Work around an Android 9 dlopen bug on x86

Fixes: #4893

This commit fixes an issue in Android 9 (API 28) 32-bit (ONLY) dynamic
loader issue where if the same library is loaded more than once, instead
of returning a handle to the already loaded copy, `dlopen` will
reinitialize the library thus resetting its state (BSS section is
cleared, global instances of C++ classes are re-instantiated etc).

When the library is reinitalized, however, our runtime is not - since
the Java `Runtime.initialize`, which ends up in our native code,
is *not* called because the library load is not part of the process
startup, when `Runtime.initialize` is invoked.  This in turn leads to
situation when certain class members initialized in `Runtime.initialize`
and assumed to be correct throughout the application life are, in fact,
invalid and lead to crashes.  In the instance of issue #4893, the
problem was that the `MonodroidRuntime::Class_getName` field was not
initialized to point to the `Java.Lang.Class.getName` Java method and
that, in turn, led to Xamarin.Android not being able to look up Java
types during application execution time.

`libmonodroid.so` is reloaded whenever Mono tries to resolve an
`__Internal` p/invoke (via a call to Xamarin.Android's
`monodroid_dlopen` fallback handler) - these p/invokes are assumed to
live in `libmonodroid` so we need to return a handle to it, so that Mono
can subsequently use `dlsym` to look up the required symbol.  Given the
situation described in the previous paragraph, we need a way to not
reload `libmonodroid` or do something else to avoid it being
reinitialized.

Since there's no portable way to check whether a DSO is already
loaded we need to resort to "something else", implemented in this
commit.  We move all the functions used in `__Internal` p/invokes to a
separate DSO which can be reloaded & reinitialized over and over again
without shredding our global state.  This is done by way of introducing
a C++ interface class, `MonoAndroidExternalAPI` which is passed to the
new DSO on load time, thus giving it a way to invoke all the necessary
internal Xamarin.Android functions the `__Internal` p/invokes need in
order to work as expected.

Author: grendello

build-tools/installers/create-installers.targets

@@ -159,6 +159,7 @@
     <_MSBuildFiles Include="$(MSBuildSrcDir)\LayoutBinding.cs" />
     <_MSBuildFiles Include="@(AndroidSupportedTargetJitAbi->'$(MSBuildSrcDir)\lib\%(Identity)\libmono-android.debug.so')"       Condition=" '$(PackageId)' != 'Microsoft.Android.Sdk' " />
     <_MSBuildFiles Include="@(AndroidSupportedTargetJitAbi->'$(MSBuildSrcDir)\lib\%(Identity)\libmono-android.release.so')"     Condition=" '$(PackageId)' != 'Microsoft.Android.Sdk' " />
+    <_MSBuildFiles Include="@(AndroidSupportedTargetJitAbi->'$(MSBuildSrcDir)\lib\%(Identity)\libmonodroid-mono-api.so')"       Condition=" '$(PackageId)' != 'Microsoft.Android.Sdk' " />
     <_MSBuildFiles Include="@(AndroidSupportedTargetJitAbi->'$(MSBuildSrcDir)\lib\%(Identity)\libmono-btls-shared.so')"         Condition=" '$(PackageId)' != 'Microsoft.Android.Sdk' " />
     <_MSBuildFiles Include="@(AndroidSupportedTargetJitAbi->'$(MSBuildSrcDir)\lib\%(Identity)\libmono-profiler-aot.so')"        Condition=" '$(PackageId)' != 'Microsoft.Android.Sdk' " />
     <_MSBuildFiles Include="@(AndroidSupportedTargetJitAbi->'$(MSBuildSrcDir)\lib\%(Identity)\libmono-profiler-log.so')"        Condition=" '$(PackageId)' != 'Microsoft.Android.Sdk' " />

samples/HelloWorld/Properties/AndroidManifest.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="1" android:versionName="1.0" package="com.xamarin.android.helloworld">
 	<uses-sdk android:minSdkVersion="21" />
-	<application android:allowBackup="true" android:icon="@mipmap/icon" android:label="@string/app_name">
+	<application android:allowBackup="true" android:icon="@mipmap/icon" android:label="@string/app_name" android:extractNativeLibs="false">
 	</application>
 </manifest>

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1958,6 +1958,7 @@ because xbuild doesn't support framework reference assemblies.
     <_AndroidNativeLibraryForFastDev Condition=" '$(_InstantRunEnabled)' == 'True' And '$(AndroidUseDebugRuntime)' == 'True' " Include="%(_TargetLibDir.Identity)\libxamarin-debug-app-helper.so" />
     <FrameworkNativeLibrary Include="%(_TargetLibDir.Identity)\libmono-android.debug.so" Condition=" '$(AndroidIncludeDebugSymbols)' == 'True' " ArchiveFileName="libmonodroid.so" />
     <FrameworkNativeLibrary Include="%(_TargetLibDir.Identity)\libmono-android.release.so" Condition=" '$(AndroidIncludeDebugSymbols)' != 'True' " ArchiveFileName="libmonodroid.so" />
+    <FrameworkNativeLibrary Include="%(_TargetLibInterpreterDir.Identity)\libmonodroid-mono-api.so"  />
     <FrameworkNativeLibrary Include="%(_TargetLibInterpreterDir.Identity)\libmono-btls-shared.so"  />
     <FrameworkNativeLibrary Include="%(_TargetLibInterpreterDir.Identity)\libmono-profiler-aot.so" Condition=" @(_EmbedProfilers->Count()) > 0 and (@(_EmbedProfilers->AnyHaveMetadataValue('Identity', 'aot')) or @(_EmbedProfilers->AnyHaveMetadataValue('Identity', 'all'))) " />
     <FrameworkNativeLibrary Include="%(_TargetLibInterpreterDir.Identity)\libmono-profiler-log.so" Condition=" '$(AndroidIncludeDebugSymbols)' == 'True' or (@(_EmbedProfilers->Count()) > 0 and (@(_EmbedProfilers->AnyHaveMetadataValue('Identity', 'log')) or @(_EmbedProfilers->AnyHaveMetadataValue('Identity', 'all')))) " />

src/monodroid/CMakeLists.txt

@@ -220,6 +220,8 @@ else()
   endif()
 endif()
 
+string(STRIP "${EXTRA_LINKER_FLAGS}" EXTRA_LINKER_FLAGS)
+
 if (ENABLE_TIMING)
   add_definitions("-DMONODROID_TIMING=1")
 endif()
@@ -277,11 +279,10 @@ set(MONODROID_SOURCES
   ${SOURCES_DIR}/debug-constants.cc
   ${SOURCES_DIR}/embedded-assemblies.cc
   ${SOURCES_DIR}/embedded-assemblies-zip.cc
-  ${SOURCES_DIR}/external-api.cc
   ${SOURCES_DIR}/globals.cc
-  ${SOURCES_DIR}/jni.c
   ${SOURCES_DIR}/logger.cc
   ${SOURCES_DIR}/monodroid-glue.cc
+  ${SOURCES_DIR}/monodroid-external-api.cc
   ${SOURCES_DIR}/osbridge.cc
   ${SOURCES_DIR}/shared-constants.cc
   ${SOURCES_DIR}/timezones.cc
@@ -291,6 +292,12 @@ set(MONODROID_SOURCES
   ${JAVA_INTEROP_SRC_PATH}/java-interop-util.cc
   )
 
+set(MONODROID_MONO_API_SOURCES
+  ${SOURCES_DIR}/jni.c
+  ${SOURCES_DIR}/external-api.cc
+  ${JAVA_INTEROP_SRC_PATH}/java-interop-util.cc
+  )
+
 if(ENABLE_NDK)
   set(MONODROID_SOURCES
 	${MONODROID_SOURCES}
@@ -306,6 +313,15 @@ if(UNIX)
     )
 endif()
 
+add_library(monodroid-mono-api SHARED ${MONODROID_MONO_API_SOURCES})
+target_link_libraries(monodroid-mono-api ${EXTRA_LINKER_FLAGS})
+set_target_properties(
+  monodroid-mono-api
+  PROPERTIES
+  COMPILE_FLAGS -fvisibility=default
+  LINK_FLAGS -fvisibility=default
+  )
+
 set(XAMARIN_APP_STUB_SOURCES ${SOURCES_DIR}/application_dso_stub.cc)
 add_library(xamarin-app SHARED ${XAMARIN_APP_STUB_SOURCES})
 if (ENABLE_NDK)

src/monodroid/jni/cpu-arch-detect.cc

@@ -225,7 +225,7 @@ get_running_on_cpu (unsigned short *running_on_cpu)
 	*running_on_cpu = CPU_KIND_UNKNOWN;
 }
 
-MONO_API void
+void
 _monodroid_detect_cpu_and_architecture (unsigned short *built_for_cpu, unsigned short *running_on_cpu, unsigned char *is64bit)
 {
 	assert (built_for_cpu);

src/monodroid/jni/embedded-assemblies.cc

@@ -597,7 +597,7 @@ EmbeddedAssemblies::md_mmap_apk_file (int fd, uint32_t offset, uint32_t size, co
 	md_mmap_info file_info;
 	md_mmap_info mmap_info;
 
-	size_t pageSize       = static_cast<size_t>(monodroid_getpagesize());
+	size_t pageSize       = static_cast<size_t>(utils.monodroid_getpagesize ());
 	uint32_t offsetFromPage  = static_cast<uint32_t>(offset % pageSize);
 	uint32_t offsetPage      = offset - offsetFromPage;
 	uint32_t offsetSize      = size + offsetFromPage;