gdb special command to print tensors (pytorch#54339)

Summary:
This is something which I wrote because it was useful during my debugging sessions, but I think it might be generally useful to other people as well so I took the liberty of proposing an official `pytorch-gdb` extension.

`pytorch-gdb` is a gdb script written in python. Currently, it contains only one command: `torch-tensor-repr`, which prints a human-readable repr of an `at::Tensor` object. Example:
```
Breakpoint 1, at::native::neg (self=...) at [...]/pytorch/aten/src/ATen/native/UnaryOps.cpp:520
520     Tensor neg(const Tensor& self) { return unary_op_impl(self, at::neg_out); }
(gdb) # the default repr of 'self' is not very useful
(gdb) p self
$1 = (const at::Tensor &) 0x7ffff72ed780: {impl_ = {target_ = 0x5555559df6e0}}
(gdb) torch-tensor-repr self
Python-level repr of self:
tensor([1., 2., 3., 4.], dtype=torch.float64)
```

The idea is that by having an official place where to put these things, `pytorch-gdb` will slowly grow other useful features and make the pytorch debugging experience nicer and faster.

Pull Request resolved: pytorch#54339

Reviewed By: bdhirsh

Differential Revision: D27253674

Pulled By: ezyang

fbshipit-source-id: dba219e126cc2fe66b2d26740f3a8e3b886e56f5

Author: antocuni

Diff for: .gdbinit

@@ -0,0 +1,14 @@
+# automatically load the pytoch-gdb extension.
+#
+# gdb automatically tries to load this file whenever it is executed from the
+# root of the pytorch repo, but by default it is not allowed to do so due to
+# security reasons. If you want to use pytorch-gdb, please add the following
+# line to your ~/.gdbinit (i.e., the .gdbinit file which is in your home
+# directory, NOT this file):
+#    add-auto-load-safe-path /path/to/pytorch/.gdbinit
+#
+# Alternatively, you can manually load the pytorch-gdb commands into your
+# existing gdb session by doing the following:
+#    (gdb) source /path/to/pytorch/tools/gdb/pytorch-gdb.py
+
+source tools/gdb/pytorch-gdb.py

Diff for: CONTRIBUTING.md

@@ -26,6 +26,7 @@
       - [Use CCache](#use-ccache)
       - [Use a faster linker](#use-a-faster-linker)
     - [C++ frontend development tips](#c-frontend-development-tips)
+    - [GDB integration](#gdb-integration)
   - [CUDA development tips](#cuda-development-tips)
   - [Windows development tips](#windows-development-tips)
     - [Known MSVC (and MSVC with NVCC) bugs](#known-msvc-and-msvc-with-nvcc-bugs)
@@ -735,6 +736,68 @@ framework, which you can read up about to learn how to configure the test runner
 submitting a new feature, we care very much that you write appropriate tests.
 Please follow the lead of the other tests to see how to write a new test case.
 
+### GDB integration
+
+If you are debugging pytorch inside GDB, you might be interested in
+[pytorch-gdb](tools/gdb/pytorch-gdb.py). This script introduces some
+pytorch-specific commands which you can use from the GDB prompt. In
+particular, `torch-tensor-repr` prints a human-readable repr of an at::Tensor
+object. Example of usage:
+
+```
+$ gdb python
+GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
+[...]
+(gdb) # insert a breakpoint when we call .neg()
+(gdb) break at::native:neg
+No source file named at::native.
+Make breakpoint pending on future shared library load? (y or [n]) y
+Breakpoint 1 (at::native:neg) pending.
+
+(gdb) run
+[...]
+>>> import torch
+>>> t = torch.tensor([1, 2, 3, 4], dtype=torch.float64)
+>>> t
+tensor([1., 2., 3., 4.], dtype=torch.float64)
+>>> t.neg()
+
+Breakpoint 1, at::native::neg (self=...) at [...]/pytorch/aten/src/ATen/native/UnaryOps.cpp:520
+520     Tensor neg(const Tensor& self) { return unary_op_impl(self, at::neg_out); }
+(gdb) # the default repr of 'self' is not very useful
+(gdb) p self
+$1 = (const at::Tensor &) @0x7ffff72ed780: {impl_ = {target_ = 0x5555559df6e0}}
+(gdb) torch-tensor-repr self
+Python-level repr of self:
+tensor([1., 2., 3., 4.], dtype=torch.float64)
+```
+
+GDB tries to automatically load `pytorch-gdb` thanks to the
+[.gdbinit](.gdbinit) at the root of the pytorch repo. Howevever, auto-loadings is disabled by default, because of security reasons:
+
+```
+$ gdb
+warning: File "/path/to/pytorch/.gdbinit" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
+To enable execution of this file add
+        add-auto-load-safe-path /path/to/pytorch/.gdbinit
+line to your configuration file "/home/YOUR-USERNAME/.gdbinit".
+To completely disable this security protection add
+        set auto-load safe-path /
+line to your configuration file "/home/YOUR-USERNAME/.gdbinit".
+For more information about this security protection see the
+"Auto-loading safe path" section in the GDB manual.  E.g., run from the shell:
+        info "(gdb)Auto-loading safe path"
+(gdb)
+```
+
+As gdb itself suggests, the best way to enable auto-loading of `pytorch-gdb`
+is to add the following line to your `~/.gdbinit` (i.e., the `.gdbinit` file
+which is in your home directory, **not** `/path/to/pytorch/.gdbinit`):
+```
+add-auto-load-safe-path /path/to/pytorch/.gdbinit
+```
+
+
 ## CUDA development tips
 
 If you are working on the CUDA code, here are some useful CUDA debugging tips:

Diff for: tools/gdb/pytorch-gdb.py

@@ -0,0 +1,52 @@
+import gdb
+import textwrap
+
+class DisableBreakpoints:
+    """
+    Context-manager to temporarily disable all gdb breakpoints, useful if
+    there is a risk to hit one during the evaluation of one of our custom
+    commands
+    """
+
+    def __enter__(self):
+        self.disabled_breakpoints = []
+        for b in gdb.breakpoints():
+            if b.enabled:
+                b.enabled = False
+                self.disabled_breakpoints.append(b)
+
+    def __exit__(self, etype, evalue, tb):
+        for b in self.disabled_breakpoints:
+            b.enabled = True
+
+class TensorRepr(gdb.Command):
+    """
+    Print a human readable representation of the given at::Tensor.
+    Usage: torch-tensor-repr EXP
+
+    at::Tensor instances do not have a C++ implementation of a repr method: in
+    pytoch, this is done by pure-Python code. As such, torch-tensor-repr
+    internally creates a Python wrapper for the given tensor and call repr()
+    on it.
+    """
+    __doc__ = textwrap.dedent(__doc__).strip()
+
+    def __init__(self):
+        gdb.Command.__init__(self, 'torch-tensor-repr',
+                             gdb.COMMAND_USER, gdb.COMPLETE_EXPRESSION)
+
+    def invoke(self, args, from_tty):
+        args = gdb.string_to_argv(args)
+        if len(args) != 1:
+            print('Usage: torch-tensor-repr EXP')
+            return
+        name = args[0]
+        with DisableBreakpoints():
+            res = gdb.parse_and_eval('torch::gdb::tensor_repr(%s)' % name)
+            print('Python-level repr of %s:' % name)
+            print(res.string())
+            # torch::gdb::tensor_repr returns a malloc()ed buffer, let's free it
+            gdb.parse_and_eval('(void)free(%s)' % int(res))
+
+TensorRepr()
+

Diff for: torch/csrc/utils.cpp

@@ -252,3 +252,59 @@ void THPPointer<THPStorage>::free() {
 }
 
 template class THPPointer<THPStorage>;
+
+namespace torch { namespace gdb {
+/* ~~~ misc debugging utilities ~~~
+ *
+ * torch::gdb::* functions are NOT meant to be called by general pytorch code,
+ * but only from within a gdb session. As such, utils.h does not contain any
+ * declaration for those.
+ */
+
+// This is a helper needed by the torch-tensor-repr gdb command.
+// Return an human-readable representation of the given Tensor. The resulting
+// string is stored into a malloc()ed buffer. The caller is responsible to
+// free() it. We use malloc() instead of new[] because it's much easier to
+// call free than delete[] from withing gdb.
+// Currently the code for computing the repr of a tensor is written in Python,
+// so we need to wrap the Tensor into a Python object first.
+char *tensor_repr(at::Tensor tensor) {
+  PyGILState_STATE gil = PyGILState_Ensure();
+  PyObject *pytensor = NULL;
+  PyObject *repr = NULL;
+  Py_ssize_t bufsize;
+  const char *buf = NULL;
+  char *result = NULL;
+
+  pytensor = THPVariable_Wrap(at::Tensor(tensor));
+  if (!pytensor)
+    goto error;
+  repr = PyObject_Repr(pytensor);
+  if (!repr)
+    goto error;
+  buf = PyUnicode_AsUTF8AndSize(repr, &bufsize);
+  if (!buf)
+    goto error;
+  result = static_cast<char*>(malloc(bufsize + 1)); // account for the trailing \0
+  if (!result) {
+    fprintf(stderr, "cannot allocate memory for the result\n");
+    goto error;
+  }
+  strcpy(result, buf);
+  Py_XDECREF(pytensor);
+  Py_XDECREF(repr);
+  PyGILState_Release(gil);
+  return result;
+
+error:
+  fprintf(stderr, "torch::gdb::tensor_repr: unexpected error\n");
+  if (PyErr_Occurred())
+    PyErr_Print();
+  Py_XDECREF(pytensor);
+  Py_XDECREF(repr);
+  free(result);
+  PyGILState_Release(gil);
+  return NULL;
+}
+
+}} // namespace torch::gdb