[coverage] Do not reset JSFunction::code post-deoptimization

When enabling any coverage mode (other than best-effort), we trigger
deoptimization of all functions on the heap.

Prior to the recent removal of the weak list of optimized functions [0],
we'd unlink optimized code from all relevant JSFunctions during the call
to DeoptimizeAll.

After the weak-list-removal, this was no longer the case, hence this [1]
change which attempts to reset the code object from the
SharedFunctionInfo for all found JSFunction objects.

But this can create a situation in which JSFunctions are set up
incorrectly s.t. they have unoptimized code but no feedback vector.

This CL fixes that by leaving JSFunction objects untouched and relying
on self-healing mechanisms (CompileLazyDeoptimizedCode) to fix up
JSFunction::code.

[0] https://crrev.com/f0acede9bb05155c25ee87e81b4b587e8a76f690
[1] https://crrev.com/c/647596/5/src/debug/debug-coverage.cc

Bug: chromium:786784, chromium:791940, v8:6637
Change-Id: I13191f4c8800a0d72894b959105189dc09ca693e
Reviewed-on: https://chromium-review.googlesource.com/813615
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49932}

src/isolate.cc

@@ -3196,9 +3196,6 @@ void Isolate::InitializeVectorListFromHeap() {
         if (!shared->IsSubjectToDebugging()) continue;
         vector->clear_invocation_count();
         vectors.emplace_back(vector, this);
-      } else if (current_obj->IsJSFunction()) {
-        JSFunction* function = JSFunction::cast(current_obj);
-        function->set_code(function->shared()->code());
       }
     }
   }

test/mjsunit/regress/regress-786784.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+function f() {
+  function g(arg) { return arg; }
+  // The closure contains a call IC slot.
+  return function() { return g(42); };
+}
+
+const a = Realm.create();
+const b = Realm.create();
+
+// Create two closures in different contexts sharing the same
+// SharedFunctionInfo (shared due to code caching).
+const x = Realm.eval(a, f.toString() + " f()");
+const y = Realm.eval(b, f.toString() + " f()");
+
+// Run the first closure to create SFI::code.
+x();
+
+// At this point, SFI::code is set and `x` has a feedback vector (`y` does not).
+
+// Enabling block code coverage deoptimizes all functions and triggers the
+// buggy code path in which we'd unconditionally replace JSFunction::code with
+// its SFI::code (but skip feedback vector setup).
+%DebugToggleBlockCoverage(true);
+
+// Still no feedback vector set on `y` but it now contains code. Run it to
+// trigger the crash when attempting to write into the non-existent feedback
+// vector.
+y();