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

[Markus Armbruster <armbru@redhat.com>]

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.

> - 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...

> 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).

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.

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.

> 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"?

> 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.

> 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

>
>> 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.

> +        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.

> 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'