feat: add external process runtime (#1799)

app/src/main/cpp/CMakeLists.txt

@@ -0,0 +1,38 @@
+
+# For more information about using CMake with Android Studio, read the
+# documentation: https://d.android.com/studio/projects/add-native-code.html.
+# For more examples on how to use CMake, see https://github.com/android/ndk-samples.
+
+# Sets the minimum CMake version required for this project.
+cmake_minimum_required(VERSION 3.22.1)
+
+# Declares the project name. The project name can be accessed via ${ PROJECT_NAME},
+# Since this is the top level CMakeLists.txt, the project name is also accessible
+# with ${CMAKE_PROJECT_NAME} (both CMake variables are in-sync within the top level
+# build script scope).
+project("prop_override")
+
+# Creates and names a library, sets it as either STATIC
+# or SHARED, and provides the relative paths to its source code.
+# You can define multiple libraries, and CMake builds them for you.
+# Gradle automatically packages shared libraries with your APK.
+#
+# In this top level CMakeLists.txt, ${CMAKE_PROJECT_NAME} is used to define
+# the target library name; in the sub-module's CMakeLists.txt, ${PROJECT_NAME}
+# is preferred for the same purpose.
+#
+# In order to load a library into your app from Java/Kotlin, you must call
+# System.loadLibrary() and pass the name of the library defined here;
+# for GameActivity/NativeActivity derived applications, the same library name must be
+# used in the AndroidManifest.xml file.
+add_library(${CMAKE_PROJECT_NAME} SHARED
+    # List C/C++ source files with relative paths to this CMakeLists.txt.
+        prop_override.cpp)
+
+# Specifies libraries CMake should link to your target library. You
+# can link libraries from various origins, such as libraries defined in this
+# build script, prebuilt third-party libraries, or Android system libraries.
+target_link_libraries(${CMAKE_PROJECT_NAME}
+    # List libraries link to the target library
+    android
+    log)

app/src/main/cpp/prop_override.cpp

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+// Library for overriding Android system properties via environment variables.
+//
+// Usage: LD_PRELOAD=prop_override.so PROP_dalvik.vm.heapsize=123M getprop dalvik.vm.heapsize
+// Output: 123M
+#include <string>
+#include <cstring>
+#include <cstdlib>
+#include <dlfcn.h>
+
+// Source: https://android.googlesource.com/platform/system/core/+/100b08a848d018eeb1caa5d5e7c7c2aaac65da15/libcutils/include/cutils/properties.h
+#define PROP_VALUE_MAX 92
+// This is the mangled name of "android::base::GetProperty".
+#define GET_PROPERTY_MANGLED_NAME "_ZN7android4base11GetPropertyERKNSt3__112basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEES9_"
+
+extern "C" typedef int (*property_get_ptr)(const char *, char *, const char *);
+typedef std::string (*GetProperty_ptr)(const std::string &, const std::string &);
+
+char *GetPropOverride(const std::string &key) {
+    auto envKey = "PROP_" + key;
+
+    return getenv(envKey.c_str());
+}
+
+// See: https://android.googlesource.com/platform/system/core/+/100b08a848d018eeb1caa5d5e7c7c2aaac65da15/libcutils/properties.cpp
+extern "C" int property_get(const char *key, char *value, const char *default_value) {
+    auto replacement = GetPropOverride(std::string(key));
+    if (replacement) {
+        int len = strnlen(replacement, PROP_VALUE_MAX);
+
+        strncpy(value, replacement, len);
+        return len;
+    }
+
+    static property_get_ptr original = NULL;
+    if (!original) {
+        // Get the address of the original function.
+        original = reinterpret_cast<property_get_ptr>(dlsym(RTLD_NEXT, "property_get"));
+    }
+
+    return original(key, value, default_value);
+}
+
+// Defining android::base::GetProperty ourselves won't work because std::string has a slightly different "path" in the NDK version of the C++ standard library.
+// We can get around this by forcing the function to adopt a specific name using the asm keyword.
+std::string GetProperty(const std::string &, const std::string &) asm(GET_PROPERTY_MANGLED_NAME);
+
+
+// See: https://android.googlesource.com/platform/system/libbase/+/1a34bb67c4f3ba0a1ea6f4f20ac9fe117ba4fe64/properties.cpp
+// This isn't used for the properties we want to override, but property_get is deprecated so that could change in the future.
+std::string GetProperty(const std::string &key, const std::string &default_value) {
+    auto replacement = GetPropOverride(key);
+    if (replacement) {
+        return std::string(replacement);
+    }
+
+    static GetProperty_ptr original = NULL;
+    if (!original) {
+        original = reinterpret_cast<GetProperty_ptr>(dlsym(RTLD_NEXT, GET_PROPERTY_MANGLED_NAME));
+    }
+
+    return original(key, default_value);
+}