If the GN variable use_icu is set and skia_use_system_icu is unset, then the build system will build ICU from the source. The repository in ../externals/icu is a mirror of the Unicode Consortium's repository.
ICU requires a platform-independent data file, which it does not provide in its repository. The data file can be built by first building the ICU tools to process the provided source data, but this is a giant hassle for building ICU in a cross-platform way (which might necessitate building ICU twice).
icu/BUILD.gn has two action build rules: get_data and assemble_data. get_data downloads the icu.dat file if it is not already downloaded. The Google Cloud Storage location of the file is content-addressed with the icu_dat_md5 GN variable found in icu.gni.
assemble_data will produce either a C++ source file (for our WASM build), an object file (for our Windows builds), or a assembly file (for everything else). These files will contain a single exported symbol, icudtNN_dat which points at a block of data containing the contents of the icu.dat file.
This build also provides the SkLoadICU function, which must be called on Windows for some reason.
When we update the ICU branch, we will use the update_icu.sh script to update the icu.gni file and produce a new .dat file for upload.
Edit DEPS file.
Run update script:
third_party/icu/update_icu.sh
Upload new data file. DATA_FILE and GS_URL will be provided by the output of the previous step.
gsutil cp DATA_FILE GS_URL
Commit:
git add DEPS third_party/icu/icu.gni git commit -m 'DEPS: update ICU' git push origin @:refs/for/master bin/sysopen https://review.skia.org/$(bin/gerrit-number @)