --git a/src/mdb_v8_context.c b/src/mdb_v8_context.c
index 03f025d..0478a7a 100644
--- a/src/mdb_v8_context.c
+++ b/src/mdb_v8_context.c
@@ -67,12 +67,13 @@ typedef struct {
 	v8scopeinfo_vartype_t	v8vti_vartype;
 	const char		*v8vti_label;
 	intptr_t		*v8vti_idx_countp;
+	intptr_t		*v8vti_offset;
 } v8scopeinfo_vartype_info_t;
 
 static v8scopeinfo_vartype_info_t v8scopeinfo_vartypes[] = {
 	{ V8SV_PARAMS, "parameter", &V8_SCOPEINFO_IDX_NPARAMS },
 	{ V8SV_STACKLOCALS, "stack local variable",
-	    &V8_SCOPEINFO_IDX_NSTACKLOCALS },
+	    &V8_SCOPEINFO_IDX_NSTACKLOCALS, &V8_SCOPEINFO_OFFSET_STACK_LOCALS },
 	{ V8SV_CONTEXTLOCALS, "context local variable",
 	    &V8_SCOPEINFO_IDX_NCONTEXTLOCALS },
 };
@@ -385,13 +386,59 @@ v8scopeinfo_iter_vars(v8scopeinfo_t *sip,
 	assert(vtip != NULL);
 	nvars = v8scopeinfo_vartype_nvars(sip, scopevartype);
 
+	/*
+	 * A ScopeInfo instance has two distinct parts:
+	 *
+	 * 1) A static part that contains information about the number of
+	 * entries for each variable type.
+	 *
+	 * 2) A dynamic part of variable size that contains the actual data for
+	 * each variable type (parameters, stack local and context local
+	 * entries' names).
+	 *
+	 * V8_SCOPEINFO_IDX_FIRST_VARS is the offset from the beginning of the
+	 * ScopeInfo layout to the start of the variable part that contains the
+	 * actual information for each variable type, so we start by skipping to
+	 * that offset.
+	 */
 	nskip = V8_SCOPEINFO_IDX_FIRST_VARS;
+
+	/*
+	 * Iterate over variable types so that we can add the offset from the
+	 * beginning of the actual data (the dynamic part) to the region of the
+	 * dynamic part that is specific to the variable type we're interested
+	 * in.
+	 */
 	for (i = 0; i < v8scopeinfo_nvartypes; i++) {
 		ogrp = &v8scopeinfo_vartypes[i];
-		if (*(ogrp->v8vti_idx_countp) >= *(vtip->v8vti_idx_countp)) {
-			continue;
+
+		/*
+		 * In the variable/dynamic part of a ScopeInfo layout, some
+		 * variable types have static metadata, e.g stack local entries
+		 * have a StackLocalFirstSlot, before the actual data. Add that
+		 * offset for each variable type, including for the one we're
+		 * interested in.
+		 */
+		if (v8scopeinfo_vartypes[i].v8vti_offset != NULL &&
+		    *(v8scopeinfo_vartypes[i].v8vti_offset) != -1) {
+			nskip += *(v8scopeinfo_vartypes[i].v8vti_offset);
+		}
+
+		/*
+		 * The current variable type is the one we're interested in,
+		 * do not add anything to the offset, we're done.
+		 */
+		if (*(ogrp->v8vti_idx_countp) == *(vtip->v8vti_idx_countp)) {
+			break;
 		}
 
+		/*
+		 * The data for the current variable type is before the one
+		 * we're interested in in the variable part of the ScopeInfo
+		 * layout.
+		 * Add the number of entries for this variable type to the
+		 * offset.
+		 */
 		nskip += v8scopeinfo_vartype_nvars(sip, ogrp->v8vti_vartype);
 	}
 
diff --git a/src/mdb_v8_impl.h b/src/mdb_v8_impl.h
index fac16e5..c4fd66a 100644
--- a/src/mdb_v8_impl.h
+++ b/src/mdb_v8_impl.h
@@ -62,6 +62,7 @@ extern intptr_t V8_SCOPEINFO_IDX_FIRST_VARS;
 extern intptr_t V8_SCOPEINFO_IDX_NCONTEXTLOCALS;
 extern intptr_t V8_SCOPEINFO_IDX_NPARAMS;
 extern intptr_t V8_SCOPEINFO_IDX_NSTACKLOCALS;
+extern intptr_t V8_SCOPEINFO_OFFSET_STACK_LOCALS;
 
 extern intptr_t V8_HeapObjectTag;
 extern intptr_t V8_HeapObjectTagMask;