Re: [PATCH 01/22] qapi/parser: Don't try to handle file errors

[John Snow <jsnow@redhat.com>]

On 4/23/21 11:46 AM, Markus Armbruster wrote:
> John Snow <jsnow@redhat.com> writes:
> 
>> The short-ish version of what motivates this patch is:
>>
>> - The parser initializer does not possess adequate context to write a
>>    good error message -- It tries to determine the caller's semantic
>>    context.
> 
> I'm not sure I get what you're trying to say here.
> 

I mean: this __init__ method does not *know* who is calling it or why. 
Of course, *we* do, because the code base is finite and nobody else but 
us is calling into it.

I mean to point out that the initializer has to do extra work (Just a 
little) to determine what the calling context is and raise an error 
accordingly.

Example: If we have a parent info context, we raise an error in the 
context of the caller. If we don't, we have to create a new presumed 
context (using the weird None SourceInfo object).

So I just mean to say:

"Let the caller, who unambiguously always has the exactly correct 
context worry about what the error message ought to be."

>> - We don't want to allow QAPISourceInfo(None, None, None) to exist.
>> - Errors made using such an object are currently incorrect.
>> - It's not technically a semantic error if we cannot open the schema
>> - There are various typing constraints that make mixing these two cases
>>    undesirable for a single special case.
> 
> These I understand.
> 
>> - The current open block in parser's initializer will leak file
>>    pointers, because it isn't using a with statement.
> 
> Uh, isn't the value returned by open() reference-counted?  @fp is the
> only reference...
> 

Yeah, eventually. O:-)

Whenever the GC runs. OK, it's not really an apocalypse error, but it 
felt strange to rewrite a try/except and then write it using bad hygiene 
on purpose in the name of a more isolated commit.

>> Here's the details in why this got written the way it did, and why a few
>> disparate issues are rolled into one commit. (They're hard to fix
>> separately without writing really weird stuff that'd be harder to
>> review.)
>>
>> The error message string here is incorrect:
>>
>>> python3 qapi-gen.py 'fake.json'
>> qapi-gen.py: qapi-gen.py: can't read schema file 'fake.json': No such file or directory
> 
> Regressed in commit 52a474180a "qapi-gen: Separate arg-parsing from
> generation" (v5.2.0).
> 

Mea Culpa. Didn't realize it wasn't tested, and I didn't realize at the 
time that the two kinds of errors here were treated differently.

> Before commit c615550df3 "qapi: Improve source file read error handling"
> (v4.2.0), it was differently bad (uncaught exception).
> 
> Commit c615550df3 explains why the funny QAPISourceInfo exists:
> 
>      Reporting open or read failure for the main schema file needs a
>      QAPISourceInfo representing "no source".  Make QAPISourceInfo cope
>      with fname=None.
> 

I am apparently not the first or the last person to dream of wanting a 
QAPISourceInfo that represents "Actually, there's no source location!"

> The commit turned QAPISourceInfo into the equivalent of a disjoint union
> of
> 
> 1. A position in a source file (.fname is a str)
> 
> 2. "Not in any source file" (.fname is None)
> 
> This is somewhat similar to struct Location in C, which has
> 
> 1. LOC_FILE: a position in a source file
> 
> 2. LOC_CMDLINE: a range of command line arguments
> 
> 3. LOC_NONE: no location information
> 
> Abstracting locations this way lets error_report() do the right thing
> whether its complaining about the command line, a monitor command, or a
> configuration file read with -readconfig.
> 
> Your patch demonstrates that qapi-gen has much less need for abstracting
> sources: we use 2. "Not in any source file" only for reading the main
> schema file.
> 

Yes. I got the impression that you didn't want to pursue more abstract 
QSI constructs based on earlier work, so going the other way and 
*removing* them seemed like the faster way to achieve a clean type 
system here.

>> In pursuing it, we find that QAPISourceInfo has a special accommodation
>> for when there's no filename.
> 
> Yes:
> 
>      def loc(self) -> str:
> -->     if self.fname is None:
> -->         return sys.argv[0]
>          ret = self.fname
>          if self.line is not None:
>              ret += ':%d' % self.line
>          return ret
> 
>>                                Meanwhile, we intend to type info.fname as
>> str; something we always have.
> 
> Do you mean "as non-optional str"?
> 

Yeah. I typed it originally as `str`, but the analyzer missed that we 
check the field to see if it's None, which is misleading.

>> To remove this, we need to not have a "fake" QAPISourceInfo object. We
> 
> We may well want to, but I doubt we *need* to.  There are almost
> certainly other ways to fix the bug.  I don't see a need to explore
> them, though.
> 

Either we build out the fake QSI into a proper subtype, or we remove it 
-- those are the two obvious options. Building it out is almost 
certainly more work than this patch.

>> also don't want to explicitly begin accommodating QAPISourceInfo being
>> None, because we actually want to eventually prove that this can never
>> happen -- We don't want to confuse "The file isn't open yet" with "This
>> error stems from a definition that wasn't defined in any file".
> 
> Yes, encoding both "poisoned source info not to be used with actual
> errors" and "'fake' source info not pointing to a source file" as None
> would be a mistake.
> 

:)

>> (An earlier series tried to create an official dummy object, but it was
>> tough to prove in review that it worked correctly without creating new
>> regressions. This patch avoids trying to re-litigate that discussion.
>>
>> We would like to first prove that we never raise QAPISemError for any
>> built-in object before we relent and add "special" info objects. We
>> aren't ready to do that yet, so crashing is preferred.)
>>
>> So, how to solve this mess?
>>
>> Here's one way: Don't try to handle errors at a level with "mixed"
>> semantic levels; i.e. don't try to handle inclusion errors (should
>> report a source line where the include was triggered) with command line
>> errors (where we specified a file we couldn't read).
>>
>> Simply remove the error handling from the initializer of the
>> parser. Pythonic! Now it's the caller's job to figure out what to do
>> about it. Handle the error in QAPISchemaParser._include() instead, where
>> we do have the correct semantic context to not need to play games with
>> the error message generation.
>>
>> Next, to re-gain a nice error at the top level, add a new try/except
>> into qapi/main.generate(). Now the error looks sensible:
> 
> Missing "again" after "sensible" ;-P
> 

okayokayokayfine

>>
>>> python3 qapi-gen.py 'fake.json'
>> qapi-gen.py: can't read schema file 'fake.json': No such file or directory
>>
>> Lastly, with this usage gone, we can remove the special type violation
>> from QAPISourceInfo, and all is well with the world.
>>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> ---
>>   scripts/qapi/main.py   |  8 +++++++-
>>   scripts/qapi/parser.py | 18 +++++++++---------
>>   scripts/qapi/source.py |  3 ---
>>   3 files changed, 16 insertions(+), 13 deletions(-)
>>
>> diff --git a/scripts/qapi/main.py b/scripts/qapi/main.py
>> index 703e7ed1ed5..70f8aa86f37 100644
>> --- a/scripts/qapi/main.py
>> +++ b/scripts/qapi/main.py
>> @@ -48,7 +48,13 @@ def generate(schema_file: str,
>>       """
>>       assert invalid_prefix_char(prefix) is None
>>   
>> -    schema = QAPISchema(schema_file)
>> +    try:
>> +        schema = QAPISchema(schema_file)
>> +    except OSError as err:
>> +        raise QAPIError(
>> +            f"can't read schema file '{schema_file}': {err.strerror}"
>> +        ) from err
>> +
>>       gen_types(schema, output_dir, prefix, builtins)
>>       gen_visit(schema, output_dir, prefix, builtins)
>>       gen_commands(schema, output_dir, prefix)
>> diff --git a/scripts/qapi/parser.py b/scripts/qapi/parser.py
>> index ca5e8e18e00..b378fa33807 100644
>> --- a/scripts/qapi/parser.py
>> +++ b/scripts/qapi/parser.py
>> @@ -40,15 +40,9 @@ def __init__(self, fname, previously_included=None, incl_info=None):
>>           previously_included = previously_included or set()
>>           previously_included.add(os.path.abspath(fname))
>>   
>> -        try:
>> -            fp = open(fname, 'r', encoding='utf-8')
>> +        # Allow the caller to catch this error.
> 
> "this error"?  I understand what you mean now, but I'm not sure I will
> in three months, when I won't have the context I have now.
> 

Yep, OK.

# May raise OSError, allow the caller to handle it.

>> +        with open(fname, 'r', encoding='utf-8') as fp:
>>               self.src = fp.read()
>> -        except IOError as e:
>> -            raise QAPISemError(incl_info or QAPISourceInfo(None, None, None),
>> -                               "can't read %s file '%s': %s"
>> -                               % ("include" if incl_info else "schema",
>> -                                  fname,
>> -                                  e.strerror))
>>   
>>           if self.src == '' or self.src[-1] != '\n':
>>               self.src += '\n'
>> @@ -129,7 +123,13 @@ def _include(self, include, info, incl_fname, previously_included):
>>           if incl_abs_fname in previously_included:
>>               return None
>>   
>> -        return QAPISchemaParser(incl_fname, previously_included, info)
>> +        try:
>> +            return QAPISchemaParser(incl_fname, previously_included, info)
>> +        except OSError as err:
>> +            raise QAPISemError(
>> +                info,
>> +                f"can't read include file '{incl_fname}': {err.strerror}"
>> +            ) from err
>>   
>>       def _check_pragma_list_of_str(self, name, value, info):
>>           if (not isinstance(value, list)
> 
> Before the patch, only IOError from open() and .read() get converted to
> QAPISemError, and therefore caught by main().
> 
> The patch widen this to anywhere in QAPISchemaParser.__init__().  Hmm.
> 

"Changed in version 3.3: EnvironmentError, IOError, WindowsError, 
socket.error, select.error and mmap.error have been merged into OSError, 
and the constructor may return a subclass."

 >>> OSError == IOError
True

(No, I didn't know this before I wrote it. I just intentionally wanted 
to catch everything that open() might return, which I had simply assumed 
was not fully captured by IOError. Better to leave it as OSError now to 
avoid misleading anyone into thinking it's more narrow than it really is.)

>> diff --git a/scripts/qapi/source.py b/scripts/qapi/source.py
>> index 03b6ede0828..1ade864d7b9 100644
>> --- a/scripts/qapi/source.py
>> +++ b/scripts/qapi/source.py
>> @@ -10,7 +10,6 @@
>>   # See the COPYING file in the top-level directory.
>>   
>>   import copy
>> -import sys
>>   from typing import List, Optional, TypeVar
>>   
>>   
>> @@ -53,8 +52,6 @@ def next_line(self: T) -> T:
>>           return info
>>   
>>       def loc(self) -> str:
>> -        if self.fname is None:
>> -            return sys.argv[0]
>>           ret = self.fname
>>           if self.line is not None:
>>               ret += ':%d' % self.line
> 
> tests/qapi-schema/test-qapi.py also needs an update.  Before the patch:
> 
>      $ PYTHONPATH=scripts python3 tests/qapi-schema/test-qapi.py nonexistent
>      tests/qapi-schema/test-qapi.py: can't read schema file 'nonexistent.json': No such file or directory
> 
> After:
> 
>      Traceback (most recent call last):
>        File "tests/qapi-schema/test-qapi.py", line 207, in <module>
>          main(sys.argv)
>        File "tests/qapi-schema/test-qapi.py", line 201, in main
>          status |= test_and_diff(test_name, dir_name, args.update)
>        File "tests/qapi-schema/test-qapi.py", line 129, in test_and_diff
>          test_frontend(os.path.join(dir_name, test_name + '.json'))
>        File "tests/qapi-schema/test-qapi.py", line 109, in test_frontend
>          schema = QAPISchema(fname)
>        File "/work/armbru/qemu/scripts/qapi/schema.py", line 852, in __init__
>          parser = QAPISchemaParser(fname)
>        File "/work/armbru/qemu/scripts/qapi/parser.py", line 44, in __init__
>          with open(fname, 'r', encoding='utf-8') as fp:
>      FileNotFoundError: [Errno 2] No such file or directory: 'nonexistent.json'
> 

Probably something that should be added to the actual battery of tests 
somehow, yeah? I can't prevent regressions in invocations that don't get 
run O:-)

--js