Add a GDB pretty printer to aid in debugging

misc/scripts/godot_gdb_pretty_print.py

@@ -0,0 +1,116 @@
+#!/usr/bin/env python3
+# Load this file to your GDB session to enable pretty-printing
+# of some Godot C++ types.
+# GDB command: source misc/scripts/godot_gdb_pretty_print.py
+#
+# To load these automatically in Visual Studio Code,
+# add the source command to the setupCommands of your configuration
+# in launch.json.
+# "setupCommands": [
+# ...
+# {
+#     "description": "Load custom pretty-printers for Godot types.",
+#     "text": "source ${workspaceRoot}/misc/scripts/godot_gdb_pretty_print.py"
+# }
+# ]
+# Other UI:s that use GDB under the hood are likely to have their own ways to achieve this.
+#
+# To debug this script it's easiest to use the interactive python from a command-line
+# GDB session. Stop at a breakpoint, then use
+# python-interactive to enter the python shell and
+# acquire a Value object using gdb.selected_frame().read_var("variable name").
+# From there you can figure out how to print it nicely.
+import re
+
+import gdb
+
+
+# Printer for Godot StringName variables.
+class GodotStringNamePrinter:
+    def __init__(self, value):
+        self.value = value
+
+    def to_string(self):
+        return self.value["_data"]["name"]["_cowdata"]["_ptr"]
+
+    # Hint that the object is string-like.
+    def display_hint(self):
+        return "string"
+
+
+# Printer for Godot String variables.
+class GodotStringPrinter:
+    def __init__(self, value):
+        self.value = value
+
+    def to_string(self):
+        return self.value["_cowdata"]["_ptr"]
+
+    # Hint that the object is string-like.
+    def display_hint(self):
+        return "string"
+
+
+# Printer for Godot Vector variables.
+class GodotVectorPrinter:
+    def __init__(self, value):
+        self.value = value
+
+    # The COW (Copy On Write) object does a bunch of pointer arithmetic to access
+    # its members.
+    # The offsets are constants on the C++ side, optimized out, so not accessible to us.
+    # I'll just hard code the observed values and hope they are the same forever.
+    # See core/templates/cowdata.h
+    SIZE_OFFSET = 8
+    DATA_OFFSET = 16
+
+    # Figures out the number of elements in the vector.
+    def get_size(self):
+        cowdata = self.value["_cowdata"]
+        if cowdata["_ptr"] == 0:
+            return 0
+        else:
+            # The ptr member of cowdata does not point to the beginning of the
+            # cowdata. It points to the beginning of the data section of the cowdata.
+            # To get to the length section, we must back up to the beginning of the struct,
+            # then move back forward to the size.
+            # cf. CowData::_get_size
+            ptr = cowdata["_ptr"].cast(gdb.lookup_type("uint8_t").pointer())
+            return int((ptr - self.DATA_OFFSET + self.SIZE_OFFSET).dereference())
+
+    # Lists children of the value, in this case the vector's items.
+    def children(self):
+        # Return nothing if ptr is null.
+        ptr = self.value["_cowdata"]["_ptr"]
+        if ptr == 0:
+            return
+        # Yield the items one by one.
+        for i in range(self.get_size()):
+            yield str(i), (ptr + i).dereference()
+
+    def to_string(self):
+        return "%s [%d]" % (self.value.type.name, self.get_size())
+
+    # Hint that the object is array-like.
+    def display_hint(self):
+        return "array"
+
+
+VECTOR_REGEX = re.compile("^Vector<.*$")
+
+
+# Tries to find a pretty printer for a debugger value.
+def lookup_pretty_printer(value):
+    if value.type.name == "StringName":
+        return GodotStringNamePrinter(value)
+    if value.type.name == "String":
+        return GodotStringPrinter(value)
+    if value.type.name and VECTOR_REGEX.match(value.type.name):
+        return GodotVectorPrinter(value)
+    return None
+
+
+# Register our printer lookup function.
+# The first parameter could be used to limit the scope of the printer
+# to a specific object file, but that is unnecessary for us.
+gdb.printing.register_pretty_printer(None, lookup_pretty_printer)