boost_fetch very slow on repeated builds

[HDembinski]

I started to re-implement CMake Support in Boost::histogram, using mincmake and Boost conforming CMake configuration. Histogram depends directly on 8 Boost libraries, but indirectly on 29. I currently use boost_fetch, but it takes very long to fetch 29 libraries every time. Is there a way to speed this up? I think once a dependency was fetched, it shouldn't be re-fetched unless the user deletes the dependency directory.

[pdimov]

At the moment I use the form of FetchContent_Populate which re-downloads each time. I should probably switch to using FetchContent_Declare and then FetchContent_Populate(name), or perhaps FetchContent_MakeAvailable.

[pdimov]

I tried splitting the FetchContent_Populate call as per the documentation of FetchContent, but each cmake reconfiguration still redownloads the libraries (w/ CMake 3.12).

[HDembinski]

Thank you, I am happy with the version in develop.

It is like you say, but the reconfigure is still faster for some reason that is not entirely clear to me.

The version with _Declare and _Populate has the advantage that FETCHCONTENT_FULLY_DISCONNECTED and FETCHCONTENT_UPDATES_DISCONNECTED are respected:

https://cmake.org/cmake/help/v3.11/module/FetchContent.html?highlight=fetchcontent

[FetchContent_Populate with options] has a number of key differences to that where only <name> is provided:

• All required population details are assumed to have been provided directly in the call to FetchContent_Populate(). Any saved details for are ignored.
• No check is made for whether content for has already been populated.
• No global property is set to record that the population has occurred.
• No global properties record the source or binary directories used for the populated content.
• The FETCHCONTENT_FULLY_DISCONNECTED and FETCHCONTENT_UPDATES_DISCONNECTED cache variables are ignored.