#23: do not rely on V8's metadata

Hardcode handling of V8_SCOPEINFO_IDX_FIRST_VARS' fallback value for
different versions of V8.
Do not rely on V8's metadata for getting
V8_SCOPEINFO_OFFSET_STACK_LOCALS' value, use V8_CONSTANT_FALLBACK
instead.

src/mdb_v8.c

@@ -361,12 +361,12 @@ static v8_constant_t v8_constants[] = {
 	    V8_CONSTANT_FALLBACK(3, 7), 2 },
 	{ &V8_SCOPEINFO_OFFSET_STACK_LOCALS,
 		"v8dbg_scopeinfo_offset_stack_locals",
-	    V8_CONSTANT_ADDED_SINCE(4, 4) },
+	    V8_CONSTANT_FALLBACK(4, 4), 1 },
 	{ &V8_SCOPEINFO_IDX_NCONTEXTLOCALS,
 	    "v8dbg_scopeinfo_idx_ncontextlocals",
 	    V8_CONSTANT_FALLBACK(3, 7), 3 },
 	{ &V8_SCOPEINFO_IDX_FIRST_VARS, "v8dbg_scopeinfo_idx_first_vars",
-	    V8_CONSTANT_FALLBACK(3, 7), 4 },
+	    V8_CONSTANT_FALLBACK(4, 5), 6 },
 };
 
 static int v8_nconstants = sizeof (v8_constants) / sizeof (v8_constants[0]);
@@ -809,6 +809,24 @@ autoconfigure(v8_cfg_t *cfgp)
 	if (V8_OFF_MAP_BIT_FIELD2 == -1)
 		V8_OFF_MAP_BIT_FIELD2 = V8_OFF_MAP_INSTANCE_ATTRIBUTES + 3;
 
+	/*
+	 * V8_SCOPEINFO_IDX_FIRST_VARS' value was 4 in V8 3.7 and up,
+	 * then 5 when StrongModeFreeVariableCount was added with
+	 * https://codereview.chromium.org/1005063002, and 6 when
+	 * ContextGlobalCount was added with
+	 * https://codereview.chromium.org/1218783005.
+	 * Since the current V8_CONSTANT_FALLBACK macro doesn't allow
+	 * us to specify different values for different V8 versions,
+	 * these are hardcoded below.
+	 */
+	if (V8_SCOPEINFO_IDX_FIRST_VARS == -1) {
+		if (v8_major > 4 || (v8_major == 4 && v8_minor >= 3)) {
+			V8_SCOPEINFO_IDX_FIRST_VARS = 5;
+		} else if (v8_major > 3 || (v8_major == 3 && v8_minor >= 7)) {
+			V8_SCOPEINFO_IDX_FIRST_VARS = 4;
+		}
+	}
+
 	return (failed ? -1 : 0);
 }
 

src/mdb_v8_context.c

@@ -387,19 +387,8 @@ v8scopeinfo_iter_vars(v8scopeinfo_t *sip,
 	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.
+	 * Skip to the start of the ScopeInfo's dynamic part. See mdb_v8_db.h
+	 * for more details on the layout of ScopeInfo objects.
 	 */
 	nskip = V8_SCOPEINFO_IDX_FIRST_VARS;
 
@@ -425,8 +414,8 @@ v8scopeinfo_iter_vars(v8scopeinfo_t *sip,
 		}
 
 		/*
-		 * The current variable type is the one we're interested in,
-		 * do not add anything to the offset, we're done.
+		 * If 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;
@@ -435,9 +424,8 @@ v8scopeinfo_iter_vars(v8scopeinfo_t *sip,
 		/*
 		 * 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.
+		 * layout. Add the number of entries for this variable type to
+		 * the offset.
 		 */
 		nskip += v8scopeinfo_vartype_nvars(sip, ogrp->v8vti_vartype);
 	}