liblzma: Include cached memory in reported memusage in threaded decoder.

This affects lzma_memusage() and lzma_memlimit_set() when used
with the threaded decompressor. Now all allocations are reported
by lzma_memusage() (so it's not misleading) and lzma_memlimit_set()
cannot lower the limit below that value.

The alternative would have been to allow lowering the limit if
doing so is possible by freeing the cached memory but since
the primary use case of lzma_memlimit_set() is to increase
memlimit after LZMA_MEMLIMIT_ERROR this simple approach
was selected.

The cached memory was always included when enforcing
the memory usage limit while decoding.

Thanks to Jia Tan.

src/liblzma/common/stream_decoder_mt.c

@@ -1834,11 +1834,27 @@ stream_decoder_mt_memconfig(void *coder_ptr, uint64_t *memusage,
 {
 	// NOTE: This function gets/sets memlimit_stop. For now,
 	// memlimit_threading cannot be modified after initialization.
+	//
+	// *memusage will include cached memory too. Excluding cached memory
+	// would be misleading and it wouldn't help the applications to
+	// know how much memory is actually needed to decompress the file
+	// because the higher the number of threads and the memlimits are
+	// the more memory the decoder may use.
+	//
+	// Setting a new limit includes the cached memory too and too low
+	// limits will be rejected. Alternative could be to free the cached
+	// memory immediately if that helps to bring the limit down but
+	// the current way is the simplest. It's unlikely that limit needs
+	// to be lowered in the middle of a file anyway; the typical reason
+	// to want a new limit is to increase after LZMA_MEMLIMIT_ERROR
+	// and even such use isn't common.
 	struct lzma_stream_coder *coder = coder_ptr;
 
 	mythread_sync(coder->mutex) {
-		*memusage = coder->mem_direct_mode + coder->mem_in_use
+		*memusage = coder->mem_direct_mode
-				+ coder->outq.mem_in_use; // FIXME?
+				+ coder->mem_in_use
+				+ coder->mem_cached
+				+ coder->outq.mem_allocated;
 	}
 
 	// If no filter chains are allocated, *memusage may be zero.
@@ -1849,7 +1865,7 @@ stream_decoder_mt_memconfig(void *coder_ptr, uint64_t *memusage,
 	*old_memlimit = coder->memlimit_stop;
 
 	if (new_memlimit != 0) {
-		if (new_memlimit < *memusage) // FIXME?
+		if (new_memlimit < *memusage)
 			return LZMA_MEMLIMIT_ERROR;
 
 		coder->memlimit_stop = new_memlimit;