rfc:zpp-conversion-rules

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PHP RFC: Coercive Types for Function Arguments

Introduction

This RFC proposes to implement a new set of rules to be used when parsing internal function arguments.

The changes proposed here are based on concerns that arose during the discussion about scalar type hints.

While the discussion focused on a dual-mode approach, with people opposing use cases about so-called 'weak' and 'strict' typing, some of us decided to search for a compromise that would satisfy both camps.

It was soon clear that the strongest argument of strict-typing proponents was that 'weak' mode was way too permissive, allowing, for instance, true or '7 years' as integer value.

As the proposed argument type hinting mechanism is aligned on the ZPP behavior, and as everyone seemed to agree it should remain so, it was decided to propose a new more restrictive ruleset to be implemented in ZPP and zend_parse_parameters().

So, this RFC presents a new ruleset for parameter parsing and implicit conversions.

Note: In the rest of the document, 'ZPP' means 'Z_PARAM macros and zend_parse_parameters()' as, whatever the final implementation, both mechanisms must remain in sync.

Proposal

Current ruleset

The ZPP ruleset authorizes implicit conversions for IS_NULL, IS_FALSE, IS_TRUE, IS_LONG, IS_DOUBLE, and IS_STRING zval types only.

Actually, it also implement a mechanism for (object -> string) but, as this one will remain unchanged, it will be ignored here.

The following table shows the current rules used to accept and convert an input zval through the ZPP layer :

Zval type
ZPP type IS_NULL IS_FALSE IS_TRUE IS_LONG IS_DOUBLE IS_STRING
bool Yes (-> false) <native> <native> Yes (1) Yes (1) Yes (2)
long Yes (-> 0) Yes (-> 0) Yes (-> 1) <native> Yes (3)
double Yes (-> 0.0) Yes (-> 0.0) Yes (-> 1.0) Yes <native> (4)
string Yes (-> “”) Yes (-> “”) Yes (-> “1”) Yes Yes <native>
  • (1) false if null, true if non null
  • (2) “” and “0” -> false, other values -> true
  • (3) Run string through is_numeric_str_function() and detect error. If double returned, convert it to long
  • (4) Run string through is_numeric_str_function() and detect error. If int returned, convert it to double

The conversion of IS_STRING to int/float is done through _is_numeric_string_ex(). Among others, this function has the following behavior :

  • Stop conversion at first non digit character with no error (this is what authorizes '7 years' as integer).

Proposed changes

  • Disable IS_FALSE/IS_NULL to anything but bool (native).
  • Disable (IS_STRING -> bool)
  • Disable IS_NULL conversion to every ZPP type. It causes IS_NULL to be systematically rejected by every Z_PARAM macro except Z_PARAM_ZVAL.
  • Disable (IS_DOUBLE -> long) if the fractional part of the input value is non null.
  • When converting from IS_STRING to long, reject conversion if string contains the representation of a number with a non null fractional part.

In _is_numeric_string_ex() :

  • Add a check for trailing characters : trailing blanks are accepted, the first non-blank character encountered raises an error (note that blanks are supported as leading or trailing chars only).

Future ruleset

This table shows a synthetic view of the resulting ruleset ('-' means 'Disabled'):

Zval type
ZPP type IS_NULL IS_FALSE IS_TRUE IS_LONG IS_DOUBLE IS_STRING
bool - <native> <native> Yes (1) Yes (1) -
long - - - <native> (2) (3)
double - - - Yes <native> (4)
string - - - Yes Yes <native>
  • (1) false if null, true if non null
  • (2) error if fractional part is non null
  • (3) Run string through is_numeric_str_function() and detect error. If double with null fractional part returned, convert to long, else error
  • (4) Run string through is_numeric_str_function() and detect error. If int returned, convert to double

and the new behavior of _is_numeric_string_ex() :

  • Ignore trailing blanks
  • Error on any non-blank trailing char

Backward Incompatible Changes

Every restriction of the ruleset causes a BC break.

Proposed PHP Version(s)

Because of BC breaks, requires a major version, so 7.0.

RFC Impact

Every internal functions are potentially impacted.

Any code converting a string to a number is potentially impacted. If this cause an unacceptable BC break, we'll create a private copy of this function specific for argument parsing. As long as it is not needed, it is better if every string to numeric conversion uses the same code.

To Opcache

None

New Constants

None

php.ini Defaults

None

Open Issues

Coming soon...

Unaffected PHP Functionality

Future Scope

The set of supported numeric strings can be extended.

String to numeric conversion can be made smarter and accept more.

Proposed Voting Choices

Requires a 2/3 majority.

Vote will be a single-choice yes/no vote.

Voting date is not planned yet.

Patches and Tests

Work in progress.

As soon as patch is available, extensive testing must be performed to evaluate overall BC breaks.

Implementation

After the project is implemented, this section should contain

  1. the version(s) it was merged to
  2. a link to the git commit(s)
  3. a link to the PHP manual entry for the feature

References

Rejected Features

Keep this updated with features that were discussed on the mail lists.

rfc/zpp-conversion-rules.1424376697.txt.gz · Last modified: 2017/09/22 13:28 (external edit)