Enable proguarded Android builds with --experimental_desugar_java8_libs in Bazel

RELNOTES: Bazel supports including select Java 8 APIs into Android apps targeting pre-Nougat Android devices with --experimental_desugar_java8_libs

PiperOrigin-RevId: 196833987

3 files changed, 120 insertions, 7 deletions

diff --git a/tools/android/BUILD.tools b/tools/android/BUILD.tools
index 592e6dc966..4828323ad7 100644
--- a/tools/android/BUILD.tools
+++ b/tools/android/BUILD.tools
@@ -149,16 +149,21 @@ genrule(
 sh_binary(
     name = "build_java8_legacy_dex",
     srcs = ["build_java8_legacy_dex.sh"],
-    data = [":java8_legacy_dex"],
+    data = [
+        "minify_java8_legacy_libs.cfg",
+        ":desugared_java8_legacy_libs",
+        ":dexer",
+        "@bazel_tools//src/tools/android/java/com/google/devtools/build/android/desugar/scan:KeepScanner",
+        "@bazel_tools//third_party/java/proguard",
+    ],
     visibility = ["//visibility:public"],
 )
 
 genrule(
     name = "java8_legacy_dex",
-    srcs = ["desugared_java8_legacy_libs"],
     outs = ["java8_legacy.dex.zip"],
-    cmd = """$(location :dexer) --dex "--output=$@" $<""",
-    tools = [":dexer"],
+    cmd = """$(location :build_java8_legacy_dex) --output $@""",
+    tools = [":build_java8_legacy_dex"],
     visibility = ["//visibility:public"],
 )
 
diff --git a/tools/android/build_java8_legacy_dex.sh b/tools/android/build_java8_legacy_dex.sh
index 7ceaea8a4c..beb2e1d508 100644
--- a/tools/android/build_java8_legacy_dex.sh
+++ b/tools/android/build_java8_legacy_dex.sh
@@ -5,7 +5,7 @@
 # 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
+#    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,
@@ -14,6 +14,45 @@
 
 set -eu
 
+RUNFILES="${RUNFILES:-$0.runfiles}"
+RUNFILES_MANIFEST_FILE="${RUNFILES_MANIFEST_FILE:-$RUNFILES/MANIFEST}"
+
+IS_WINDOWS=false
+case "$(uname | tr [:upper:] [:lower:])" in
+msys*|mingw*|cygwin*)
+  IS_WINDOWS=true
+esac
+
+if "$IS_WINDOWS" && ! type rlocation &> /dev/null; then
+  function rlocation() {
+    # Use 'sed' instead of 'awk', so if the absolute path ($2) has spaces, it
+    # will be printed completely.
+    local result="$(grep "$1" "${RUNFILES_MANIFEST_FILE}" | head -1)"
+    # If the entry has a space, it is a mapping from a runfiles-path to absolute
+    # path, otherwise it resolves to itself.
+    echo "$result" | grep -q " " \
+        && echo "$result" | sed 's/^[^ ]* //' \
+        || echo "$result"
+  }
+fi
+
+# Find helper artifacts:
+#   Windows (in MANIFEST):  <repository_name>/<path/to>/file
+#   Linux/MacOS (symlink):  ${RUNFILES}/<repository_name>/<path/to>/file
+if "$IS_WINDOWS"; then
+  INPUT="$(rlocation "[^/]*/tools/android/desugared_java8_legacy_libs.jar")"
+  CONFIG="$(rlocation "[^/]*/tools/android/minify_java8_legacy_libs.cfg")"
+  SCAN="$(rlocation "[^/]*/src/tools/android/java/com/google/devtools/build/android/desugar/scan/KeepScanner")"
+  PG="$(rlocation "[^/]*/third_party/java/proguard/proguard")"
+  DEXER="$(rlocation "[^/]*/tools/android/dexer")"
+else
+  INPUT="$(find "${RUNFILES}" -path "*/tools/android/desugared_java8_legacy_libs.jar" | head -1)"
+  CONFIG="$(find "${RUNFILES}" -path "*/tools/android/minify_java8_legacy_libs.cfg" | head -1)"
+  SCAN="$(find "${RUNFILES}" -path "*/src/tools/android/java/com/google/devtools/build/android/desugar/scan/KeepScanner" | head -1)"
+  PG="$(find "${RUNFILES}" -path "*/third_party/java/proguard/proguard" | head -1)"
+  DEXER="$(find "${RUNFILES}" -path "*/tools/android/dexer" | head -1)"
+fi
+
 android_jar=
 binary_jar=
 dest=
@@ -30,5 +69,29 @@ while [[ "$#" -gt 0 ]]; do
   esac
 done
 
-# TODO(b/77339644): Support minification
-cp "$(dirname "$0")/java8_legacy.dex.zip" "${dest}"
+todex="${INPUT}"
+if [[ -n "${binary_jar}" ]]; then
+  tmpdir=$(mktemp -d)
+  trap "rm -rf ${tmpdir}" EXIT
+
+  # Minification requested
+  # 1. compute -keep rules from binary
+  seeds="${tmpdir}/seeds.cfg"
+  "${SCAN}" \
+      --input "${binary_jar}" \
+      --classpath_entry "${todex}" \
+      --bootclasspath_entry "${android_jar}" \
+      --keep_file "${seeds}"
+
+  # 2. proguard with -keep rules generated above and standard config file.
+  # Use app's android.jar as -libraryjar.
+  todex="${tmpdir}/proguarded.jar"
+  "${PG}" \
+      -injars "${INPUT}" \
+      -outjars "${todex}" \
+      -libraryjars "${android_jar}" \
+      "@${CONFIG}" \
+      "@${seeds}"
+fi
+# Convert .jar file to .dex
+"${DEXER}" --dex "--output=${dest}" "${todex}"
diff --git a/tools/android/minify_java8_legacy_libs.cfg b/tools/android/minify_java8_legacy_libs.cfg
new file mode 100644
index 0000000000..74aa802d77
--- /dev/null
+++ b/tools/android/minify_java8_legacy_libs.cfg
@@ -0,0 +1,45 @@
+# Copyright 2018 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
+
+# Proguard configuration for :build_java8_legacy_dex
+
+# Shrink only
+-dontoptimize
+-dontobfuscate
+-dontpreverify
+
+-dontskipnonpubliclibraryclasses
+
+-keepattributes SourceFile,LineNumberTable,*Annotation*
+
+# When keeping enums, keep all their values.
+-keepclassmembers enum * {
+    public static **[] values();
+}
+
+# Keep Serializable members magically invoked by the runtime.
+-keepclassmembers class * implements java.io.Serializable {
+    static final long serialVersionUID;
+    private static final java.io.ObjectStreamField[] serialPersistentFields;
+    private void writeObject(java.io.ObjectOutputStream);
+    private void readObject(java.io.ObjectInputStream);
+    java.lang.Object writeReplace();
+    java.lang.Object readResolve();
+}
+
+# Keep Externalizable members magically invoked by the runtime.
+-keepclassmembers class * implements java.io.Externalizable {
+  <init>();  # No-arg constructor required per spec
+  public void readExternal(java.io.ObjectInput);
+  public void writeExternal(java.io.ObjectOutput);
+}