Standalone libprotobuf.so cross compiled with Android NDK

[milinddeore]

My build platform is ubuntu 22.4 and I have set the build configuration as suggested in the document, but it always pick the linux toolchain.

Android NDK: r25c
picked latest clone of protobuf on 25-oct-2024

My configuration looks as below:

diff --git a/BUILD.bazel b/BUILD.bazel
index 97d1ef84b..78bd10e40 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -647,3 +647,22 @@ filegroup(
     srcs = glob(["**/*.bzl"]),
     visibility = ["//visibility:public"],
 )
+
+
+platform(
+    name = "android_arm64",
+    constraint_values = [
+        "@platforms//os:android",
+        "@platforms//cpu:arm64",
+    ],
+)
+
+toolchain(
+    name = "my_android_toolchain",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
+    toolchain = "@androidndk//:toolchain",
+    target_compatible_with = [
+        "@platforms//cpu:arm64",
+        "@platforms//os:android",
+    ],
+)
diff --git a/WORKSPACE b/WORKSPACE
index f84687db9..783310f46 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -1,5 +1,11 @@
 workspace(name = "com_google_protobuf")

+android_ndk_repository(
+    name = "androidndk",
+    path = "/home/ved/PROJECT/android-ndk-r21e",  # Path to NDK installation
+    api_level = 21,  # Minimum API level you are targeting
+)
+
 # An explicit self-reference to work around changes in Bazel 7.0
 # See https://github.com/bazelbuild/bazel/issues/19973#issuecomment-1787814450
 # buildifier: disable=duplicated-name
@@ -275,3 +281,4 @@ load("@rules_buf//buf:repositories.bzl", "rules_buf_dependencies", "rules_buf_to
 rules_buf_dependencies()

 rules_buf_toolchains(version = "v1.32.1")

Even at in the build command, explicitly specifying the toolchain but it never pick it.

$ bazel build //src/google/protobuf:protobuf --android_platforms=@androidndk//:android_arm64 --crosstool_top=@androidndk//:toolchain

Following commands returns empty result:

$ bazel query 'kind(android_platform, @androidndk//...)'
Starting local Bazel server and connecting to it...
DEBUG: /home/ved/.cache/bazel/_bazel_ved/59b4c1376de3e22977e2bd4801fb92a1/external/rules_ruby/ruby/private/toolchains/ruby_runtime.bzl:297:14: WARNING: no system ruby available, builds against system ruby will fail
DEBUG: /home/ved/.cache/bazel/_bazel_ved/59b4c1376de3e22977e2bd4801fb92a1/external/system_ruby/bundle.bzl:3:10: WARNING: no system ruby found for bundle
DEBUG: Rule 'com_google_protobuf_v25.0' indicated that a canonical reproducible form can be obtained by modifying arguments integrity = "sha256-e+7ZxRHWMs/3wirACU3Xcg5VAVMDnV2n4Fm8zrSIR0o="
DEBUG: Repository com_google_protobuf_v25.0 instantiated at:
  /home/ved/PROJECT/protobuf/WORKSPACE:242:13: in <toplevel>
Repository rule http_archive defined at:
  /home/ved/.cache/bazel/_bazel_ved/59b4c1376de3e22977e2bd4801fb92a1/external/bazel_tools/tools/build_defs/repo/http.bzl:387:31: in <toplevel>
INFO: Empty results

The resultant libraries is compiled using linux toolchain:

bazel build //src/google/protobuf:protobuf --android_platforms=@androidndk//:android_arm64 --crosstool_top=@androidndk//:toolchain
Starting local Bazel server and connecting to it...
DEBUG: /home/ved/.cache/bazel/_bazel_ved/59b4c1376de3e22977e2bd4801fb92a1/external/rules_ruby/ruby/private/toolchains/ruby_runtime.bzl:297:14: WARNING: no system ruby available, builds against system ruby will fail
DEBUG: /home/ved/.cache/bazel/_bazel_ved/59b4c1376de3e22977e2bd4801fb92a1/external/system_ruby/bundle.bzl:3:10: WARNING: no system ruby found for bundle
DEBUG: Rule 'com_google_protobuf_v25.0' indicated that a canonical reproducible form can be obtained by modifying arguments integrity = "sha256-e+7ZxRHWMs/3wirACU3Xcg5VAVMDnV2n4Fm8zrSIR0o="
DEBUG: Repository com_google_protobuf_v25.0 instantiated at:
  /home/ved/PROJECT/protobuf/WORKSPACE:242:13: in <toplevel>
Repository rule http_archive defined at:
  /home/ved/.cache/bazel/_bazel_ved/59b4c1376de3e22977e2bd4801fb92a1/external/bazel_tools/tools/build_defs/repo/http.bzl:387:31: in <toplevel>
DEBUG: Rule 'rules_buf_toolchains' indicated that a canonical reproducible form can be obtained by modifying arguments sha256 = "f7e50b227bf171158ff4a80d22f274d0195487b618e0c4a57b3b0741a52453c2"
DEBUG: Repository rules_buf_toolchains instantiated at:
  /home/ved/PROJECT/protobuf/WORKSPACE:283:21: in <toplevel>
  /home/ved/.cache/bazel/_bazel_ved/59b4c1376de3e22977e2bd4801fb92a1/external/rules_buf/buf/internal/toolchain.bzl:197:26: in rules_buf_toolchains
Repository rule buf_download_releases defined at:
  /home/ved/.cache/bazel/_bazel_ved/59b4c1376de3e22977e2bd4801fb92a1/external/rules_buf/buf/internal/toolchain.bzl:170:40: in <toplevel>
INFO: Analyzed target //src/google/protobuf:protobuf (159 packages loaded, 1071 targets configured).
INFO: Found 1 target...
Target //src/google/protobuf:protobuf up-to-date:
  bazel-bin/src/google/protobuf/libprotobuf.a
  bazel-bin/src/google/protobuf/libprotobuf.so
INFO: Elapsed time: 119.992s, Critical Path: 24.67s
INFO: 284 processes: 93 internal, 191 linux-sandbox.
INFO: Build completed successfully, 284 total actions

ved@ved-Lenovo:~/PROJECT/protobuf$ file bazel-bin/src/google/protobuf/libprotobuf.so
bazel-bin/src/google/protobuf/libprotobuf.so: ELF 64-bit LSB shared object, x86-64, version 1 (GNU/Linux), dynamically linked, BuildID[sha1]=72c40c7ab934dc05d7bf3368d196c864a345bb8e, not stripped