Fix truncated decode of mp3 files on import

The MAD decoder will not decode the final frame in an mp3 stream unless
it has a small amount of padding afterwards (MAD_BUFFER_GUARD bytes,
actually 8). Without this, it loses sync before returning any decoded
data from the final frame. The result is that the imported audio is
truncated by up to 1152 samples.

This commit addresses that by using a final round of input callback
calls to provide the last MAD_BUFFER_GUARD bytes after the underlying
file has reached eof. The logic is based on madplay.

src/import/ImportMP3.cpp

@@ -95,12 +95,14 @@ extern "C" {
 struct private_data {
    wxFile *file;            /* the file containing the mp3 data we're feeding the encoder */
    unsigned char *inputBuffer;
+   int inputBufferFill;     /* amount of data in inputBuffer */
    TrackFactory *trackFactory;
    TrackHolders channels;
    ProgressDialog *progress;
    unsigned numChannels;
    int updateResult;
    bool id3checked;
+   bool eof;      /* having supplied both underlying file and guard pad data */
 };
 
 class MP3ImportPlugin final : public ImportPlugin
@@ -216,11 +218,13 @@ int MP3ImportFileHandle::Import(TrackFactory *trackFactory, TrackHolders &outTra
 
    mPrivateData.file        = mFile.get();
    mPrivateData.inputBuffer = new unsigned char [INPUT_BUFFER_SIZE];
+   mPrivateData.inputBufferFill = 0;
    mPrivateData.progress    = mProgress.get();
    mPrivateData.updateResult= eProgressSuccess;
    mPrivateData.id3checked  = false;
    mPrivateData.numChannels = 0;
    mPrivateData.trackFactory= trackFactory;
+   mPrivateData.eof         = false;
 
    mad_decoder_init(&mDecoder, &mPrivateData, input_cb, 0, 0, output_cb, error_cb, 0);
 
@@ -404,7 +408,10 @@ enum mad_flow input_cb(void *_data, struct mad_stream *stream)
    if(data->updateResult != eProgressSuccess)
       return MAD_FLOW_STOP;
 
-   if(data->file->Eof()) {
+   if (data->eof) {
+      /* different from data->File->Eof(), this means the underlying
+         file has reached eof *and* we have subsequently supplied the
+         final padding zeros */
       return MAD_FLOW_STOP;
    }
 
@@ -431,20 +438,41 @@ enum mad_flow input_cb(void *_data, struct mad_stream *stream)
     *  mad_stream_buffer()"
     *           -- Rob Leslie, on the mad-dev mailing list */
 
-   unsigned int unconsumedBytes;
+   int unconsumedBytes;
-   if(stream->next_frame) {
+   if (stream->next_frame) {
-      unconsumedBytes = data->inputBuffer + INPUT_BUFFER_SIZE - stream->next_frame;
+      /* we must use inputBufferFill instead of INPUT_BUFFER_SIZE here
+         because the final buffer of the file may be only partially
+         filled, and we would otherwise be providing too much input
+         after eof */
+      unconsumedBytes = data->inputBuffer + data->inputBufferFill
+	  - stream->next_frame;
       memmove(data->inputBuffer, stream->next_frame, unconsumedBytes);
    }
    else
       unconsumedBytes = 0;
 
+   if (data->file->Eof() &&
+       (unconsumedBytes + MAD_BUFFER_GUARD < INPUT_BUFFER_SIZE)) {
+       
+      /* supply the requisite MAD_BUFFER_GUARD zero bytes to ensure
+         the final frame gets decoded properly, then finish */
+       
+      memset(data->inputBuffer + unconsumedBytes, 0, MAD_BUFFER_GUARD);
+      mad_stream_buffer
+          (stream, data->inputBuffer, MAD_BUFFER_GUARD + unconsumedBytes);
+      
+      data->eof = true; /* so on next call, we will tell mad to stop */
+      
+      return MAD_FLOW_CONTINUE;
+   }
 
    off_t read = data->file->Read(data->inputBuffer + unconsumedBytes,
                                  INPUT_BUFFER_SIZE - unconsumedBytes);
 
    mad_stream_buffer(stream, data->inputBuffer, read + unconsumedBytes);
 
+   data->inputBufferFill = int(read + unconsumedBytes);
+
    return MAD_FLOW_CONTINUE;
 }