FFmpeg: поиск невозможен с файловым дескриптором на Android Q

можно открыть дескриптор файла, используя протокол канала, как описано

Мне любопытно, почему необходимо открывать дескриптор файла через протокол канала? проигрыватель sView открывает дескриптор файла с помощью пользовательского AVIOContext, который доступен для поиска, по крайней мере на более старых протестированных версиях Android. Вот псевдокод, открывающий AVFormatContext с пользовательским AVIOContext.

    int aFileDescriptor = myResMgr->openFileDescriptor(theFileToLoad);
    AVFormatContext* aFormatCtx = avformat_alloc_context();
    StAVIOContext myAvioContext;
    if(!myAvioContext.openFromDescriptor(aFileDescriptor, "rb")) {
       // error
    }

    aFormatCtx->pb = myAvioContext.getAvioContext();
    int avErrCode = avformat_open_input(&aFormatCtx, theFileToLoad, NULL, NULL);

Ниже приведена попытка извлечения упрощенного определения класса StAVIOFileContext.

//! Wrapper over AVIOContext for passing the custom I/O.
class StAVIOContext {
public:
  //! Main constructor.
  StAVIOContext() {
    const int aBufferSize = 32768;
    unsigned char* aBufferIO = (unsigned char* )av_malloc(aBufferSize + AV_INPUT_BUFFER_PADDING_SIZE);
    AVIOContext* myAvioCtx = avio_alloc_context (aBufferIO, aBufferSize, 0, this, readCallback, writeCallback, seekCallback);
  }

  //! Destructor.
  virtual ~StAVIOContext() {
    close();
    if (myAvioCtx != NULL) { av_free (myAvioCtx); }
  }

  //! Close the file.
  void close() {
    if(myFile != NULL) {
        fclose(myFile);
        myFile = NULL;
    }
  }

  //! Associate a stream with a file that was previously opened for low-level I/O.
  //! The associated file will be automatically closed on destruction.
  bool openFromDescriptor(int theFD, const char* theMode) {
    close();
  #ifdef _WIN32
    myFile = ::_fdopen(theFD, theMode);
  #else
    myFile =  ::fdopen(theFD, theMode);
  #endif
    return myFile != NULL;
  }

  //! Access AVIO context.
  AVIOContext* getAvioContext() const { return myAvioCtx; }

public:

  //! Virtual method for reading the data.
  virtual int read (uint8_t* theBuf,
                    int theBufSize) {
    if(myFile == NULL) { return -1; }

    int aNbRead = (int )::fread(theBuf, 1, theBufSize, myFile);
    if(aNbRead == 0 && feof(myFile) != 0) { return AVERROR_EOF; }
    return aNbRead;
  }

  //! Virtual method for writing the data.
  virtual int write (uint8_t* theBuf,
                     int theBufSize) {
    if(myFile == NULL) { return -1; }
    return (int )::fwrite(theBuf, 1, theBufSize, myFile);
  }

  //! Virtual method for seeking to new position.
  virtual int64_t seek (int64_t theOffset,
                        int theWhence) {
    if(theWhence == AVSEEK_SIZE || myFile == NULL) { return -1; }
  #ifdef _WIN32
    bool isOk = ::_fseeki64(myFile, theOffset, theWhence) == 0;
  #else
    bool isOk =    ::fseeko(myFile, theOffset, theWhence) == 0;
  #endif
    if(!isOk) { return -1; }
  #ifdef _WIN32
    return ::_ftelli64(myFile);
  #else
    return ::ftello(myFile);
  #endif
  }

private:
  //! Callback for reading the data.
  static int readCallback(void* theOpaque,
                          uint8_t* theBuf,
                          int  theBufSize) {
    return theOpaque != NULL
         ? ((StAVIOContext* )theOpaque)->read(theBuf, theBufSize)
         : 0;
  }

  //! Callback for writing the data.
  static int writeCallback(void* theOpaque,
                           uint8_t* theBuf,
                           int theBufSize) {
    return theOpaque != NULL
         ? ((StAVIOContext* )theOpaque)->write(theBuf, theBufSize)
         : 0;
  }

  //! Callback for seeking to new position.
  static int64_t seekCallback(void*   theOpaque,
                              int64_t theOffset,
                              int     theWhence) {
    return theOpaque != NULL
        ? ((StAVIOContext* )theOpaque)->seek(theOffset, theWhence)
        : -1;
  }

protected:
  AVIOContext* myAvioCtx;
  FILE* myFile;
};

пользовательский протокол может обрабатывать Uri как content://com.android.providers.downloads.documents/document/msf%3A62 или content://com.android.externalstorage.documents/document/primary%3ADownload%2Ftranscode.aac

Вот C код, открывающий такой Uri (проверки ошибок скрыты для краткости):

int get_fd_from_content(const char *content, int access) {

    static jclass    android_net_Uri;
    static jmethodID android_net_Uri_parse = 0;
    static jmethodID android_content_Context_getContentResolver = 0;
    static jmethodID android_content_ContentResolver_openFileDescriptor = 0;
    static jmethodID android_os_ParcelFileDescriptor_getFd = 0;

    int fd = -1;

    JNIEnv *env;
    int ret = (*globalVm)->GetEnv(globalVm, (void **)&env, JNI_VERSION_1_6);

    android_net_Uri_parse = get_static_method_id(env, "android/net/Uri", "parse", "(Ljava/lang/String;)Landroid/net/Uri;", &android_net_Uri);
    android_content_Context_getContentResolver = get_method_id(env, "android/content/Context", "getContentResolver", "()Landroid/content/ContentResolver;");
    android_content_ContentResolver_openFileDescriptor = get_method_id(env, "android/content/ContentResolver", "openFileDescriptor", "(Landroid/net/Uri;Ljava/lang/String;)Landroid/os/ParcelFileDescriptor;");
    android_os_ParcelFileDescriptor_getFd = get_method_id(env, "android/os/ParcelFileDescriptor", "getFd", "()I"));

    const char *fmode = "r";
    if (access & (O_WRONLY | O_RDWR)) {
        fmode = "w";
    }

    LOGI("get_fd_from_content" " \"%s\" fd from %s", fmode, content);

    jstring uriString = (*env)->NewStringUTF(env, content);
    jstring fmodeString = (*env)->NewStringUTF(env, fmode);
    jobject uri = (*env)->CallStaticObjectMethod(env, android_net_Uri, android_net_Uri_parse, uriString);
    jobject contentResolver = (*env)->CallObjectMethod(env, appContext, android_content_Context_getContentResolver);
    jobject parcelFileDescriptor = (*env)->CallObjectMethod(env, contentResolver, android_content_ContentResolver_openFileDescriptor, uri, fmodeString);

    fd = (*env)->CallIntMethod(env, parcelFileDescriptor, android_os_ParcelFileDescriptor_getFd);

    (*env)->DeleteLocalRef(env, uriString);
    (*env)->DeleteLocalRef(env, fmodeString);
    (*env)->DeleteLocalRef(env, uri);
    (*env)->DeleteLocalRef(env, contentResolver);
    (*env)->DeleteLocalRef(env, parcelFileDescriptor);

    return fd;
}