Discuss limitations on types based on field def

[mnot]

Fixes #2393.

[mnot]

It's likely because the field is ignored when parsing fails, but 8941 s 2 allows a field to specify other error handling behaivour.

[reschke]

How so?

8941, Section 4.2 says:

If parsing fails -- including when calling another algorithm -- the entire field value MUST be ignored (i.e., treated as if the field were not present in the section). This is intentionally strict, to improve interoperability and safety, and specifications referencing this document are not allowed to loosen this requirement.

[mnot]

Section 2 has a bit more nuance:

When parsing fails, the entire field is ignored (see Section 4.2); in most situations, violating field-specific constraints should have the same effect. Thus, if a header is defined as an Item and required to be an Integer, but a String is received, the field will by default be ignored. If the field requires different error handling, this should be explicitly specified

[reschke]

You refer to:

If the field requires different error handling, this should be explicitly specified

?

My reading is that this applies to the preceding text about what happens when parsing does not fail, but additional constraints are not met.

If this is not the case, we should consider this to be an inconsistency in requirements and treat it as spec bug.

EDIT: opened #2399

[reschke]

I think "likely discarded" is very misleading; it's a requirement to discard.

[martinthomson]

A suggestion you might consider... I think that it would be helpful to include a more complete example here.

There is a hazard here in that any inconsistency how processing occurs creates interesting effects. That is, the field is destroyed if a Date if observed. That might lead to Heisenbugs.

Let's say you have a WAF that uses SF to parse, but it only looks at certain things. Maybe it doesn't look at parameters. This might ignore a Date on something like:

Example: ?0;t=@4098543601

Code that processes the t parameter might (rightfully) discard the field if it sees the Date.

[reschke]

(with two nits)