Cannot perform frame jump after handled exception

[dkrystki]

For given code:

def fun():
    a = 1  # <---- jump location

    try:
        b = 1 / 0
    except ZeroDivisionError as e:
        pass

    c = 3  # current location


if __name__ == "__main__":
    fun()

Jumping from current location to jump location yields ValueError: can't jump from within an exception handler

Your environment

• CPython versions tested on: 3.11.0a7
• Operating system and architecture: Linux q 5.4.0-42-generic x64

Works as expected for python < 3.11

[serhiy-storchaka]

@markshannon

[markshannon]

@dkrystki Do you have a full reproducer?
You haven't said how you are performing the jump.

[dkrystki]

For the presented example I performed jump using PyCharm.
Not sure if I know how to reproduce it using settrace (I can try) but under the hood PyCharm is simply setting f_lineno to perform a jump.

Edit: For my library purposes I am setting f_lineno manually and it is causing the same issue

[dkrystki]

I managed to make a full reproducer:

import sys

def fun():
    a = 1  # <---- jump location

    try:
        b = 1 / 0
    except ZeroDivisionError as e:
        pass

    c = 3  # current location


def trace(frame, event, arg):
    if event == "line" and frame.f_lineno == 11:
        frame.f_lineno = 4

    return trace


sys.settrace(trace)

if __name__ == "__main__":
    fun()

Traceback:

Traceback (most recent call last):
  File "<cwd>/cakeshop.py", line 24, in <module>
    fun()
    ^^^^^
  File "<cwd>/cakeshop.py", line 11, in fun
    c = 3  # current location
        ^
  File "<cwd>/cakeshop.py", line 11, in fun
    c = 3  # current location
        ^
  File "<cwd>/cakeshop.py", line 16, in trace
    frame.f_lineno = 4
    ^^^^^^^^^^^^^^
ValueError: can't jump from within an exception handler

[serhiy-storchaka]

diff --git a/Lib/test/test_sys_settrace.py b/Lib/test/test_sys_settrace.py
index b1c8f6f80a..34126464bc 100644
--- a/Lib/test/test_sys_settrace.py
+++ b/Lib/test/test_sys_settrace.py
@@ -2217,6 +2217,16 @@ def test_jump_backwards_into_try_except_block(output):
             raise
         output.append(6)
 
+    @jump_test(7, 1, [1, 3, 6, 1, 3, 6])
+    def test_jump_backwards_over_try_except_block(output):
+        output.append(1)
+        try:
+            output.append(3)
+            1/0
+        except ZeroDivisionError:
+            output.append(6)
+        x = 7
+
     # 'except' with a variable creates an implicit finally block
     @jump_test(5, 7, [4], (ValueError, 'within'))
     def test_no_jump_between_except_blocks_2(output):

[serhiy-storchaka]

All works if add anything above or below the assignment.

[sweeneyde]

For what it's worth, this is the problematic result from the reproducer.

Bytecode:

3           0 RESUME                   0

4           2 LOAD_CONST               1 (1)
            4 STORE_FAST               0 (a)

6           6 NOP

7           8 LOAD_CONST               1 (1)
           10 LOAD_CONST               2 (0)
           12 BINARY_OP               11 (/)
           16 STORE_FAST               1 (b)

11          18 LOAD_CONST               3 (3)
           20 STORE_FAST               3 (c)
           22 LOAD_CONST               0 (None)
           24 RETURN_VALUE
      >>   26 PUSH_EXC_INFO

8          28 LOAD_GLOBAL              0 (ZeroDivisionError)
           40 CHECK_EXC_MATCH
           42 POP_JUMP_FORWARD_IF_FALSE    13 (to 70)
           44 STORE_FAST               2 (e)

9          46 POP_EXCEPT
           48 LOAD_CONST               0 (None)
           50 STORE_FAST               2 (e)
           52 DELETE_FAST              2 (e)

11          54 LOAD_CONST               3 (3)
           56 STORE_FAST               3 (c)
           58 LOAD_CONST               0 (None)
           60 RETURN_VALUE
           62 LOAD_CONST               0 (None)
           64 STORE_FAST               2 (e)
           66 DELETE_FAST              2 (e)
           68 RERAISE                  1

8     >>   70 RERAISE                  0
      >>   72 COPY                     3
           74 POP_EXCEPT
           76 RERAISE                  1
ExceptionTable:
8 to 16 -> 26 [0]
26 to 44 -> 72 [1] lasti
62 to 70 -> 72 [1] lasti

Result from mark_stacks():

int64_t *stacks = {
     [0] =  0,  [1] =  0,  [2] =  3,  [3] =  0,  [4] =  0,
     [5] =  3,  [6] = 15,  [7] =  3,  [8] =  3,  [9] =  0,
    [10] =  3, [11] =  0, [12] =  3, [13] = -2, [14] = -2,
    [15] = -2, [16] = -2, [17] = -2, [18] = -2, [19] = -2,
    [20] = -2, [21] = -2, [22] = -2, [23] = -2, [24] = -2,
    [25] = -2, [26] = -2, [27] = -2, [28] = -2, [29] = -2,
    [30] = -2, [31] = -2, [32] = -2, [33] = -2, [34] = -2,
    [35] = -2, [36] = -2, [37] = -2, [38] = -2, [39] = -2,
};

Here, -2 means UNINITIALIZED. This is a result of the following code in mark_stacks:

        for (i = 0; i < len; i++) {
            int64_t next_stack = stacks[i];
            if (next_stack == UNINITIALIZED) {
                continue;
            }
            // ...
        }

After RETURN_VALUE, the rest of the stacks are UNINITIALIZED and remain UNINITIALIZED as they're skipped over. I don't know the correct solution.

BTW, bisected to adcd220

[markshannon]

I'm making this a deferred blocker.
I don't think it needs to block the next beta, but I definitely want to fix it before release

[markshannon]

We need to use the exception handling table to fill in the stacks for exception handlers.
Fiddly, but not too difficult.

[pablogsal]

Bumping this to release-blocker

[iritkatriel]

We need to use the exception handling table to fill in the stacks for exception handlers.
Fiddly, but not too difficult.

The issue here is actually not jumping from within an exception handler to code outside of the handler (I'm not sure we want to allow that). Rather, it's jumping between two points that are outside of the handler.

If it wasn't for the inline-small-exit-blocks optimisation, there would be a jump over the except block that mark_stacks follows. But the c = 3 line is duplicated, so the except case has its own copy, which is unreachable in normal control flow:

  1           0 RESUME                   0

  2           2 LOAD_CONST               1 (1)
              4 STORE_FAST               0 (a)

  3           6 NOP

  4           8 LOAD_CONST               1 (1)
             10 LOAD_CONST               2 (0)
             12 BINARY_OP               11 (/)
             16 STORE_FAST               1 (b)

  7          18 LOAD_CONST               3 (3)          <==   c = 3 for no-exception case
             20 STORE_FAST               3 (c)
             22 LOAD_CONST               0 (None)
             24 RETURN_VALUE
        >>   26 PUSH_EXC_INFO

  5          28 LOAD_GLOBAL              0 (ZeroDivisionError)
             40 CHECK_EXC_MATCH
             42 POP_JUMP_FORWARD_IF_FALSE    13 (to 70)
             44 STORE_FAST               2 (e)

  6          46 POP_EXCEPT
             48 LOAD_CONST               0 (None)
             50 STORE_FAST               2 (e)
             52 DELETE_FAST              2 (e)

  7          54 LOAD_CONST               3 (3)            <==   c = 3 for handled-exception case
             56 STORE_FAST               3 (c)
             58 LOAD_CONST               0 (None)
             60 RETURN_VALUE
             62 LOAD_CONST               0 (None)
             64 STORE_FAST               2 (e)
             66 DELETE_FAST              2 (e)
             68 RERAISE                  1

  5     >>   70 RERAISE                  0
        >>   72 COPY                     3
             74 POP_EXCEPT
             76 RERAISE                  1

Thinking..

[iritkatriel]

This confirms what I wrote above - the problem goes away if we make this change:

diff --git a/Python/compile.c b/Python/compile.c
index 5ae1e34520..5e56d4eb22 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -8978,7 +8978,7 @@ jump_thread(struct instr *inst, struct instr *target, int opcode)
 }
 
 /* Maximum size of basic block that should be copied in optimizer */
-#define MAX_COPY_SIZE 4
+#define MAX_COPY_SIZE 0
 
 /* Optimization */
 static int

[iritkatriel]

All works if add anything above or below the assignment.

Indeed - then the size of the block exceeds the threshold for the optimisation.

[iritkatriel]

This optimisation is very old, and I wanted to check that it is still worth having.

I ran pyperformance a few times on my Mac with and without it, and my results are that removing it actually makes python a little faster. This is not a shock if it's true - adding more code to save one jump per function is not an obvious improvement.

If this is repeatable then the fix for this issue is obvious.

Another option is to leave the optimisation for blocks without line numbers (the implicit 'return None's) but disable it for small blocks that do have line numbers. I want to get numbers from our benchmarking machine (which is not working for me at the moment for some reason) before we make a decision.

[tiran]

Since the optimization does not have a noticeable negative impact and in fact improves performance on your system, then it is reasonable to remove it and get rid of one more release blocker. In worst case we can reconsider the optimization for next beta or 3.12.

[iritkatriel]

The PR is ready now. I went for the smaller change of disabling the optimisation only for blocks that have a line number.

[esc]

This optimisation is very old, and I wanted to check that it is still worth having.

@iritkatriel : Just to double check, is this the optimization introduced in:

https://bugs.python.org/issue44626

From what I can tell this came with Python 3.10, or am I mistaken?

[esc]

This optimisation is very old, and I wanted to check that it is still worth having.

@iritkatriel : Just to double check, is this the optimization introduced in:
https://bugs.python.org/issue44626
From what I can tell this came with Python 3.10, or am I mistaken?

No that PR just moved it around.

@iritkatriel : Thank you for answering my question. I have a followup questions about this. I will post the question here, but do let me know if this is not the appropriate place. I am asking, because I only discovered this bytecode optimization being triggered on Python 3.10. Maybe you can shed some light as to why I hadn't seen this before, e.g. on 3.9. Allow me to illustrate:

Using the following example:

def test(a):
    b = 0
    if a == 1:
        b = 3
    elif a == 2:
        b = 4
    else:
        b = 5
    return b

Using https://godbolt.org/ I get the following for 3.9

  2           0 LOAD_CONST               1 (0)
              2 STORE_FAST               1 (b)

  3           4 LOAD_FAST                0 (a)
              6 LOAD_CONST               2 (1)
              8 COMPARE_OP               2 (==)
             10 POP_JUMP_IF_FALSE       18

  4          12 LOAD_CONST               3 (3)
             14 STORE_FAST               1 (b)
             16 JUMP_FORWARD            18 (to 36)

  5     >>   18 LOAD_FAST                0 (a)
             20 LOAD_CONST               4 (2)
             22 COMPARE_OP               2 (==)
             24 POP_JUMP_IF_FALSE       32

  6          26 LOAD_CONST               5 (4)
             28 STORE_FAST               1 (b)
             30 JUMP_FORWARD             4 (to 36)

  8     >>   32 LOAD_CONST               6 (5)
             34 STORE_FAST               1 (b)

  9     >>   36 LOAD_FAST                1 (b)
             38 RETURN_VALUE

and then for 3.10:

  2           0 LOAD_CONST               1 (0)
              2 STORE_FAST               1 (b)

  3           4 LOAD_FAST                0 (a)
              6 LOAD_CONST               2 (1)
              8 COMPARE_OP               2 (==)
             10 POP_JUMP_IF_FALSE       10 (to 20)

  4          12 LOAD_CONST               3 (3)
             14 STORE_FAST               1 (b)

  9          16 LOAD_FAST                1 (b)
             18 RETURN_VALUE

  5     >>   20 LOAD_FAST                0 (a)
             22 LOAD_CONST               4 (2)
             24 COMPARE_OP               2 (==)
             26 POP_JUMP_IF_FALSE       18 (to 36)

  6          28 LOAD_CONST               5 (4)
             30 STORE_FAST               1 (b)

  9          32 LOAD_FAST                1 (b)
             34 RETURN_VALUE

  8     >>   36 LOAD_CONST               6 (5)
             38 STORE_FAST               1 (b)

  9          40 LOAD_FAST                1 (b)
             42 RETURN_VALUE

From what I can tell, the return block is only being inlined on Python 3.10 only?

[iritkatriel]

Possibly it was added in 3.10. You can dig in the code if you want. The compiler has changed quite a lot between 3.9 and now.

[esc]

Possibly it was added in 3.10. You can dig in the code if you want. The compiler has changed quite a lot between 3.9 and now.

Yes, I am aware of the changes to the compiler in 3.10. Thank you for your answer, my guess is that the code has been there for some time, but due to the refactoring in 3.10 it actually started triggering.

[markshannon]

Unfortunately #94592 doesn't fix this.
The problem applies to jumping from any block that is only reachable through an exception handler.
For example, this test fails.

    @jump_test(7, 1, [1, 6, 1, 6])
    def test_jump_over_try_except_early_return(output):
        output.append(1)
        try:
            1 / 0
            return
        except ZeroDivisionError as e:
            output.append(6)
        pass # This block is dominated by the except handler

Code like this is much less likely than the original example, but I think we should fix this properly by marking the stacks for exception handlers in mark_stacks

[iritkatriel]

That we are not tracking the exception handler is a separate (and known) issue. The problem here was that we could not tell where an exception handler ends.

[iritkatriel]

The issue of exception handlers is tracked here: #94739

[markshannon]

The jump from the example I give is from the pass, which is outside of the except handler.

[iritkatriel]

I see, will check.

[iritkatriel]

This fails too:

    @jump_test(1, 3, [1, 3])
    def test_jump_over_return(output):
        output.append(1)
        return 12
        output.append(3)

[markshannon]

That should fail. output.append(3) is unreachable

[iritkatriel]

You mean, the compiler can tell that it's unreachable.

If you allow the debugger to jump around you can do all kinds of impossible things (like get to the pass in your example without an exception being raised). I'm not sure what the semantics of these debugger jumps are, really.

[markshannon]

I'm not sure what the semantics of these debugger jumps are, really.

No one is.

In my mind, they are:

• Never crash the interpreter.
• If it is possible to make the jump, then do so.
• Present a meaningful error message if the jump is impossible.

[iritkatriel]

We understand the issue, and it's the subject of #94739 so I'll close this again (the original problem reported here was resolved).