Make the settings attribute of Skylark maven_jar and maven_aar a label.

Fixes bazelbuild#2117.

The settings attribute allows you to optional specify a custom Maven
settings.xml file as a label. Previously, this attribute was an absolute path.
As an absolute path it cannot really be used to specify a file in the workspace
because each developer may install the workspace in a different location.
And if the settings.xml cannot be included in the workspace, the developers may
as well use one of Maven's default locations for settings.xml.

Now the attribute is a label. An example use case is:

    $ cat WORKSPACE
    load("@bazel_tools//tools/build_defs/repo:maven_rules.bzl")
    maven_jar(
        name = "guava",
        artifact = "com.google.guava:guava:19.0",
        settings = "//:my_custom_settings.xml",
    )
    $ cat BUILD
    java_library(
        srcs = glob(["**/*.java"]),
        deps = ["@guava//jar"],
    )
    $ cat my_custom_settings.xml
    <settings xmlns="http://maven.apache.org/SETTINGS/1.0.0
                     https://maven.apache.org/xsd/settings-1.0.0.xsd">
      <mirrors>
        <mirror>
          <id>planetmirror.com</id>
          <name>PlanetMirror Australia</name>
          <url>http://downloads.planetmirror.com/pub/maven2</url>
          <mirrorOf>central</mirrorOf>
        </mirror>
      </mirrors>
    </settings>

Note that `settings = "my_custom_settings.xml"` will not work as the workspace
rule will not know to look in the main workspace.

RELNOTES[INC]: Skylark maven_jar and maven_aar settings attribute is now a label so
it can be checked into your workspace.

--
PiperOrigin-RevId: 140861633
MOS_MIGRATED_REVID=140861633

src/test/shell/bazel/maven_skylark_test.sh

@@ -46,9 +46,12 @@ EOF
 }
 
 # This function takes an optional url argument: mirror. If one is passed, a
-# mirror of maven central will be set for the url.
+# mirror of maven central will be set for the url. It also creates a `BUILD`
+# file in the same directory as $local_maven_settings so that it can be used as
+# a label in WORKSPACE.
 function setup_local_maven_settings_xml() {
-  local_maven_settings_xml=$(pwd)/settings.xml
+  local_maven_settings_xml=settings.xml
+  touch $(pwd)/BUILD
   cat > $local_maven_settings_xml <<EOF
 <!-- # DO NOT EDIT: automatically generated settings.xml for maven_dependency_plugin -->
 <settings xmlns="http://maven.apache.org/SETTINGS/1.0.0"
@@ -89,7 +92,7 @@ maven_jar(
     name = 'endangered',
     artifact = "com.example.carnivore:carnivore:$version",
     sha1 = '$sha1',
-    settings = '$local_maven_settings_xml',
+    settings = '//:$local_maven_settings_xml',
 )
 
 bind(name = 'mongoose', actual = '@endangered//jar')
@@ -132,7 +135,7 @@ maven_aar(
     name = "herbivore",
     artifact = "com.example.carnivore:herbivore:1.21",
     sha1 = "$sha1",
-    settings = "$local_maven_settings_xml",
+    settings = "//:$local_maven_settings_xml",
 )
 EOF
   bazel build //java/com/app || fail "Expected build to succeed"
@@ -155,7 +158,7 @@ load("@bazel_tools//tools/build_defs/repo:maven_rules.bzl", "maven_jar")
 maven_jar(
     name = 'endangered',
     artifact = "com.example.carnivore:carnivore:$version",
-    settings = '$local_maven_settings_xml',
+    settings = '//:$local_maven_settings_xml',
 )
 
 bind(name = 'mongoose', actual = '@endangered//jar')
@@ -176,7 +179,7 @@ load("@bazel_tools//tools/build_defs/repo:maven_rules.bzl", "maven_jar")
 maven_jar(
     name = 'endangered',
     artifact = "com.example.carnivore:carnivore:$version",
-    settings = '$local_maven_settings_xml',
+    settings = '//:$local_maven_settings_xml',
 )
 
 bind(name = 'mongoose', actual = '@endangered//jar')
@@ -202,7 +205,7 @@ maven_jar(
     name = 'endangered',
     artifact = "com.example.carnivore:carnivore:1.24",
     sha1 = '$wrong_sha1',
-    settings = '$local_maven_settings_xml',
+    settings = '//:$local_maven_settings_xml',
 )
 
 bind(name = 'mongoose', actual = '@endangered//jar')
@@ -225,7 +228,7 @@ maven_jar(
     name = 'endangered',
     artifact = "com.example.carnivore:carnivore:$version",
     server = "attr_not_implemented",
-    settings = "$local_maven_settings_xml",
+    settings = "//:$local_maven_settings_xml",
 )
 
 bind(name = 'mongoose', actual = '@endangered//jar')

tools/build_defs/repo/maven_rules.bzl

@@ -181,8 +181,9 @@ def _file_exists(ctx, filename):
 def _mvn_download(ctx, paths, fully_qualified_name):
   # If a custom settings file exists, we'll use that. If not, Maven will use the default settings.
   mvn_flags = ""
-  if hasattr(ctx.attr, "settings") and ctx.attr.settings != "":
-    mvn_flags += "-s %s " % ctx.attr.settings
+  if hasattr(ctx.attr, "settings") and ctx.attr.settings != None:
+    ctx.symlink(ctx.attr.settings, "settings.xml")
+    mvn_flags += "-s %s " % "settings.xml"
 
   # dependency:get step. Downloads the artifact into the local repository.
   mvn_get = MVN_PLUGIN + ":get"
@@ -262,7 +263,7 @@ _common_maven_rule_attrs = {
         mandatory = True,
     ),
     "sha1": attr.string(default = ""),
-    "settings": attr.string(default = "")
+    "settings": attr.label(default = None)
 }
 
 def _maven_jar_impl(ctx):