Introduce class target_stack

Currently, the target stack is represented by a singly linked list,
with target_ops having a pointer to the target beneath.  This poses a
problem for multi-process / multi-target debugging.  In that case, we
will naturally want multiple instances of target stacks.  E.g., one
stack for inferior 1 which is debugging a core file, and another
target stack for inferior 2 which is debugging a remote process.  The
problem then is in finding a target's "beneath" target, if we consider
that for some target_ops types, we'll be sharing a single target_ops
instance between several inferiors.  For example, so far, I found no
need to have multiple instances of the spu_multiarch_target /
exec_target / dummy_target targets.

Thus this patch, which changes the target stack representation to an
array of pointers.  For now, there's still a single global instance of
this new target_stack class, though further down in the multi-target
work, each inferior will have its own instance.

gdb/ChangeLog:
2018-06-07  Pedro Alves  <palves@redhat.com>

	* target.h (target_ops) <beneath>: Now a method.  All references
	updated.
	(class target_stack): New.
	* target.c (g_target_stack): New.
	(g_current_top_target): Delete.
	(current_top_target): Get the top target out of g_target_stack.
	(target_stack::push, target_stack::unpush): New.
	(push_target, unpush_target): Reimplement.
	(target_is_pushed): Reimplement in terms of g_target_stack.
	(target_ops::beneath, target_stack::find_beneath): New.

3 files changed, 119 insertions, 62 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index a8da90b..424b7af 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,18 @@
 2018-06-07  Pedro Alves  <palves@redhat.com>
 
+	* target.h (target_ops) <beneath>: Now a method.  All references
+	updated.
+	(class target_stack): New.
+	* target.c (g_target_stack): New.
+	(g_current_top_target): Delete.
+	(current_top_target): Get the top target out of g_target_stack.
+	(target_stack::push, target_stack::unpush): New.
+	(push_target, unpush_target): Reimplement.
+	(target_is_pushed): Reimplement in terms of g_target_stack.
+	(target_ops::beneath, target_stack::find_beneath): New.
+
+2018-06-07  Pedro Alves  <palves@redhat.com>
+
 	* target.h (find_target_beneath): Delete declaration.
 	* target.c (find_target_beneath): Delete definition.
 	* aix-thread.c: All callers of find_target_beneath adjusted to
diff --git a/gdb/target.c b/gdb/target.c
index cc4f81e..a1fe8f9 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -119,16 +119,18 @@ static std::unordered_map<const target_info *, target_open_ftype *>
 static struct target_ops *the_dummy_target;
 static struct target_ops *the_debug_target;
 
+/* The target stack.  */
+
+static target_stack g_target_stack;
+
 /* Top of target stack.  */
 /* The target structure we are currently using to talk to a process
    or file or whatever "inferior" we have.  */
 
-static target_ops *g_current_top_target;
-
 target_ops *
 current_top_target ()
 {
-  return g_current_top_target;
+  return g_target_stack.top ();
 }
 
 /* Command list for target.  */
@@ -631,50 +633,47 @@ default_execution_direction (struct target_ops *self)
 to_execution_direction must be implemented for reverse async");
 }
 
-/* Push a new target type into the stack of the existing target accessors,
-   possibly superseding some of the existing accessors.
-
-   Rather than allow an empty stack, we always have the dummy target at
-   the bottom stratum, so we can call the function vectors without
-   checking them.  */
+/* See target.h.  */
 
 void
-push_target (struct target_ops *t)
+target_stack::push (target_ops *t)
 {
-  struct target_ops **cur;
-
-  /* Find the proper stratum to install this target in.  */
-  for (cur = &g_current_top_target; (*cur) != NULL; cur = &(*cur)->m_beneath)
+  /* If there's already a target at this stratum, remove it.  */
+  if (m_stack[t->to_stratum] != NULL)
     {
-      if ((int) (t->to_stratum) >= (int) (*cur)->to_stratum)
-	break;
+      target_ops *prev = m_stack[t->to_stratum];
+      m_stack[t->to_stratum] = NULL;
+      target_close (prev);
     }
 
-  /* If there's already targets at this stratum, remove them.  */
-  /* FIXME: cagney/2003-10-15: I think this should be popping all
-     targets to CUR, and not just those at this stratum level.  */
-  while ((*cur) != NULL && t->to_stratum == (*cur)->to_stratum)
-    {
-      /* There's already something at this stratum level.  Close it,
-         and un-hook it from the stack.  */
-      struct target_ops *tmp = (*cur);
+  /* Now add the new one.  */
+  m_stack[t->to_stratum] = t;
 
-      (*cur) = (*cur)->m_beneath;
-      tmp->m_beneath = NULL;
-      target_close (tmp);
-    }
+  if (m_top < t->to_stratum)
+    m_top = t->to_stratum;
+}
+
+/* See target.h.  */
 
-  /* We have removed all targets in our stratum, now add the new one.  */
-  t->m_beneath = (*cur);
-  (*cur) = t;
+void
+push_target (struct target_ops *t)
+{
+  g_target_stack.push (t);
 }
 
-/* Remove a target_ops vector from the stack, wherever it may be.
-   Return how many times it was removed (0 or 1).  */
+/* See target.h.  */
 
 int
 unpush_target (struct target_ops *t)
 {
+  return g_target_stack.unpush (t);
+}
+
+/* See target.h.  */
+
+bool
+target_stack::unpush (target_ops *t)
+{
   struct target_ops **cur;
   struct target_ops *tmp;
 
@@ -682,31 +681,30 @@ unpush_target (struct target_ops *t)
     internal_error (__FILE__, __LINE__,
 		    _("Attempt to unpush the dummy target"));
 
-  /* Look for the specified target.  Note that we assume that a target
-     can only occur once in the target stack.  */
+  gdb_assert (t != NULL);
+
+  /* Look for the specified target.  Note that a target can only occur
+     once in the target stack.  */
 
-  for (cur = &g_current_top_target; (*cur) != NULL; cur = &(*cur)->m_beneath)
+  if (m_stack[t->to_stratum] != t)
     {
-      if ((*cur) == t)
-	break;
+      /* If T wasn't pushed, quit.  Only open targets should be
+	 closed.  */
+      return false;
     }
 
-  /* If we don't find target_ops, quit.  Only open targets should be
-     closed.  */
-  if ((*cur) == NULL)
-    return 0;			
-
   /* Unchain the target.  */
-  tmp = (*cur);
-  (*cur) = (*cur)->m_beneath;
-  tmp->m_beneath = NULL;
+  m_stack[t->to_stratum] = NULL;
+
+  if (m_top == t->to_stratum)
+    m_top = t->beneath ()->to_stratum;
 
   /* Finally close the target.  Note we do this after unchaining, so
      any target method calls from within the target_close
      implementation don't end up in T anymore.  */
   target_close (t);
 
-  return 1;
+  return true;
 }
 
 /* Unpush TARGET and assert that it worked.  */
@@ -751,13 +749,7 @@ pop_all_targets (void)
 int
 target_is_pushed (struct target_ops *t)
 {
-  for (target_ops *cur = current_top_target ();
-       cur != NULL;
-       cur = cur->beneath ())
-    if (cur == t)
-      return 1;
-
-  return 0;
+  return g_target_stack.is_pushed (t);
 }
 
 /* Default implementation of to_get_thread_local_address.  */
@@ -2625,7 +2617,7 @@ target_thread_address_space (ptid_t ptid)
 target_ops *
 target_ops::beneath () const
 {
-  return m_beneath;
+  return g_target_stack.find_beneath (this);
 }
 
 void
@@ -3213,16 +3205,25 @@ default_thread_architecture (struct target_ops *ops, ptid_t ptid)
 
 /* See target.h.  */
 
-struct target_ops *
-find_target_at (enum strata stratum)
+target_ops *
+target_stack::find_beneath (const target_ops *t) const
 {
-  for (target_ops *t = current_top_target (); t != NULL; t = t->beneath ())
-    if (t->to_stratum == stratum)
-      return t;
+  /* Look for a non-empty slot at stratum levels beneath T's.  */
+  for (int stratum = t->to_stratum - 1; stratum >= 0; --stratum)
+    if (m_stack[stratum] != NULL)
+      return m_stack[stratum];
 
   return NULL;
 }
 
+/* See target.h.  */
+
+struct target_ops *
+find_target_at (enum strata stratum)
+{
+  return g_target_stack.at (stratum);
+}
+
 
 
 /* See target.h  */
diff --git a/gdb/target.h b/gdb/target.h
index 9dd29a6..18c4a84 100644
--- a/gdb/target.h
+++ b/gdb/target.h
@@ -61,7 +61,11 @@ struct inferior;
    of variables any more (the file target is handling them and they
    never get to the process target).  So when you push a file target,
    it goes into the file stratum, which is always below the process
-   stratum.  */
+   stratum.
+
+   Note that rather than allow an empty stack, we always have the
+   dummy target at the bottom stratum, so we can call the target
+   methods without checking them.  */
 
 #include "target/target.h"
 #include "target/resume.h"
@@ -425,7 +429,6 @@ struct target_info
 struct target_ops
   {
     /* To the target under this one.  */
-    target_ops *m_beneath;
     target_ops *beneath () const;
 
     /* Free resources associated with the target.  Note that singleton
@@ -1268,6 +1271,46 @@ extern void set_native_target (target_ops *target);
    NULL.  */
 extern target_ops *get_native_target ();
 
+/* Type that manages a target stack.  See description of target stacks
+   and strata at the top of the file.  */
+
+class target_stack
+{
+public:
+  target_stack () = default;
+  DISABLE_COPY_AND_ASSIGN (target_stack);
+
+  /* Push a new target into the stack of the existing target
+     accessors, possibly superseding some existing accessor.  */
+  void push (target_ops *t);
+
+  /* Remove a target from the stack, wherever it may be.  Return true
+     if it was removed, false otherwise.  */
+  bool unpush (target_ops *t);
+
+  /* Returns true if T is pushed on the target stack.  */
+  bool is_pushed (target_ops *t) const
+  { return at (t->to_stratum) == t; }
+
+  /* Return the target at STRATUM.  */
+  target_ops *at (strata stratum) const { return m_stack[stratum]; }
+
+  /* Return the target at the top of the stack.  */
+  target_ops *top () const { return at (m_top); }
+
+  /* Find the next target down the stack from the specified target.  */
+  target_ops *find_beneath (const target_ops *t) const;
+
+private:
+  /* The stratum of the top target.  */
+  enum strata m_top {};
+
+  /* The stack, represented as an array, with one slot per stratum.
+     If no target is pushed at some stratum, the corresponding slot is
+     null.  */
+  target_ops *m_stack[(int) debug_stratum + 1] {};
+};
+
 /* The ops structure for our "current" target process.  This should
    never be NULL.  If there is no target, it points to the dummy_target.  */