1370779 - stylo: percentages serialized with some float "rounding errors"

Status: RESOLVED FIXED

(Core :: CSS Parsing and Computation, defect, P2)

Description

[Boris Zbarsky [:bzbarsky]]

Attachment: baz.html

Consider the attached testcase.  In Gecko, the two percentages on each line are the same.  In stylo, the following lines have them different:

15% 15.000001%;
27% 27.000002%;
30% 30.000002%;
53% 52.999996%;
54% 54.000004%;
59% 58.999996%;
60% 60.000004%;

This was caught by some mochitests.  Specifically, this causes dom/base/test/test_bug338679.html to fail.  Good thing it happened to use "15" as the value...

If I had to guess, servo is taking floats, converting to double, then serializing the double.

Comment 1

[Boris Zbarsky [:bzbarsky]]

Actually, looks like Gecko just asks the float-to-string machinery to produce 6 decimal digits of precision, rounding the last one; this is the general behavior of nsTSubstring_CharT::AppendFloat (the version that takes 32-bit floats).  The errors above are all in the 8th digit, so they get rounded away.

Comment 2

[Boris Zbarsky [:bzbarsky]]

Filed https://github.com/servo/servo/issues/17205

Comment 3

[Xidorn Quan [:xidorn] UTC+11]

A potential fix at https://github.com/servo/rust-cssparser/pull/173

Comment 4

[Xidorn Quan [:xidorn] UTC+11]

This seems to affect several tests, so bump to P2.

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/a203d42085ea
Bump version of cssparser.

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/04080bc695e3
followup - Bump version of cssparser again to 0.19.4.

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/e16dba457260
followup 2 - Downgrade cssparser to 0.19.2.

Comment 8

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/6b0ed8c31532
followup 3 - Update several web-platform test expectation.

Comment 9

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

https://hg.mozilla.org/mozilla-central/rev/a203d42085ea
https://hg.mozilla.org/mozilla-central/rev/04080bc695e3
https://hg.mozilla.org/mozilla-central/rev/e16dba457260
https://hg.mozilla.org/mozilla-central/rev/6b0ed8c31532