ruby: Fix object cache lookups on 32-bit platforms (#13494)

#13204 refactored the Ruby object cache to use a key of `LL2NUM(key_val)` instead of `LL2NUM(key_val >> 2)`. On 32-bit systems, `LL2NUM(key_val)` returns inconsistent results because a large value has to be stored as a Bignum on the heap. This causes cache lookups to fail.

This commit restores the previous behavior of using `ObjectCache_GetKey`, which discards the lower 2 bits, which are zero. This enables a key to be stored as a Fixnum on both 32 and 64-bit platforms.

As https://patshaughnessy.net/2014/1/9/how-big-is-a-bignum describes, a Fixnum uses:

* 1 bit for the `FIXNUM_FLAG`.
* 1 bit for the sign flag.

Therefore the largest possible Fixnum value on a 64-bit value is 4611686018427387903 (2^62 - 1). On a 32-bit system, the largest value  is 1073741823 (2^30 - 1).

For example, a possible VALUE pointer address on a 32-bit system:

0xff5b4af8 => 4284173048

Dropping the lower 2 bits makes up for the loss of range to these flags. In the example above, we see that shifting by 2 turns the value into a 30-bit number, which can be represented as a Fixnum:

(0xff5b4af8 >> 2) => 1071043262

This bug can also manifest on a 64-bit system if the upper bits are 0xff.

Closes #13481

Closes #13494

COPYBARA_INTEGRATE_REVIEW=#13494 from stanhu:sh-fix-ruby-protobuf-32bit d63122a
FUTURE_COPYBARA_INTEGRATE_REVIEW=#13494 from stanhu:sh-fix-ruby-protobuf-32bit d63122a
PiperOrigin-RevId: 557189479

.github/workflows/test_ruby.yml

@@ -45,6 +45,39 @@ jobs:
           bazel-cache: ruby_linux/${{ matrix.ruby }}_${{ matrix.bazel }}
           bazel: test //ruby/... //ruby/tests:ruby_version --test_env=KOKORO_RUBY_VERSION --test_env=BAZEL=true ${{ matrix.ffi == 'FFI' && '--//ruby:ffi=enabled --test_env=PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION=FFI' || '' }}
 
+  linux-32bit:
+    name: Linux 32-bit
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout pending changes
+        uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3.3.0
+        with:
+          ref: ${{ inputs.safe-checkout }}
+          submodules: recursive
+
+      - name: Cross compile protoc for i386
+        id: cross-compile
+        uses: protocolbuffers/protobuf-ci/cross-compile-protoc@v2
+        with:
+          image: us-docker.pkg.dev/protobuf-build/containers/common/linux/bazel:6.3.0-91a0ac83e968068672bc6001a4d474cfd9a50f1d
+          credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
+          architecture: linux-i386
+
+      - name: Run tests
+        uses: protocolbuffers/protobuf-ci/docker@v2
+        with:
+          image: i386/ruby:2.7.3-buster
+          skip-staleness-check: true
+          credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
+          command: >-
+            /bin/bash -cex '
+            gem install bundler;
+            cd /workspace/ruby;
+            bundle;
+            PROTOC=/workspace/${{ steps.cross-compile.outputs.protoc }} rake;
+            rake clobber_package gem;
+            PROTOC=/workspace/${{ steps.cross-compile.outputs.protoc }} rake test'
+
   linux-aarch64:
     name: Linux aarch64
     runs-on: ubuntu-latest

ruby/ext/google/protobuf_c/protobuf.c

@@ -276,17 +276,25 @@ static void ObjectCache_Init(VALUE protobuf) {
   rb_const_set(protobuf, rb_intern("SIZEOF_VALUE"), INT2NUM(SIZEOF_VALUE));
 }
 
-VALUE ObjectCache_TryAdd(const void *key, VALUE val) {
+static VALUE ObjectCache_GetKey(const void *key) {
   VALUE key_val = (VALUE)key;
   PBRUBY_ASSERT((key_val & 3) == 0);
-  return rb_funcall(weak_obj_cache, item_try_add, 2, LL2NUM(key_val), val);
+  // Ensure the key can be stored as a Fixnum since 1 bit is needed for
+  // FIXNUM_FLAG and 1 bit is needed for the sign bit.
+  VALUE new_key = LL2NUM(key_val >> 2);
+  PBRUBY_ASSERT(FIXNUM_P(new_key));
+  return new_key;
+}
+
+VALUE ObjectCache_TryAdd(const void *key, VALUE val) {
+  VALUE key_val = ObjectCache_GetKey(key);
+  return rb_funcall(weak_obj_cache, item_try_add, 2, key_val, val);
 }
 
 // Returns the cached object for this key, if any. Otherwise returns Qnil.
 VALUE ObjectCache_Get(const void *key) {
-  VALUE key_val = (VALUE)key;
-  PBRUBY_ASSERT((key_val & 3) == 0);
-  return rb_funcall(weak_obj_cache, item_get, 1, LL2NUM(key_val));
+  VALUE key_val = ObjectCache_GetKey(key);
+  return rb_funcall(weak_obj_cache, item_get, 1, key_val);
 }
 
 /*