Don't go back to the interpreter after the end of pseudo main when debugging

Fix a long-standing bug where if you step off of the end of pseudo main we'd segfault in the interpreter loop. To get this we have to have a breakpoint in the final TC in pseudo main, and have that called from another TC, then Step or Next off the end. We'd end up with a PC of zero after the ret, which makes sense, but the logic after interpreting a block of code to stay in the interpreter when debugging was blind to this case.

hphp/runtime/base/execution_context.h

@@ -687,6 +687,7 @@ private:
   bool prepareArrayArgs(ActRec* ar, ArrayData* args);
   void recordCodeCoverage(PC pc);
   bool isReturnHelper(uintptr_t address);
+  void switchModeForDebugger();
   int m_coverPrevLine;
   HPHP::Unit* m_coverPrevUnit;
   Array m_evaledArgs;

hphp/runtime/vm/bytecode.cpp

@@ -7134,22 +7134,23 @@ void VMExecutionContext::dispatch() {
   }
 }
 
-void VMExecutionContext::dispatchN(int numInstrs) {
-  dispatchImpl<LimitInstrs | BreakOnCtlFlow>(numInstrs);
-  // We are about to go back to Jit, check whether we should
-  // stick with interpreter
-  if (DEBUGGER_FORCE_INTR) {
+// We are about to go back to translated code, check whether we should
+// stick with the interpreter. NB: if we've just executed a return
+// from pseudomain, then there's no PC and no more code to interpret.
+void VMExecutionContext::switchModeForDebugger() {
+  if (DEBUGGER_FORCE_INTR && (getPC() != 0)) {
     throw VMSwitchMode();
   }
 }
 
+void VMExecutionContext::dispatchN(int numInstrs) {
+  dispatchImpl<LimitInstrs | BreakOnCtlFlow>(numInstrs);
+  switchModeForDebugger();
+}
+
 void VMExecutionContext::dispatchBB() {
   dispatchImpl<BreakOnCtlFlow>(0);
-  // We are about to go back to Jit, check whether we should
-  // stick with interpreter
-  if (DEBUGGER_FORCE_INTR) {
-    throw VMSwitchMode();
-  }
+  switchModeForDebugger();
 }
 
 void VMExecutionContext::recordCodeCoverage(PC pc) {

hphp/test/quick/debugger/flow_pseudomain.php

@@ -0,0 +1,5 @@
+<?php
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+$x = 1;
+echo $x;

hphp/test/quick/debugger/flow_pseudomain.php.expectf

@@ -0,0 +1,13 @@
+Program %s/flow_pseudomain.php loaded. Type '[r]un' or '[c]ontinue' to go.
+break 5
+Breakpoint 1 set on line 5 of %s/flow_pseudomain.php
+But wont break until file %s/flow_pseudomain.php has been loaded.
+continue
+Breakpoint 1 reached on line 5 of %s/flow_pseudomain.php
+   4 $x = 1;
+   5 echo $x;
+   6 (END)
+
+step
+1Program %s/flow_pseudomain.php exited normally.
+quit

hphp/test/quick/debugger/flow_pseudomain.php.in

@@ -0,0 +1,3 @@
+break 5
+continue
+step