<i18n dev> RFR: 8323699: MessageFormat.toPattern() generates non-equivalent MessageFormat pattern [v3]

[Archie Cobbs]

On Fri, 19 Jan 2024 23:30:43 GMT, Archie Cobbs <acobbs at openjdk.org> wrote:

>> Please consider this fix to ensure that going from `MessageFormat` to pattern string via `toPattern()` and then back via `new MessageFormat()` results in a format that is equivalent to the original.
>> 
>> The quoting and escaping rules for `MessageFormat` pattern strings are really tricky. I admit not completely understanding them. At a high level, they work like this: The normal way one would "nest" strings containing special characters is with straightforward recursive escaping like with the `bash` command line. For example, if you want to echo `a "quoted string" example` then you enter `echo "a "quoted string" example"`. With this scheme it's always the "outer" layer's job to (un)escape special characters as needed. That is, the echo command never sees the backslash characters.
>> 
>> In contrast, with `MessageFormat` and friends, nested subformat pattern strings are always provided "pre-escaped". So to build an "outer" string (e.g., for `ChoiceFormat`) the "inner" subformat pattern strings are more or less just concatenated, and then only the `ChoiceFormat` option separator characters (e.g., `<`, `#`, `|`, etc.) are escaped.
>> 
>> The "pre-escape" escaping algorithm escapes `{` characters, because `{` indicates the beginning of a format argument. However, it doesn't escape `}` characters. This is OK because the format string parser treats any "extra" closing braces (where "extra" means not matching an opening brace) as plain characters.
>> 
>> So far, so good... at least, until a format string containing an extra closing brace is nested inside a larger format string, where the extra closing brace, which was previously "extra", can now suddenly match an opening brace in the outer pattern containing it, thus truncating it by "stealing" the match from some subsequent closing brace.
>> 
>> An example is the `MessageFormat` string `"{0,choice,0.0#option A: {1}|1.0#option B: {1}'}'}"`. Note the second option format string has a trailing closing brace in plain text. If you create a `MessageFormat` with this string, you see a trailing `}` only with the second option.
>> 
>> However, if you then invoke `toPattern()`, the result is `"{0,choice,0.0#option A: {1}|1.0#option B: {1}}}"`. Oops, now because the "extra" closing brace is no longer quoted, it matches the opening brace at the beginning of the string, and the following closing  brace, which was the previous match, is now just plain text in the outer `MessageFormat` string.
>> 
>> As a result, invoking `f.format(new ...
>
> Archie Cobbs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add @implNote to Javadoc for toPattern().

Adding an additional note of explanation here just for the record (this is copied from the CSR):

> We need to ask how do we know it is safe to quote the unquoted curly brace characters in the subformat patterns? If curly braces are not special to the subformat, then quoting them clearly does no harm. So we only need worry about subformat patterns where curly braces are special. But the only subformat pattern strings supported by `MessageFormat` are for `DecimalFormat`, `SimpleDateFormat`, and `ChoiceFormat`, and curly braces are not special for any of these classes, so we're good.
> 
> However, it should be noted that there is some confusing special logic that clouds this question. If the string that results from evaluating a `ChoiceFormat` subformat of a `MessageFormat` contains an opening curly brace, then a new `MessageFormat` is created from that string and evaluated, and _that_ string replaces the original. This behavior doesn't impact how subformats should be quoted, only how their results are interpreted at "run time".

-------------

PR Comment: https://git.openjdk.org/jdk/pull/17416#issuecomment-1901291484