Fix LZMA_SYNC_FLUSH handling in LZ and LZMA encoders.

That code is now almost completely in LZ coder, where
it can be shared with other LZ77-based algorithms in
future.

src/liblzma/lz/lz_encoder.c

@@ -141,7 +141,7 @@ lzma_lz_encoder_reset(lzma_lz_encoder *lz, lzma_allocator *allocator,
 		const uint8_t *preset_dictionary,
 		size_t preset_dictionary_size)
 {
-	lz->sequence = SEQ_RUN;
+	lz->sequence = SEQ_START;
 	lz->uncompressed_size = uncompressed_size;
 	lz->temp_size = 0;
 
@@ -477,24 +477,42 @@ lzma_lz_encode(lzma_coder *coder, lzma_allocator *allocator,
 		coder->lz.temp_size = 0;
 	}
 
-	if (coder->lz.sequence == SEQ_FLUSH_END) {
+	switch (coder->lz.sequence) {
+	case SEQ_START:
+		assert(coder->lz.read_pos == coder->lz.write_pos);
+
+		// If there is no new input data and LZMA_SYNC_FLUSH is used
+		// immediatelly after previous LZMA_SYNC_FLUSH finished or
+		// at the very beginning of the input stream, we return
+		// LZMA_STREAM_END immediatelly. Writing a flush marker
+		// to the very beginning of the stream or right after previous
+		// flush marker is not allowed by the LZMA stream format.
+		if (*in_pos == in_size && action == LZMA_SYNC_FLUSH)
+			return LZMA_STREAM_END;
+
+		coder->lz.sequence = SEQ_RUN;
+		break;
+
+	case SEQ_FLUSH_END:
 		// During an earlier call to this function, flushing was
 		// otherwise finished except some data was left pending
 		// in coder->lz.buffer. Now we have copied all that data
 		// to the output buffer and can return LZMA_STREAM_END.
-		coder->lz.sequence = SEQ_RUN;
+		coder->lz.sequence = SEQ_START;
 		assert(action == LZMA_SYNC_FLUSH);
 		return LZMA_STREAM_END;
-	}
 
-	if (coder->lz.sequence == SEQ_END) {
+	case SEQ_END:
 		// This is like the above flushing case, but for finishing
 		// the encoding.
 		//
 		// NOTE: action is not necesarily LZMA_FINISH; it can
-		// be LZMA_SYNC_FLUSH too in case it is used at the
-		// end of the stream with known Uncompressed Size.
+		// be LZMA_RUN or LZMA_SYNC_FLUSH too in case it is used
+		// at the end of the stream with known Uncompressed Size.
 		return action != LZMA_RUN ? LZMA_STREAM_END : LZMA_OK;
+
+	default:
+		break;
 	}
 
 	while (*out_pos < out_size
@@ -511,7 +529,7 @@ lzma_lz_encode(lzma_coder *coder, lzma_allocator *allocator,
 				assert(action == LZMA_SYNC_FLUSH);
 				if (coder->lz.temp_size == 0) {
 					// Flushing was finished successfully.
-					coder->lz.sequence = SEQ_RUN;
+					coder->lz.sequence = SEQ_START;
 				} else {
 					// Flushing was otherwise finished,
 					// except that some data was left

src/liblzma/lz/lz_encoder.h

@@ -30,6 +30,7 @@
 typedef struct lzma_lz_encoder_s lzma_lz_encoder;
 struct lzma_lz_encoder_s {
 	enum {
+		SEQ_START,
 		SEQ_RUN,
 		SEQ_FLUSH,
 		SEQ_FLUSH_END,

src/liblzma/lzma/lzma_encoder.c

@@ -157,31 +157,8 @@ lzma_lzma_encode(lzma_coder *coder, uint8_t *restrict out,
 	// Initialize the stream if no data has been encoded yet.
 	if (!coder->is_initialized) {
 		if (coder->lz.read_pos == coder->lz.read_limit) {
-			switch (coder->lz.sequence) {
-			case SEQ_RUN:
-				// Cannot initialize, because there is
-				// no input data.
-				return false;
-
-			case SEQ_FLUSH:
-				// Nothing to flush. There cannot be a flush
-				// marker when no data has been processed
-				// yet (file format doesn't allow it, and
-				// it would be just waste of space).
-				return true;
-
-			case SEQ_FINISH:
-				// We are encoding an empty file. No need
-				// to initialize the encoder.
 			assert(coder->lz.write_pos == coder->lz.read_pos);
-				break;
-
-			default:
-				// We never get here.
-				assert(0);
-				return true;
-			}
-
+			assert(coder->lz.sequence == SEQ_FINISH);
 		} else {
 			// Do the actual initialization.
 			uint32_t len;