Fix Python completion when using the "complete" command

This patch is related to PR python/16699, and is an improvement over the
patch posted here:

  <https://sourceware.org/ml/gdb-patches/2014-03/msg00301.html>

Keith noticed that, when using the "complete" command on GDB to complete
a Python command, some strange things could happen.  In order to
understand what can go wrong, I need to explain how the Python
completion mechanism works.

When the user requests a completion of a Python command by using TAB,
GDB will first try to determine the right set of "brkchars" that will be
used when doing the completion.  This is done by actually calling the
"complete" method of the Python class.  Then, when we already know the
"brkchars" that will be used, we call the "complete" method again, for
the same values.

If you read the thread mentioned above, you will see that one of the
design decisions was to make the "cmdpy_completer_helper" (which is the
function the does the actual calling of the "complete" method) cache the
first result of the completion, since this result will be used in the
second call, to do the actual completion.

The problem is that the "complete" command does not process the
brkchars, and the current Python completion mechanism (improved by the
patch mentioned above) relies on GDB trying to determine the brkchars,
and then doing the completion itself.  Therefore, when we use the
"complete" command instead of doing a TAB-completion on GDB, there is a
scenario where we can use the invalid cache of a previous Python command
that was completed before.  For example:

  (gdb) A <TAB>
  (gdb) complete B
  B value1
  B value10
  B value2
  B value3
  B value4
  B value5
  B value6
  B value7
  B value8
  B value9
  (gdb) B <TAB>
  comp1   comp2   comp4   comp6   comp8
  comp10  comp3   comp5   comp7   comp9

Here, we see that "complete B " gave a different result than "B <TAB>".
The reason for that is because "A <TAB>" was called before, and its
completion results were "value*", so when GDB tried to "complete B " it
wrongly answered with the results for A.  The problem here is using a
wrong cache (A's cache) for completing B.

We tried to come up with a solution that would preserve the caching
mechanism, but it wasn't really possible.  So I decided to completely
remove the cache, and doing the method calling twice for every
completion.  This is not optimal, but I do not think it will impact
users noticeably.

It is worth mentioning another small issue that I found.  The code was
doing:

  wordobj = PyUnicode_Decode (word, sizeof (word), host_charset (), NULL);

which is totally wrong, because using "sizeof" here will lead to always
the same result.  So I changed this to use "strlen".  The testcase also
catches this problem.

Keith kindly expanded the existing testcase to cover the problem
described above, and everything is passing.

gdb/ChangeLog:
2015-04-08  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR python/16699
	* python/py-cmd.c (cmdpy_completer_helper): Adjust function to not
	use a caching mechanism.  Adjust comments and code to reflect
	that.  Replace 'sizeof' by 'strlen' when fetching 'wordobj'.
	(cmdpy_completer_handle_brkchars): Adjust call to
	cmdpy_completer_helper.  Call Py_XDECREF for 'resultobj'.
	(cmdpy_completer): Likewise.

gdb/testsuite/ChangeLog:
2015-04-08  Keith Seitz  <keiths@redhat.com>

	PR python/16699
	* gdb.python/py-completion.exp: New tests for completion.
	* gdb.python/py-completion.py (CompleteLimit1): New class.
	(CompleteLimit2): Likewise.
	(CompleteLimit3): Likewise.
	(CompleteLimit4): Likewise.
	(CompleteLimit5): Likewise.
	(CompleteLimit6): Likewise.
	(CompleteLimit7): Likewise.

1 files changed, 52 insertions, 67 deletions

diff --git a/gdb/python/py-cmd.c b/gdb/python/py-cmd.c
index 1d89912..017d0b6 100644
--- a/gdb/python/py-cmd.c
+++ b/gdb/python/py-cmd.c
@@ -210,85 +210,70 @@ cmdpy_function (struct cmd_list_element *command, char *args, int from_tty)
 /* Helper function for the Python command completers (both "pure"
    completer and brkchar handler).  This function takes COMMAND, TEXT
    and WORD and tries to call the Python method for completion with
-   these arguments.  It also takes HANDLE_BRKCHARS_P, an argument to
-   identify whether it is being called from the brkchar handler or
-   from the "pure" completer.  In the first case, it effectively calls
-   the Python method for completion, and records the PyObject in a
-   static variable (used as a "cache").  In the second case, it just
-   returns that variable, without actually calling the Python method
-   again.  This saves us one Python method call.
-
-   The reason for this two step dance is that we need to know the set
-   of "brkchars" to use early on, before we actually try to perform
-   the completion.  But if a Python command supplies a "complete"
-   method then we have to call that method first: it may return as its
-   result the kind of completion to perform and that will in turn
-   specify which brkchars to use.  IOW, we need the result of the
-   "complete" method before we actually perform the completion.
-
-   It is important to mention that this function is built on the
-   assumption that the calls to cmdpy_completer_handle_brkchars and
-   cmdpy_completer will be subsequent with nothing intervening.  This
-   is true for our completer mechanism.
+   these arguments.
+
+   This function is usually called twice: once when we are figuring out
+   the break characters to be used, and another to perform the real
+   completion itself.  The reason for this two step dance is that we
+   need to know the set of "brkchars" to use early on, before we
+   actually try to perform the completion.  But if a Python command
+   supplies a "complete" method then we have to call that method
+   first: it may return as its result the kind of completion to
+   perform and that will in turn specify which brkchars to use.  IOW,
+   we need the result of the "complete" method before we actually
+   perform the completion.  The only situation when this function is
+   not called twice is when the user uses the "complete" command: in
+   this scenario, there is no call to determine the "brkchars".
+
+   Ideally, it would be nice to cache the result of the first call (to
+   determine the "brkchars") and return this value directly in the
+   second call (to perform the actual completion).  However, due to
+   the peculiarity of the "complete" command mentioned above, it is
+   possible to put GDB in a bad state if you perform a TAB-completion
+   and then a "complete"-completion sequentially.  Therefore, we just
+   recalculate everything twice for TAB-completions.
 
    This function returns the PyObject representing the Python method
    call.  */
 
 static PyObject *
 cmdpy_completer_helper (struct cmd_list_element *command,
-			const char *text, const char *word,
-			int handle_brkchars_p)
+			const char *text, const char *word)
 {
   cmdpy_object *obj = (cmdpy_object *) get_cmd_context (command);
   PyObject *textobj, *wordobj;
-  /* This static variable will server as a "cache" for us, in order to
-     store the PyObject that results from calling the Python
-     function.  */
-  static PyObject *resultobj = NULL;
+  PyObject *resultobj;
 
-  if (handle_brkchars_p)
+  if (obj == NULL)
+    error (_("Invalid invocation of Python command object."));
+  if (!PyObject_HasAttr ((PyObject *) obj, complete_cst))
     {
-      /* If we were called to handle brkchars, it means this is the
-	 first function call of two that will happen in a row.
-	 Therefore, we need to call the completer ourselves, and cache
-	 the return value in the static variable RESULTOBJ.  Then, in
-	 the second call, we can just use the value of RESULTOBJ to do
-	 our job.  */
-      if (resultobj != NULL)
-	Py_DECREF (resultobj);
-
-      resultobj = NULL;
-      if (obj == NULL)
-	error (_("Invalid invocation of Python command object."));
-      if (!PyObject_HasAttr ((PyObject *) obj, complete_cst))
-	{
-	  /* If there is no complete method, don't error.  */
-	  return NULL;
-	}
-
-      textobj = PyUnicode_Decode (text, strlen (text), host_charset (), NULL);
-      if (textobj == NULL)
-	error (_("Could not convert argument to Python string."));
-      wordobj = PyUnicode_Decode (word, sizeof (word), host_charset (), NULL);
-      if (wordobj == NULL)
-	{
-	  Py_DECREF (textobj);
-	  error (_("Could not convert argument to Python string."));
-	}
+      /* If there is no complete method, don't error.  */
+      return NULL;
+    }
 
-      resultobj = PyObject_CallMethodObjArgs ((PyObject *) obj, complete_cst,
-					      textobj, wordobj, NULL);
+  textobj = PyUnicode_Decode (text, strlen (text), host_charset (), NULL);
+  if (textobj == NULL)
+    error (_("Could not convert argument to Python string."));
+  wordobj = PyUnicode_Decode (word, strlen (word), host_charset (), NULL);
+  if (wordobj == NULL)
+    {
       Py_DECREF (textobj);
-      Py_DECREF (wordobj);
-      if (!resultobj)
-	{
-	  /* Just swallow errors here.  */
-	  PyErr_Clear ();
-	}
+      error (_("Could not convert argument to Python string."));
+    }
 
-      Py_XINCREF (resultobj);
+  resultobj = PyObject_CallMethodObjArgs ((PyObject *) obj, complete_cst,
+					  textobj, wordobj, NULL);
+  Py_DECREF (textobj);
+  Py_DECREF (wordobj);
+  if (!resultobj)
+    {
+      /* Just swallow errors here.  */
+      PyErr_Clear ();
     }
 
+  Py_XINCREF (resultobj);
+
   return resultobj;
 }
 
@@ -308,7 +293,7 @@ cmdpy_completer_handle_brkchars (struct cmd_list_element *command,
 
   /* Calling our helper to obtain the PyObject of the Python
      function.  */
-  resultobj = cmdpy_completer_helper (command, text, word, 1);
+  resultobj = cmdpy_completer_helper (command, text, word);
 
   /* Check if there was an error.  */
   if (resultobj == NULL)
@@ -338,8 +323,7 @@ cmdpy_completer_handle_brkchars (struct cmd_list_element *command,
 
  done:
 
-  /* We do not call Py_XDECREF here because RESULTOBJ will be used in
-     the subsequent call to cmdpy_completer function.  */
+  Py_XDECREF (resultobj);
   do_cleanups (cleanup);
 }
 
@@ -357,7 +341,7 @@ cmdpy_completer (struct cmd_list_element *command,
 
   /* Calling our helper to obtain the PyObject of the Python
      function.  */
-  resultobj = cmdpy_completer_helper (command, text, word, 0);
+  resultobj = cmdpy_completer_helper (command, text, word);
 
   /* If the result object of calling the Python function is NULL, it
      means that there was an error.  In this case, just give up and
@@ -419,6 +403,7 @@ cmdpy_completer (struct cmd_list_element *command,
 
  done:
 
+  Py_XDECREF (resultobj);
   do_cleanups (cleanup);
 
   return result;