Bug 1585159 - Allow the shell to invoke a last-ditch GC r=jonco

I want to add this option to the gc() shell builtin to write a test case for
this bug.

Differential Revision: https://phabricator.services.mozilla.com/D48291

--HG--
extra : moz-landing-system : lando

js/src/builtin/TestingFunctions.cpp

@@ -468,14 +468,27 @@ static bool GC(JSContext* cx, unsigned argc, Value* vp) {
     }
   }
 
-  bool shrinking = false;
+  JSGCInvocationKind gckind = GC_NORMAL;
+  JS::GCReason reason = JS::GCReason::API;
   if (args.length() >= 2) {
     Value arg = args[1];
     if (arg.isString()) {
+      bool shrinking = false;
+      bool last_ditch = false;
       if (!JS_StringEqualsLiteral(cx, arg.toString(), "shrinking",
                                   &shrinking)) {
         return false;
       }
+      if (!JS_StringEqualsLiteral(cx, arg.toString(), "last-ditch",
+                                  &last_ditch)) {
+        return false;
+      }
+      if (shrinking) {
+        gckind = GC_SHRINK;
+      } else if (last_ditch) {
+        gckind = GC_SHRINK;
+        reason = JS::GCReason::LAST_DITCH;
+      }
     }
   }
 
@@ -489,8 +502,7 @@ static bool GC(JSContext* cx, unsigned argc, Value* vp) {
     JS::PrepareForFullGC(cx);
   }
 
-  JSGCInvocationKind gckind = shrinking ? GC_SHRINK : GC_NORMAL;
-  JS::NonIncrementalGC(cx, gckind, JS::GCReason::API);
+  JS::NonIncrementalGC(cx, gckind, reason);
 
   char buf[256] = {'\0'};
 #ifndef JS_MORE_DETERMINISTIC
@@ -6155,12 +6167,13 @@ static bool PCCountProfiling_ScriptContents(JSContext* cx, unsigned argc,
 // clang-format off
 static const JSFunctionSpecWithHelp TestingFunctions[] = {
     JS_FN_HELP("gc", ::GC, 0, 0,
-"gc([obj] | 'zone' [, 'shrinking'])",
-"  Run the garbage collector. When obj is given, GC only its zone.\n"
-"  If 'zone' is given, GC any zones that were scheduled for\n"
-"  GC via schedulegc.\n"
-"  If 'shrinking' is passed as the optional second argument, perform a\n"
-"  shrinking GC rather than a normal GC."),
+"gc([obj] | 'zone' [, ('shrinking' | 'last-ditch') ])",
+"  Run the garbage collector.\n"
+"  The first parameter describes which zones to collect: if an object is\n"
+"  given, GC only its zone. If 'zone' is given, GC any zones that were\n"
+"  scheduled via schedulegc.\n"
+"  The second parameter is optional and may be 'shrinking' to perform a\n"
+"  shrinking GC or 'last-ditch' for a shrinking, last-ditch GC."),
 
     JS_FN_HELP("minorgc", ::MinorGC, 0, 0,
 "minorgc([aboutToOverflow])",