CaptivePortalAdvanced: Fix compatibility with Android

[engycz]

Android refuses to show page with missing Content-Length header.
Prepare page data to String and send it with server.send

[d-a-v]

Approving since it solves a compatibility issue.

As said, several other strings could be PROGMEMed, I guess a(nother) general pass over all examples may fix that.

A comment would be welcome for readers to understand that in that particular HTTP case, special care with String/PROGMEM should be considered to avoid ram waste and malloc holes.

[Misiu]

Will this get merged as part of 2.5.0?

[devyte]

You're doing several String concats in a row, which leads to mem fragmentation. I suggest figuring out an approximate upper bound to the resulting String size, and using Page.reserve(blah) beforehand. Given that this is an example, I think it would help users who play with this code.

A bit of explanation at this point:
C-style strings, i.e.: "blah", that are identical are unified during the build process.into a single one that is held in RAM.
Moving multiple C-style strings to flash breaks that unification, i.e.: the build process can't unify the ones declared to be in flash. If all instances of a string are moved to flash, then the bin size increases by the string size * number of string instances, and I think plus a pointer (4 bytes) to each instance.
To avoid the problem, you have to manually unify the strings, e.g.: declare a single one globally to be in flash, then use it everywhere.

There is a certain string size threshold for deciding whether to move to flash. I think the following, but this should be verified:

• at 4 bytes (3 chars + null term) it makes no difference (move string to flash saves 4 bytes, but creates a pointer which is also 4 bytes)
• strings bigger than 4 bytes (or 3 chars) should be moved to flash, but it should be done unifying all instances.
• strings smaller than 4 bytes (or 3 chars) should not be moved to flash. It doesn't help and increases bin size.

Again, the previous should not be taken at face value, and should be verified.

[d-a-v]

Tested, working, thanks!

[Misiu]

@d-a-v what about @devyte comment? This is just a hint (to consider) or some additional changes are required?

[d-a-v]

It is a general comment on String usage.
We will address this in all examples eventually.