Timestamp validation is technically wrong (Java 12 test failures)

[webern]

There was a bug in DateTimeFormatter.ISO_INSTANT that was fixed in Java 12. Here is the link to that bug:
https://bugs.openjdk.java.net/browse/JDK-8166138

This change exposes certain tests in

smithy-model/src/test/java/software/amazon/smithy/model/validation/NodeValidationVisitorTest.java

which rely on the incorrect, pre Java 12 behavior of DateTimeFormatter.ISO_INSTANT
For example, it is asserted that the following string should fail timestamp validation:

2000-01-12T22:11:12+01:02

In fact, this is a valid RFC 3389 time representation.
In Java versions prior to 12, DateTimeFormatter.ISO_INSTANT incorrectly throws for this parse, and the test which relies on this incorrect behavior passes.
In Java version 12, the Java SDK bug was fixed and the test fails because the string is a valid timestamp and Java no longer throws on the parse.

Bottom line: there are some tests that fail in Java 12.

[mtdowling]

Good find! We might have to just switch to a regex based validator. RFC 3339 isn't that complex based on the ABNF (we only support date-time):

date-fullyear   = 4DIGIT
date-month      = 2DIGIT  ; 01-12
date-mday       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on
                            ; month/year
time-hour       = 2DIGIT  ; 00-23
time-minute     = 2DIGIT  ; 00-59
time-second     = 2DIGIT  ; 00-58, 00-59, 00-60 based on leap second
                            ; rules
time-secfrac    = "." 1*DIGIT
time-numoffset  = ("+" / "-") time-hour ":" time-minute
time-offset     = "Z" / time-numoffset

partial-time    = time-hour ":" time-minute ":" time-second
                    [time-secfrac]
full-date       = date-fullyear "-" date-month "-" date-mday
full-time       = partial-time time-offset

date-time       = full-date "T" full-time

[mtdowling]

Looking at this again, Smithy doesn't actually timestamp offsets, all we need to do is validate that timestamp strings end with "Z". c5aa1d7