xz: Fix semi-busy-waiting in xz --flush-timeout.

When input blocked, xz --flush-timeout=1 would wake up every
millisecond and initiate flushing which would have nothing to
flush and thus would just waste CPU time. The fix disables the
timeout when no input has been seen since the previous flush.

src/xz/coder.c

@@ -729,6 +729,10 @@ coder_normal(file_pair *pair)
 
 				// Set the time of the most recent flushing.
 				mytime_set_flush_time();
+
+				// Mark that we haven't seen any new input
+				// since the previous flush.
+				pair->src_has_seen_input = false;
 			} else {
 				// Start a new Block after LZMA_FULL_BARRIER.
 				if (opt_block_list == NULL) {

src/xz/file_io.c

@@ -749,6 +749,7 @@ io_open_src(const char *src_name)
 		.src_fd = -1,
 		.dest_fd = -1,
 		.src_eof = false,
+		.src_has_seen_input = false,
 		.dest_try_sparse = false,
 		.dest_pending_sparse = 0,
 	};
@@ -1133,10 +1134,15 @@ io_read(file_pair *pair, io_buf *buf, size_t size)
 
 #ifndef TUKLIB_DOSLIKE
 			if (IS_EAGAIN_OR_EWOULDBLOCK(errno)) {
-				const io_wait_ret ret = io_wait(pair,
-						mytime_get_flush_timeout(),
-						true);
-				switch (ret) {
+				// Disable the flush-timeout if no input has
+				// been seen since the previous flush and thus
+				// there would be nothing to flush after the
+				// timeout expires (avoids busy waiting).
+				const int timeout = pair->src_has_seen_input
+						? mytime_get_flush_timeout()
+						: -1;
+
+				switch (io_wait(pair, timeout, true)) {
 				case IO_WAIT_MORE:
 					continue;
 
@@ -1160,6 +1166,7 @@ io_read(file_pair *pair, io_buf *buf, size_t size)
 		}
 
 		pos += (size_t)(amount);
+		pair->src_has_seen_input = true;
 	}
 
 	return pos;

src/xz/file_io.h

@@ -49,6 +49,10 @@ typedef struct {
 	/// True once end of the source file has been detected.
 	bool src_eof;
 
+	/// For --flush-timeout: True if at least one byte has been read
+	/// since the previous flush or the start of the file.
+	bool src_has_seen_input;
+
 	/// If true, we look for long chunks of zeros and try to create
 	/// a sparse file.
 	bool dest_try_sparse;