Skylark implementations of http_archive, new_http_archive, and http_file rules.

RELNOTES: Skylark implementations of http_archive, new_http_archive, and http_file.

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+# Copyright 2016 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Rules for downloading files and archives over HTTP.
+
+### Setup
+
+To use these rules, load them in your `WORKSPACE` file as follows:
+
+```python
+load(
+    "@bazel_tools//tools/build_defs/repo:http.bzl",
+    "http_archive",
+    "http_file",
+)
+```
+
+These rules are improved versions of the native http rules and will eventually
+replace the native rules.
+"""
+
+
+def _http_archive_impl(ctx):
+  """Implementation of the http_archive rule."""
+  if ctx.attr.build_file and ctx.attr.build_file_content:
+    ctx.fail("Only one of build_file and build_file_content can be provided.")
+
+  # TODO(dzc,jart): Implement fallback URLs when available.
+  if len(ctx.attr.urls) > 1:
+    ctx.fail("Multiple urls are not yet supported.")
+  url = ctx.attr.urls[0]
+  ctx.download_and_extract(url, "", ctx.attr.sha256, ctx.attr.type,
+                           ctx.attr.strip_prefix)
+  ctx.file("WORKSPACE", "workspace(name = \"{name}\")\n".format(name=ctx.name))
+  if ctx.attr.build_file:
+    ctx.symlink(ctx.attr.build_file, "BUILD")
+  else:
+    ctx.file("BUILD", ctx.attr.build_file_content)
+
+
+_HTTP_FILE_BUILD = """
+package(default_visibility = ["//visibility:public"])
+
+filegroup(
+    name = "file",
+    srcs = ["downloaded"],
+)
+"""
+
+def _http_file_impl(ctx):
+  """Implementation of the http_file rule."""
+  # TODO(dzc,jart): Implement fallback URLs when available.
+  if len(ctx.attr.urls) > 1:
+    ctx.fail("Multiple urls are not yet supported.")
+  url = ctx.attr.urls[0]
+  ctx.download(url, "file/downloaded", ctx.attr.sha256, ctx.attr.executable)
+  ctx.file("WORKSPACE", "workspace(name = \"{name}\")".format(name=ctx.name))
+  ctx.file("file/BUILD", _HTTP_FILE_BUILD)
+
+
+_http_archive_attrs = {
+    "urls": attr.string_list(mandatory=True),
+    "sha256": attr.string(),
+    "strip_prefix": attr.string(),
+    "type": attr.string(),
+    "build_file": attr.label(),
+    "build_file_content": attr.string(),
+}
+
+
+http_archive = repository_rule(
+    implementation = _http_archive_impl,
+    attrs = _http_archive_attrs,
+)
+"""Downloads a Bazel repository as a compressed archive file, decompresses it,
+and makes its targets available for binding.
+
+The repository should already contain a BUILD file. If it does not, use
+`new_http_archive` instead.
+
+It supports the following file extensions: `"zip"`, `"jar"`, `"war"`,
+`"tar.gz"`, `"tgz"`, `"tar.xz"`, and `tar.bz2`.
+
+Examples:
+  Suppose the current repository contains the source code for a chat program,
+  rooted at the directory `~/chat-app`. It needs to depend on an SSL library
+  which is available from http://example.com/openssl.zip. This `.zip` file
+  contains the following directory structure:
+
+  ```
+  WORKSPACE
+  src/
+    openssl.cc
+    openssl.h
+  ```
+
+  In the local repository, the user creates a `openssl.BUILD` file which
+  contains the following target definition:
+
+  ```python
+  cc_library(
+      name = "openssl-lib",
+      srcs = ["src/openssl.cc"],
+      hdrs = ["src/openssl.h"],
+  )
+  ```
+
+  Targets in the `~/chat-app` repository can depend on this target if the
+  following lines are added to `~/chat-app/WORKSPACE`:
+
+  ```python
+  http_archive(
+      name = "my_ssl",
+      urls = ["http://example.com/openssl.zip"],
+      sha256 = "03a58ac630e59778f328af4bcc4acb4f80208ed4",
+      build_file = "openssl.BUILD",
+  )
+  ```
+
+  Then targets would specify `@my_ssl//:openssl-lib` as a dependency.
+
+Args:
+  name: A unique name for this rule.
+  build_file: The file to use as the `BUILD` file for this repository.
+
+    Either `build_file` or `build_file_content` can be specified.
+
+    This attribute is a label relative to the main workspace. The file does not
+    need to be named `BUILD`, but can be (something like `BUILD.new-repo-name`
+    may work well for distinguishing it from the repository's actual `BUILD`
+    files.
+
+  build_file_content: The content for the BUILD file for this repository.
+
+    Either `build_file` or `build_file_content` can be specified.
+  sha256: The expected SHA-256 of the file downloaded.
+
+    This must match the SHA-256 of the file downloaded. _It is a security risk
+    to omit the SHA-256 as remote files can change._ At best omitting this
+    field will make your build non-hermetic. It is optional to make development
+    easier but should be set before shipping.
+  strip_prefix: A directory prefix to strip from the extracted files.
+
+    Many archives contain a top-level directory that contains all of the useful
+    files in archive. Instead of needing to specify this prefix over and over
+    in the `build_file`, this field can be used to strip it from all of the
+    extracted files.
+
+    For example, suppose you are using `foo-lib-latest.zip`, which contains the
+    directory `foo-lib-1.2.3/` under which there is a `WORKSPACE` file and are
+    `src/`, `lib/`, and `test/` directories that contain the actual code you
+    wish to build. Specify `strip_prefix = "foo-lib-1.2.3"` to use the
+    `foo-lib-1.2.3` directory as your top-level directory.
+
+    Note that if there are files outside of this directory, they will be
+    discarded and inaccessible (e.g., a top-level license file). This includes
+    files/directories that start with the prefix but are not in the directory
+    (e.g., `foo-lib-1.2.3.release-notes`). If the specified prefix does not
+    match a directory in the archive, Bazel will return an error.
+  type: The archive type of the downloaded file.
+
+    By default, the archive type is determined from the file extension of the
+    URL. If the file has no extension, you can explicitly specify one of the
+    following: `"zip"`, `"jar"`, `"war"`, `"tar.gz"`, `"tgz"`, `"tar.xz"`,
+    or `tar.bz2`.
+  urls: A URL to a file that will be made available to Bazel.
+
+    This must be an file, http or https URL. Redirections are followed.
+    Authentication is not supported.
+"""
+
+
+http_file = repository_rule(
+    implementation = _http_file_impl,
+    attrs = {
+        "executable": attr.bool(),
+        "sha256": attr.string(),
+        "urls": attr.string_list(mandatory=True),
+    },
+)
+"""Downloads a file from a URL and makes it available to be used as a file
+group.
+
+Examples:
+  Suppose you need to have a debian package for your custom rules. This package
+  is available from http://example.com/package.deb. Then you can add to your
+  WORKSPACE file:
+
+  ```python
+  http_file(
+      name = "my_deb",
+      urls = ["http://example.com/package.deb"],
+      sha256 = "03a58ac630e59778f328af4bcc4acb4f80208ed4",
+  )
+  ```
+
+  Targets would specify `@my_deb//file` as a dependency to depend on this file.
+
+Args:
+  name: A unique name for this rule.
+  executable: If the downloaded file should be made executable. Defaults to
+    False.
+  sha256: The expected SHA-256 of the file downloaded.
+
+    This must match the SHA-256 of the file downloaded. _It is a security risk
+    to omit the SHA-256 as remote files can change._ At best omitting this
+    field will make your build non-hermetic. It is optional to make development
+    easier but should be set before shipping.
+  urls: A URL to a file that will be made available to Bazel.
+
+    This must be an file, http, or https URL. Redirections are followed.
+    Authentication is not supported.
+"""