py/modsys: Add optional sys.tracebacklimit attribute.

With behaviour as per CPython.

Signed-off-by: Damien George <damien@micropython.org>

docs/library/sys.rst

@@ -144,6 +144,14 @@ Constants
 
    Standard output `stream`.
 
+.. data:: tracebacklimit
+
+   A mutable attribute holding an integer value which is the maximum number of traceback
+   entries to store in an exception.  Set to 0 to disable adding tracebacks.  Defaults
+   to 1000.
+
+   Note: this is not available on all ports.
+
 .. data:: version
 
    Python language version that this implementation conforms to, as a string.

ports/unix/variants/coverage/mpconfigvariant.h

@@ -34,6 +34,7 @@
 #define MICROPY_REPL_EMACS_WORDS_MOVE  (1)
 #define MICROPY_REPL_EMACS_EXTRA_WORDS_MOVE (1)
 #define MICROPY_WARNINGS_CATEGORY      (1)
+#define MICROPY_MODULE_ATTR_DELEGATION (1)
 #define MICROPY_MODULE_GETATTR         (1)
 #define MICROPY_PY_DELATTR_SETATTR     (1)
 #define MICROPY_PY_ALL_INPLACE_SPECIAL_METHODS (1)
@@ -44,6 +45,7 @@
 #define MICROPY_PY_BUILTINS_HELP       (1)
 #define MICROPY_PY_BUILTINS_HELP_MODULES (1)
 #define MICROPY_PY_SYS_GETSIZEOF       (1)
+#define MICROPY_PY_SYS_TRACEBACKLIMIT  (1)
 #define MICROPY_PY_MATH_CONSTANTS      (1)
 #define MICROPY_PY_MATH_FACTORIAL      (1)
 #define MICROPY_PY_URANDOM_EXTRA_FUNCS (1)

py/modsys.c

@@ -185,6 +185,9 @@ MP_DEFINE_CONST_FUN_OBJ_1(mp_sys_settrace_obj, mp_sys_settrace);
 
 #if MICROPY_PY_SYS_ATTR_DELEGATION
 STATIC const uint16_t sys_mutable_keys[] = {
+    #if MICROPY_PY_SYS_TRACEBACKLIMIT
+    MP_QSTR_tracebacklimit,
+    #endif
     MP_QSTRnull,
 };
 

py/mpconfig.h

@@ -1377,10 +1377,15 @@ typedef double mp_float_t;
 #define MICROPY_PY_SYS_STDIO_BUFFER (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)
 #endif
 
+// Whether to provide sys.tracebacklimit mutable attribute
+#ifndef MICROPY_PY_SYS_TRACEBACKLIMIT
+#define MICROPY_PY_SYS_TRACEBACKLIMIT (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EVERYTHING)
+#endif
+
 // Whether the sys module supports attribute delegation
 // This is enabled automatically when needed by other features
 #ifndef MICROPY_PY_SYS_ATTR_DELEGATION
-#define MICROPY_PY_SYS_ATTR_DELEGATION (0)
+#define MICROPY_PY_SYS_ATTR_DELEGATION (MICROPY_PY_SYS_TRACEBACKLIMIT)
 #endif
 
 // Whether to provide "uerrno" module

py/mpstate.h

@@ -41,6 +41,9 @@
 // variable, but in the future it is hoped that the state can become local.
 
 enum {
+    #if MICROPY_PY_SYS_TRACEBACKLIMIT
+    MP_SYS_MUTABLE_TRACEBACKLIMIT,
+    #endif
     MP_SYS_MUTABLE_NUM,
 };
 

py/objexcept.c

@@ -575,6 +575,16 @@ void mp_obj_exception_add_traceback(mp_obj_t self_in, qstr file, size_t line, qs
     // append this traceback info to traceback data
     // if memory allocation fails (eg because gc is locked), just return
 
+    #if MICROPY_PY_SYS_TRACEBACKLIMIT
+    mp_int_t max_traceback = MP_OBJ_SMALL_INT_VALUE(MP_STATE_VM(sys_mutable[MP_SYS_MUTABLE_TRACEBACKLIMIT]));
+    if (max_traceback <= 0) {
+        return;
+    } else if (self->traceback_data != NULL && self->traceback_len >= max_traceback * TRACEBACK_ENTRY_LEN) {
+        self->traceback_len -= TRACEBACK_ENTRY_LEN;
+        memmove(self->traceback_data, self->traceback_data + TRACEBACK_ENTRY_LEN, self->traceback_len * sizeof(self->traceback_data[0]));
+    }
+    #endif
+
     if (self->traceback_data == NULL) {
         self->traceback_data = m_new_maybe(size_t, TRACEBACK_ENTRY_LEN);
         if (self->traceback_data == NULL) {

py/runtime.c

@@ -141,6 +141,10 @@ void mp_init(void) {
     MP_STATE_THREAD(current_code_state) = NULL;
     #endif
 
+    #if MICROPY_PY_SYS_TRACEBACKLIMIT
+    MP_STATE_VM(sys_mutable[MP_SYS_MUTABLE_TRACEBACKLIMIT]) = MP_OBJ_NEW_SMALL_INT(1000);
+    #endif
+
     #if MICROPY_PY_BLUETOOTH
     MP_STATE_VM(bluetooth) = MP_OBJ_NULL;
     #endif

tests/basics/sys_tracebacklimit.py

@@ -0,0 +1,78 @@
+# test sys.tracebacklimit
+
+try:
+    try:
+        import usys as sys
+        import uio as io
+    except ImportError:
+        import sys
+        import io
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+try:
+    sys.tracebacklimit = 1000
+except AttributeError:
+    print("SKIP")
+    raise SystemExit
+
+if hasattr(sys, "print_exception"):
+    print_exception = sys.print_exception
+else:
+    import traceback
+
+    print_exception = lambda e, f: traceback.print_exception(None, e, sys.exc_info()[2], file=f)
+
+
+def print_exc(e):
+    buf = io.StringIO()
+    print_exception(e, buf)
+    s = buf.getvalue()
+    for l in s.split("\n"):
+        # Remove filename.
+        if l.startswith("  File "):
+            l = l.split('"')
+            print(l[0], l[2])
+        # uPy and CPy tracebacks differ in that CPy prints a source line for
+        # each traceback entry.  In this case, we know that offending line
+        # has 4-space indent, so filter it out.
+        elif not l.startswith("    "):
+            print(l)
+
+
+def f0():
+    raise ValueError("value")
+
+
+def f1():
+    f0()
+
+
+def f2():
+    f1()
+
+
+def f3():
+    f2()
+
+
+def ftop():
+    try:
+        f3()
+    except ValueError as er:
+        print_exc(er)
+
+
+ftop()
+
+for limit in range(4, -2, -1):
+    print("limit", limit)
+    sys.tracebacklimit = limit
+    ftop()
+
+
+# test deleting the attribute
+print(hasattr(sys, "tracebacklimit"))
+del sys.tracebacklimit
+print(hasattr(sys, "tracebacklimit"))

tests/run-tests.py

@@ -504,6 +504,7 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
         skip_tests.add("basics/del_local.py")  # requires checking for unbound local
         skip_tests.add("basics/exception_chain.py")  # raise from is not supported
         skip_tests.add("basics/scope_implicit.py")  # requires checking for unbound local
+        skip_tests.add("basics/sys_tracebacklimit.py")  # requires traceback info
         skip_tests.add("basics/try_finally_return2.py")  # requires raise_varargs
         skip_tests.add("basics/unboundlocal.py")  # requires checking for unbound local
         skip_tests.add("extmod/uasyncio_event.py")  # unknown issue

tests/unix/extra_coverage.py.exp

@@ -45,8 +45,8 @@ utime           utimeq
 argv            atexit          byteorder       exc_info
 exit            getsizeof       implementation  maxsize
 modules         path            platform        print_exception
-stderr          stdin           stdout          version
+stderr          stdin           stdout          tracebacklimit
-version_info
+version         version_info
 ementation
 # attrtuple
 (start=1, stop=2, step=3)