libexpr: add a gdb script with value pretty printers

Change-Id: Ia4a6feb194c73dddf468c78287962713bef582f7

lix/libexpr/value.cc

@@ -81,4 +81,147 @@ void Value::mkPath(const SourcePath & path)
     *this = Value(NewValueAs::path, path);
 }
 
+#ifndef __APPLE__
+[[gnu::section(".debug_gdb_scripts"), gnu::used]]
+static const char printer_script[] =
+    "\4"
+    R"(lix-value-printer
+class ValuePrinter(gdb.ValuePrinter):
+    def __init__(self, val):
+          self._val = val
+          self._t_thunk = gdb.lookup_type('nix::Value::Thunk').pointer()
+          self._t_app = gdb.lookup_type('nix::Value::App').pointer()
+          self._t_int = gdb.lookup_type('intptr_t')
+          self._t_string = gdb.lookup_type('nix::Value::String').pointer()
+          self._t_attrs = gdb.lookup_type('nix::Bindings').pointer()
+          self._t_list = gdb.lookup_type('nix::Value::List').pointer()
+          self._t_aux = gdb.lookup_type('nix::Value::Acb').pointer()
+
+    def _addr(self, t = None):
+          addr = self._val['raw'] & ~self._val['TAG_MASK']
+          return addr.cast(t).referenced_value() if t else addr
+
+    def _tag(self):
+          return self._val['raw'] & self._val['TAG_MASK']
+    def _v_int(self):
+          return self._val['raw'].cast(self._t_int) >> self._val['TAG_BITS']
+
+    def to_string(self):
+          if self._tag() == 7:
+                return self._addr(self._t_aux)
+          elif not self.children():
+                return f'undecoded {self._val['raw'].format_string(format='x')}'
+          return None
+
+    def children(self):
+          match self._tag():
+                case 0:
+                      return [('thunk', self._addr(self._t_thunk))]
+                case 1:
+                      return [('app', self._addr(self._t_app))]
+                case 2:
+                      return [('int', self._v_int())]
+                case 3:
+                      return [('bool', (self._val['raw'] >> self._val['TAG_BITS']) != 0)]
+                case 4:
+                      return [('string', self._addr(self._t_string))]
+                case 5:
+                      return [('attrs', self._addr(self._t_attrs))]
+                case 6:
+                      return [('list', self._addr(self._t_list))]
+                case _:
+                      return []
+
+class ThunkPrinter(gdb.ValuePrinter):
+    def __init__(self, val):
+          self._val = val
+          self._t_value = gdb.lookup_type('nix::Value')
+          self._t_env = gdb.lookup_type('nix::Env').pointer()
+
+    def children(self):
+          if self._val['expr'] == 0:
+                return [('result', self._val['_result'].cast(self._t_value))]
+          else:
+                return [
+                      ('env', self._val['_env'].cast(self._t_env).referenced_value()),
+                      ('expr', self._val['expr'].referenced_value()),
+                ]
+
+class AppPrinter(gdb.ValuePrinter):
+    def __init__(self, val):
+          self._val = val
+          self._t_value = gdb.lookup_type('nix::Value')
+
+    def children(self):
+          if self._val['_n'] + 1 == 0:
+                return [('result', self._val['_left'].cast(self._t_value))]
+          else:
+                n = int(self._val['_n'])
+                arr = self._t_value.array(0, n - 1)
+                return [
+                      ('fn', self._val['_left'].cast(self._t_value)),
+                      ('n', n),
+                      ('args', self._val['_args'][0].cast(arr)),
+                ]
+
+class AcbPrinter(gdb.ValuePrinter):
+    def __init__(self, val):
+          self._val = val
+          self._t_external = gdb.lookup_type('nix::Value::External').pointer()
+          self._t_float = gdb.lookup_type('nix::Value::Float').pointer()
+          self._t_primop = gdb.lookup_type('nix::Value::PrimOp')
+          self._t_primopdetails = gdb.lookup_type('nix::PrimOpDetails')
+          self._t_lambda = gdb.lookup_type('nix::Value::Lambda').pointer()
+          self._t_int = gdb.lookup_type('nix::Value::Int').pointer()
+
+    def _addr(self, t = None):
+          addr = self._val['raw'] & ~self._val['TAG_MASK']
+          return addr.cast(t).referenced_value() if t else addr
+
+    def _tag(self):
+          return self._val['raw'] & self._val['TAG_MASK']
+
+    def to_string(self):
+          match self._tag():
+                case 2:
+                      return 'null'
+                case 3:
+                      op = self._val.cast(self._t_primop).cast(self._t_primopdetails)
+                      return f'primop {op['name']}'
+                case _:
+                      return None
+
+    def children(self):
+          match self._tag():
+                case 0:
+                      return [('external', self._addr(self._t_external))]
+                case 1:
+                      return [('float', self._addr(self._t_float)['value'])]
+                case 4:
+                      return [('lambda', self._addr(self._t_lambda))]
+                case 5:
+                      return [('int', self._addr(self._t_int)['value'])]
+                case _:
+                      return []
+
+def value_lookup_function(val):
+    lookup_tag = val.type.tag
+    if lookup_tag is None:
+        return None
+    if lookup_tag == 'nix::Value':
+        return ValuePrinter(val)
+    elif lookup_tag == 'nix::Value::Thunk':
+        return ThunkPrinter(val)
+    elif lookup_tag == 'nix::Value::App':
+        return AppPrinter(val)
+    elif lookup_tag == 'nix::Value::Acb':
+        return AcbPrinter(val)
+    return None
+
+def register_printers(objfile):
+      objfile.pretty_printers.append(value_lookup_function)
+
+register_printers(gdb.current_objfile())
+)";
+#endif
 }