Make GDB compile with Python 3 on MinGW

PyFile_FromString and PyFile_AsFile have been removed in Python 3.
There is no obvious replacement that works here, and we can't just
pass our FILE* to a DLL in Windows because it may use a different
C runtime.

So we just call a Python function which reads and executes file
contents. Care must be taken to execute it in the context of
__main__.

Tested by inverting the ifdef and running the testsuite on Debian
Linux (even without the patch, I failed at running the testsuite
on Windows). I did test with both Python 2 and 3.

gdb/ChangeLog:

2019-08-22  Christian Biesinger  <cbiesinger@google.com>

	* python/lib/gdb/__init__.py (_execute_file): New function.
	* python/python.c (python_run_simple_file): Call gdb._execute_file
	on Windows.

3 files changed, 46 insertions, 9 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index b6d200d..ac84245 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2019-08-22  Christian Biesinger  <cbiesinger@google.com>
+
+	* python/lib/gdb/__init__.py (_execute_file): New function.
+	* python/python.c (python_run_simple_file): Call gdb._execute_file
+	on Windows.
+
 2019-08-22  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* f-exp.y (yylex): Remove is_a_field_of_this local variable, and
diff --git a/gdb/python/lib/gdb/__init__.py b/gdb/python/lib/gdb/__init__.py
index 8af9e47..9c23970 100644
--- a/gdb/python/lib/gdb/__init__.py
+++ b/gdb/python/lib/gdb/__init__.py
@@ -106,6 +106,32 @@ def _execute_unwinders(pending_frame):
 
     return None
 
+def _execute_file(filepath):
+    """This function is used to replace Python 2's PyRun_SimpleFile.
+
+    Loads and executes the given file.
+
+    We could use the runpy module, but its documentation says:
+    "Furthermore, any functions and classes defined by the executed code are
+    not guaranteed to work correctly after a runpy function has returned."
+    """
+    globals = sys.modules['__main__'].__dict__
+    set_file = False
+    # Set file (if not set) so that the imported file can use it (e.g. to
+    # access file-relative paths). This matches what PyRun_SimpleFile does.
+    if not hasattr(globals, '__file__'):
+        globals['__file__'] = filepath
+        set_file = True
+    try:
+        with open(filepath, 'rb') as file:
+            # We pass globals also as locals to match what Python does
+            # in PyRun_SimpleFile.
+            compiled = compile(file.read(), filepath, 'exec')
+            exec(compiled, globals, globals)
+    finally:
+        if set_file:
+            del globals['__file__']
+
 
 # Convenience variable to GDB's python directory
 PYTHONDIR = os.path.dirname(os.path.dirname(__file__))
diff --git a/gdb/python/python.c b/gdb/python/python.c
index 162470d..cca7c4c 100644
--- a/gdb/python/python.c
+++ b/gdb/python/python.c
@@ -323,9 +323,8 @@ python_interactive_command (const char *arg, int from_tty)
    A FILE * from one runtime does not necessarily operate correctly in
    the other runtime.
 
-   To work around this potential issue, we create on Windows hosts the
-   FILE object using Python routines, thus making sure that it is
-   compatible with the Python library.  */
+   To work around this potential issue, we run code in Python to load
+   the script.  */
 
 static void
 python_run_simple_file (FILE *file, const char *filename)
@@ -339,15 +338,21 @@ python_run_simple_file (FILE *file, const char *filename)
   /* Because we have a string for a filename, and are using Python to
      open the file, we need to expand any tilde in the path first.  */
   gdb::unique_xmalloc_ptr<char> full_path (tilde_expand (filename));
-  gdbpy_ref<> python_file (PyFile_FromString (full_path.get (), (char *) "r"));
-  if (python_file == NULL)
+
+  if (gdb_python_module == nullptr
+      || ! PyObject_HasAttrString (gdb_python_module, "_execute_file"))
+    error (_("Installation error: gdb._execute_file function is missing"));
+
+  gdbpy_ref<> return_value
+    (PyObject_CallMethod (gdb_python_module, "_execute_file", "s",
+			  full_path.get ()));
+  if (return_value == nullptr)
     {
-      gdbpy_print_stack ();
-      error (_("Error while opening file: %s"), full_path.get ());
+      /* Use PyErr_PrintEx instead of gdbpy_print_stack to better match the
+         behavior of the non-Windows codepath.  */
+      PyErr_PrintEx(0);
     }
 
-  PyRun_SimpleFile (PyFile_AsFile (python_file.get ()), filename);
-
 #endif /* _WIN32 */
 }