gdb: Introduce setting construct within cmd_list_element

cmd_list_element can contain a pointer to data that can be set and / or
shown.  This is achieved with the void* VAR member which points to the
data that can be accessed, while the VAR_TYPE member (of type enum
var_types) indicates how to interpret the data pointed to.

With this pattern, the user of the cmd_list_element needs to know what
is the storage type associated with a given VAR_TYPES in order to do
the proper casting.  No automatic safeguard is available to prevent
miss-use of the pointer.  Client code typically looks something like:

	switch (c->var_type)
	{
	  case var_zuinteger:
	    unsigned int v = *(unsigned int*) c->var;
	    ...
	    break;
	  case var_boolean:
	    bool v = *(bool *) c->var;
	    ...
	    break;
	  ...
	}

This patch proposes to add an abstraction around the var_types and void*
pointer pair.  The abstraction is meant to prevent the user from having
to handle the cast and verify that the data is read or written as a type
that is coherent with the setting's var_type.  This is achieved by
introducing the struct setting which exposes a set of templated get /
set member functions.  The template parameter is the type of the
variable that holds the referred variable.

Using those accessors allows runtime checks to be inserted in order to
ensure that the data pointed to has the expected type.  For example,
instantiating the member functions with bool will yield something
similar to:

	const bool &get<bool> () const
	{
	  gdb_assert (m_var_type == var_boolean);
	  gdb_assert (m_var != nullptr);
	  return *static_cast<bool *> (m_var);
	}
	void set<bool> (const bool &var)
	{
	  gdb_assert (m_var_type == var_boolean);
	  gdb_assert (m_var != nullptr);
	  *static_cast<bool *> (m_var) = var;
	}

Using the new abstraction, our initial example becomes:

	switch (c->var_type)
	{
	  case var_zuinteger:
	    unsigned int v = c->var->get<unsigned int> ();
	    ...
	    break;
	  case var_boolean:
	    bool v = c->var->get<bool> ();
	    ...
	    break;
	  ...
	}

While the call site is still similar, the introduction of runtime checks
help ensure correct usage of the data.

In order to avoid turning the bulk of add_setshow_cmd_full into a
templated function, and following a suggestion from Pedro Alves, a
setting can be constructed from a pre validated type erased reference to
a variable.  This is what setting::erased_args is used for.

Introducing an opaque abstraction to describe a setting will also make
it possible to use callbacks to retrieve or set the value of the setting
on the fly instead of pointing to a static chunk of memory.  This will
be done added in a later commit.

Given that a cmd_list_element may or may not reference a setting, the
VAR and VAR_TYPES members of the struct are replaced with a
gdb::optional<setting> named VAR.

Few internal function signatures have been modified to take into account
this new abstraction:

-The functions value_from_setting, str_value_from_setting and
 get_setshow_command_value_string used to have a 'cmd_list_element *'
 parameter but only used it for the VAR and VAR_TYPE member. They now
 take a 'const setting &' parameter instead.
- Similarly, the 'void *' and a 'enum var_types' parameters of
  pascm_param_value and gdbpy_parameter_value have been replaced with a
  'const setting &' parameter.

No user visible change is expected after this patch.

Tested on GNU/Linux x86_64, with no regression noticed.

Co-authored-by: Simon Marchi <simon.marchi@polymtl.ca>
Change-Id: Ie1d08c3ceb8b30b3d7bf1efe036eb8acffcd2f34

1 files changed, 143 insertions, 0 deletions

diff --git a/gdb/command.h b/gdb/command.h
index baf3440..e7b8110 100644
--- a/gdb/command.h
+++ b/gdb/command.h
@@ -125,6 +125,149 @@ typedef enum var_types
   }
 var_types;
 
+/* Return true if a setting of type VAR_TYPE is backed with type T.
+
+   This function is left without definition intentionally.  This template is
+   specialized for all valid types that are used to back var_types.  Therefore
+   if one tries to instantiate this un-specialized template it means the T
+   parameter is not a type used to back a var_type and it is most likely a
+   programming error.  */
+template<typename T>
+bool var_type_uses (var_types var_type) = delete;
+
+/* Return true if a setting of type T is backed by a bool variable.  */
+template<>
+inline bool var_type_uses<bool> (var_types t)
+{
+  return t == var_boolean;
+};
+
+/* Return true if a setting of type T is backed by a auto_boolean variable.
+*/
+template<>
+inline bool var_type_uses<enum auto_boolean> (var_types t)
+{
+  return t == var_auto_boolean;
+}
+
+/* Return true if a setting of type T is backed by an unsigned int variable.
+*/
+template<>
+inline bool var_type_uses<unsigned int> (var_types t)
+{
+  return (t == var_uinteger || t == var_zinteger || t == var_zuinteger);
+}
+
+/* Return true if a setting of type T is backed by an int variable.  */
+template<>
+inline bool var_type_uses<int> (var_types t)
+{
+  return (t == var_integer || t == var_zinteger
+	  || t == var_zuinteger_unlimited);
+}
+
+/* Return true if a setting of type T is backed by a char * variable.  */
+template<>
+inline bool var_type_uses<char *> (var_types t)
+{
+  return (t == var_string || t == var_string_noescape
+	  || t == var_optional_filename || t == var_filename);
+}
+
+/* Return true if a setting of type T is backed by a const char * variable.
+*/
+template<>
+inline bool var_type_uses<const char *> (var_types t)
+{
+  return t == var_enum;
+}
+
+/* Interface for getting and setting a setting's value.
+
+   The underlying data can be of any VAR_TYPES type.  */
+struct setting
+{
+  /* Create a setting backed by a variable of type T.
+
+     Type T must match the var type VAR_TYPE (see VAR_TYPE_USES).  */
+  template<typename T>
+  setting (var_types var_type, T *var)
+    : m_var_type (var_type), m_var (var)
+  {
+    gdb_assert (var != nullptr);
+    gdb_assert (var_type_uses<T> (var_type));
+  }
+
+  /* A setting can also be constructed with a pre-validated
+     type-erased variable.  Use the following function to
+     validate & type-erase said variable/function pointers.  */
+
+  struct erased_args
+  {
+    void *var;
+  };
+
+  template<typename T>
+  static erased_args erase_args (var_types var_type, T *var)
+  {
+    gdb_assert (var_type_uses<T> (var_type));
+
+    return {var};
+  }
+
+  /* Create a setting backed by pre-validated type-erased args.
+     ERASED_VAR's fields' real types must match the var type VAR_TYPE
+     (see VAR_TYPE_USES).  */
+  setting (var_types var_type, const erased_args &args)
+    : m_var_type (var_type),
+      m_var (args.var)
+  {
+  }
+
+  /* Access the type of the current setting.  */
+  var_types type () const
+  {
+    return m_var_type;
+  }
+
+  /* Return the current value.
+
+     The template parameter T is the type of the variable used to store the
+     setting.  */
+  template<typename T>
+  const T &get () const
+  {
+    gdb_assert (var_type_uses<T> (m_var_type));
+    gdb_assert (m_var != nullptr);
+
+    return *static_cast<const T *> (m_var);
+  }
+
+  /* Sets the value of the setting to V.
+
+     The template parameter T indicates the type of the variable used to
+     store the setting.
+
+     The var_type of the setting must match T.  */
+  template<typename T>
+  void set (const T &v)
+  {
+    /* Check that the current instance is of one of the supported types for
+       this instantiation.  */
+    gdb_assert (var_type_uses<T> (m_var_type));
+
+    *static_cast<T *> (m_var) = v;
+  }
+
+private:
+  /* The type of the variable M_VAR is pointing to.  */
+  var_types m_var_type;
+
+  /* Pointer to the enclosed variable.  The type of the variable is encoded
+     in M_VAR_TYPE.  */
+  void *m_var;
+};
+
 /* This structure records one command'd definition.  */
 struct cmd_list_element;