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Fixed the native crash happening on Android. #116

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Fixed the native crash happening on Android. #116

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069e1b2
Fixed the native crash happening on Android.
MohitMaliDeveloper Jan 17, 2025
5fd942d
Removed the redundant check to check whether the native object is nul…
MohitMaliDeveloper Jan 20, 2025
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2 changes: 1 addition & 1 deletion lib/src/main/cpp/libzim/archive.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -183,7 +183,7 @@ METHOD(jboolean, hasIllustration, jint size) {
GETTER(jlongArray, getIllustrationSizes)

METHOD(jobject, getEntryByPath__Ljava_lang_String_2, jstring path) {
return BUILD_WRAPPER("org/kiwix/libzim/Entry", THIS->getEntryByPath(TO_C(path)));
return BUILD_WRAPPER("org/kiwix/libzim/Entry", SAFE_THIS->getEntryByPath(TO_C(path)));
} CATCH_EXCEPTION(nullptr)

METHOD(jobject, getEntryByPath__I, jint index) {
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13 changes: 13 additions & 0 deletions lib/src/main/cpp/macros.h
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Expand Up @@ -27,6 +27,16 @@

#define THIS getPtr<NATIVE_TYPE>(env, thisObj)

#define SAFE_THIS \
({ \
auto ptr = getPtr<NATIVE_TYPE>(env, thisObj); \
if (!ptr) { \
throwException(env, "java/lang/IllegalStateException", "The native object has already been disposed."); \
return nullptr; \
} \
ptr; \
})
Comment on lines +31 to +38
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I can't see how this could work in the pre-C++-14 language standard that I am more or less familiar with. SAFE_THIS has to be an expression but it doesn't seem to expand to code that can be considered an expression according to the language specification. I tried to quickly skim through https://en.cppreference.com/w/cpp/language/expressions but couldn't find anything that allows to consider this code valid in modern c++.

In other words, I am puzzled how this code compiles (in the context of the change in lib/src/main/cpp/libzim/archive.cpp).

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@MohitMaliFtechiz MohitMaliFtechiz Jan 18, 2025 •
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@veloman-yunkan This code is made by @mgautierfr, he can better explain this, and i am not very much familiar with C++.

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I think that @veloman-yunkan understand how it works :)

#define FOO BAR make every instance of FOO literally replaced by BAR.

So something like THIS->getEntryByPath(...) is replaced by getPtr<NATIVE_TYPE>(env, thisObj)->getEntryByPath(...). And it compiles because getPtr is a function returning a pointer.

With your define of SAFE_THIS we have SAFE_THIS->getEntryByPath(...) replaced by:

 ({ 
        auto ptr = getPtr<NATIVE_TYPE>(env, thisObj); 
        if (!ptr) { 
            throwException(env, "java/lang/IllegalStateException", "The native object has already been disposed."); 
            return nullptr; 
        } 
        ptr; 
    })->getEntryByPath(...)

And, as @veloman-yunkan, I don't know how this code can be compiled.

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With your define of SAFE_THIS we have SAFE_THIS->getEntryByPath(...) replaced by:

@mgautierfr SAFE_THIS also uses the getPtr<NATIVE_TYPE>(env, thisObj) to get the pointer, and if this method returns the nullptr it throws the error which will be caught in the archive.cpp.

METHOD(jobject, getEntryByPath__Ljava_lang_String_2, jstring path) {
  return BUILD_WRAPPER("org/kiwix/libzim/Entry", SAFE_THIS->getEntryByPath(TO_C(path)));
} CATCH_EXCEPTION(nullptr)

Also, in the archive.cpp SAFE_THIS returns the archive.

Screenshot from 2025-01-21 17-41-04

I think that's why this code was compiled without any error.

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SAFE_THIS is not a function. It is a macro (ie. textual replacement)

It appears that we are facing a statement expression (https://stackoverflow.com/questions/8344101/what-is-it-called-when-a-block-returns-a-value) which is not a standard C++ code but a gcc extension.

This explain why it works. However, I not really confident to use a non standard c++ here, especially as we have a better alternative.

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Ohh okay got it. @mgautierfr So we can remove this SAFE_THIS as it is non-standard. In this PR I have updated the getPtr method if that is null it returns the nullptr which we can handle before calling the functions on this.

e.g. The updated getPtr method is:

// Return a shared_ptr for the handle
template<typename T>
shared_ptr<T> getPtr(JNIEnv* env, jobject thisObj, const char* handleName = "nativeHandle")
{
  jclass thisClass = env->GetObjectClass(thisObj);
  jfieldID fidNumber = env->GetFieldID(thisClass, handleName, "J");
  auto handle = reinterpret_cast<shared_ptr<T>*>(env->GetLongField(thisObj, fidNumber));
    if (handle == nullptr) {
        return nullptr;
    }
  return *handle;
}

and we are getting this like this in archive.cpp:

METHOD(jobject, getEntryByPath__Ljava_lang_String_2, jstring path) {
  if (THIS == nullptr) {
     throwException(env, "java/lang/IllegalStateException", "The Archive object has already been disposed.");
     return nullptr;
  }
  return BUILD_WRAPPER("org/kiwix/libzim/Entry", THIS->getEntryByPath(TO_C(path)));
} CATCH_EXCEPTION(nullptr)

and will update the other code to use this approach. By using this approach we can handle the native crash for now without upgrading the minimum SDK version. If you have a better approach to not writing this check in all places, something like extension function that avoids writing this check everywhere please let me know.

In the screenshot: I have compiled the code with this new approach and tried to explicitly dispose the archive and then tried to access the content on the disposed archive and instead of crashing the program, it normally throws the exception. That we are already handling in Kiwix app.

image

I am not in favor of upgrading the minimum SDK to 33 because we will lose most of the devices, SDK 33 means Android 13 which is one of the most recent versions of Android.


#define METHOD0(retType, name) \
JNIEXPORT retType JNICALL BUILD_METHOD(TYPENAME, name) ( \
JNIEnv* env, jobject thisObj) try
Expand Down Expand Up @@ -59,4 +69,7 @@ catch(const zim::ZimFileFormatError& e) { \
} catch(const std::exception& e) { \
throwException(env, "java/lang/Exception", e.what()); \
return RET; \
} catch(...) { \
throwException(env, "java/lang/Error", "Unknown native exception occurred."); \
return RET; \
}
3 changes: 3 additions & 0 deletions lib/src/main/cpp/utils.h
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Original file line number Diff line number Diff line change
Expand Up @@ -104,6 +104,9 @@ shared_ptr<T> getPtr(JNIEnv* env, jobject thisObj, const char* handleName = "nat
jclass thisClass = env->GetObjectClass(thisObj);
jfieldID fidNumber = env->GetFieldID(thisClass, handleName, "J");
auto handle = reinterpret_cast<shared_ptr<T>*>(env->GetLongField(thisObj, fidNumber));
if (handle == nullptr) {
return nullptr;
}
return *handle;
}
#define GET_PTR(NATIVE_TYPE) getPtr<NATIVE_TYPE>(env, thisObj)
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