trace: fix SIGSEGV after pop 2

Calling `trace_pop_target()` invalidates the linked list of
`target_stack_node_t`s by `free()`ing the node and its `p_target`.

But `p_stack_top` may still points at that now freed node. Entering the
debugger will crash as it will start from `p_stack_top`.

1. Explicitly invalidate `p_target` by setting it to `NULL`.
2. Explicitly pop the top node from `p_stack_top` before calling
   `trace_pop_target()`.

Closes: #158
Fixes: a86f3c0 ("trace: fix SIGSEGV after pop")
Signed-off-by: Philipp Hahn <[email protected]>

Author: Philipp Hahn

src/remake.c

@@ -456,7 +456,7 @@ update_file_1 (struct file *file, unsigned int depth,
         }
 
       DBF (DB_VERBOSE, _("File '%s' was considered already.\n"));
-      p_stack_top = p_stack_top->p_parent;
+      p_stack_top = p_stack_top ? p_stack_top->p_parent : NULL;
       trace_pop_target(p_call_stack);
       return 0;
     }
@@ -468,12 +468,12 @@ update_file_1 (struct file *file, unsigned int depth,
       break;
     case cs_running:
       DBF (DB_VERBOSE, _("Still updating file '%s'.\n"));
-      p_stack_top = p_stack_top->p_parent;
+      p_stack_top = p_stack_top ? p_stack_top->p_parent : NULL;
       trace_pop_target(p_call_stack);
       return 0;
     case cs_finished:
       DBF (DB_VERBOSE, _("Finished updating file '%s'.\n"));
-      p_stack_top = p_stack_top->p_parent;
+      p_stack_top = p_stack_top ? p_stack_top->p_parent : NULL;
       trace_pop_target(p_call_stack);
       return file->update_status;
     default:
@@ -710,7 +710,7 @@ update_file_1 (struct file *file, unsigned int depth,
       set_command_state (file, cs_deps_running);
       --depth;
       DBF (DB_VERBOSE, _("The prerequisites of '%s' are being made.\n"));
-      p_stack_top = p_stack_top->p_parent;
+      p_stack_top = p_stack_top ? p_stack_top->p_parent : NULL;
       trace_pop_target(p_call_stack);
       return 0;
     }
@@ -732,7 +732,7 @@ update_file_1 (struct file *file, unsigned int depth,
         OS (error, NILF,
             _("Target '%s' not remade because of errors."), file->name);
 
-      p_stack_top = p_stack_top->p_parent;
+      p_stack_top = p_stack_top ? p_stack_top->p_parent : NULL;
       trace_pop_target(p_call_stack);
       return dep_status;
     }
@@ -843,7 +843,7 @@ update_file_1 (struct file *file, unsigned int depth,
         }
 
       notice_finished_file (file);
-      p_stack_top = p_stack_top->p_parent;
+      p_stack_top = p_stack_top ? p_stack_top->p_parent : NULL;
       trace_pop_target(p_call_stack);
 
       /* Since we don't need to remake the file, convert it to use the
@@ -877,7 +877,7 @@ update_file_1 (struct file *file, unsigned int depth,
       DBF (DB_VERBOSE, _("Recipe of '%s' is being run.\n"));
       if ( file->tracing & BRK_AFTER_CMD || i_debugger_stepping )
 	  enter_debugger(p_call_stack, file, 0, DEBUG_BRKPT_AFTER_CMD);
-      p_stack_top = p_stack_top->p_parent;
+      p_stack_top = p_stack_top ? p_stack_top->p_parent : NULL;
       trace_pop_target(p_call_stack);
       return 0;
     }
@@ -901,7 +901,7 @@ update_file_1 (struct file *file, unsigned int depth,
   if ( file->tracing & BRK_AFTER_CMD || i_debugger_stepping )
       enter_debugger(p_call_stack, file, 0, DEBUG_BRKPT_AFTER_CMD);
 
-  p_stack_top = p_stack_top->p_parent;
+  p_stack_top = p_stack_top ? p_stack_top->p_parent : NULL;
   trace_pop_target(p_call_stack);
   return file->update_status;
 }