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Re: [PATCH v2 4/8] qapi/error: Change assertion

From: John Snow
Subject: Re: [PATCH v2 4/8] qapi/error: Change assertion
Date: Fri, 16 Apr 2021 14:24:23 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 4/16/21 2:17 AM, Markus Armbruster wrote:
John Snow <jsnow@redhat.com> writes:

On 4/15/21 3:00 AM, Markus Armbruster wrote:
John Snow <jsnow@redhat.com> writes:

On 3/30/21 1:18 PM, John Snow wrote:

Realizing now that this commit topic is wrong :)

A prior version modified the assertion, I decided it was less churn to
simply add one.

I think ideally we'd have no assertions here and we'd rely on the type
hints, but I don't think I can prove that this is safe until after part
6, so I did this for now instead.

Eventually, we'll be able to prove that 'info.line' must be an int and
is never None at static analysis time, and this assert can go
away. Until then, it's a type error to assume that self.info is not

Signed-off-by: John Snow <jsnow@redhat.com>
    scripts/qapi/error.py | 1 +
    1 file changed, 1 insertion(+)

diff --git a/scripts/qapi/error.py b/scripts/qapi/error.py
index d179a3bd0c..d0bc7af6e7 100644
--- a/scripts/qapi/error.py
+++ b/scripts/qapi/error.py
@@ -25,6 +25,7 @@ def __init__(self, info, msg, col=None):
            self.col = col
           def __str__(self):
+        assert self.info is not None
            loc = str(self.info)
            if self.col is not None:
                assert self.info.line is not None

Please show us the revised commit message.  I'm asking because the
patch's purpose isn't quite clear to me.  Peeking ahead at PATCH 7, I
see that info becomes Optional[QAPISourceInfo].  This means something
passes None for info (else you wouldn't make it optional).  None info
Works (in the sense of "doesn't crash") as long as col is also None.
After the patch, it doesn't.  I'm not saying that's bad, only that I
don't quite understand what you're trying to accomplish here.


If you recall, I tried to enforce that QAPISourceInfo was *never* None
by creating a special value for QSI that represented "No Source
Info". Ultimately, that idea didn't go through and we solidified that
the 'info' parameter that gets passed around can sometimes be None.

Hence, this property is Optional[QAPISourceInfo].

Since I know you wanted to crash messily in the case that we allowed a
None-info to leak into a context like this, I added the new assertion
to make sure this remains the case.

(since str(None) evaluates to "None", it seemed like there was already
the implicit assumption that info would never be None. Plus, if 'col'
is set, mypy notices that we are performing an unsafe check on
self.info.line, which had to be remedied.)

Later in the series, after schema.py is typed, it may be possible to
remove these assertions as we may be able to rely on the strict typing
to prove that this situation can never occur. However, since schema.py
is not yet typed, we can't yet.

Alright. So given that, I think what you'd like to see for a commit
message is:

qapi/error: assert QAPISourceInfo is not None

We implicitly assume that self.info will never be None, as the error
reporting function will not produce meaningful output in this case,
and will crash if self.col was set. Assert that self.info is not None
in order to formalize this assumption.

We can not yet change the type of the initializer to prove that this
is provably true at static analysis time until the rest of this
library is fully typed.

Let's insert another paragraph to make the intent even clearer, and
adjust the remainder for it:

   qapi/error: assert QAPISourceInfo is not None

   Built-in stuff is not parsed from a source file, and therefore have no
   QAPISourceInfo.  If such None info was used for reporting an error,
   built-in stuff would be broken.  Programming error.  Instead of
   reporting a confusing error with bogus source location then, we better

   We currently crash only if self.col was set.  Assert that self.info is
   not None in order to crash reliably.

   We can not yet change the type of the initializer to prove this cannot
   happen at static analysis time before the remainder of the code is
   fully typed.

Note I also replaced "this library" because I'm not sure what it means.

Wiggly-brain, wiggly-mouth. I meant "package". I get these terms consistently wrong, and I need to really make a very direct effort to treat them precisely correctly in the future.

MODULE: A single Python file. It may be a script, or serve only as a library meant for consumption by other scripts.

PACKAGE: A collection of one or more Python modules. QAPI is a package because it's a folder with an __init__.py file, containing several modules.

LIBRARY: No formal definition; however, I think of it as a Python module that is meant primarily to be consumed by another script or entry-point. It has some implications for things like Python's hierarchical logging library where the distinction is important for how logger setup is handled. Modules that use the "if __name__ == '__main__'" stanza serve dual purpose as a script *and* library.

In this case, I meant "package". I think I accidentally avoid this term because I don't want to imply that it is a "distributed package" in the sense of a "PyPI package" and my brain skips a clock cycle and picks another random word.

Bad habit. :(

What do you think?

TLDR: I took your phrasing wholesale. Thanks!


(Random aside on patch submission process: I do dislike how when I change the topic of a commit, I lose out on git-backport-diff functionality. I wish I had a persistent ID for commits that survived beyond title changes. Sometimes I debate scripting adding some kind of Patch-GUID: <...> tag, but I don't know if that would look like undesirable clutter in the git log to everyone else. Maybe a "WAS: old-topic-name-here" in the comment section would suffice well enough?)

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