Support multiple mirror URLs for external repos

This change improves upon 4c67807964e37cfd55bbcda4c6374fcc480bcecc.

- A urls attribute has been added to the native workspace rules, with
  the exception of maven_jar and git_repository. The Skylark repository
  API also supports multiple URLs now.

- The earlier mirrors in the list are preferred. Failover will happen
  automatically in parallel.

- The first 32kB of data is checked before choosing a mirror in order
  to evade captive portals.

- If one's Internet goes down or a download times out, then the
  download will resume automatically where it left off, provided the
  server supports RFC7233 for that particular file. Please note that
  GitHub does not support this for archive snapshots. Files should
  always be mirrored to a CDN, e.g. GCS, because they support this.

- A semaphore is now used on downloads so only 8 can happen at once.

Fixes #1814
Fixes #2131
Fixes #2008
Fixes #1968
Fixes #1717
Fixes #943
Wont fix #1194
Fixes tensorflow/tensorflow#5933
Fixes tensorflow/tensorflow#5924
Fixes tensorflow/tensorflow#5924
Fixes tensorflow/tensorflow#5432
See #1607
See #821
See tensorflow/tensorflow#5080
See tensorflow/tensorflow#5029
See tensorflow/tensorflow#4583
See tensorflow/tensorflow#4058

RELNOTES: A urls attribute has been added to repository rules to support multiple mirror URLs for reliably downloading files.

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MOS_MIGRATED_REVID=140495736
39 files changed
tree: e5f84250381e2e82020c1e730af8f7f6dceeaf88
  1. examples/
  2. scripts/
  3. site/
  4. src/
  5. third_party/
  6. tools/
  7. .gitattributes
  8. .gitignore
  9. AUTHORS
  10. BUILD
  11. CHANGELOG.md
  12. combine_distfiles.sh
  13. compile.sh
  14. CONTRIBUTING.md
  15. CONTRIBUTORS
  16. ISSUE_TEMPLATE.md
  17. LICENSE.txt
  18. README.md
  19. WORKSPACE
README.md

Bazel (Beta)

{Fast, Correct} - Choose two

Bazel is a build tool that builds code quickly and reliably. It is used to build the majority of Google‘s software, and thus it has been designed to handle build problems present in Google’s development environment, including:

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