xz: Add preliminary support for --flush-timeout=TIMEOUT.

When --flush-timeout=TIMEOUT is used, xz will use
LZMA_SYNC_FLUSH if read() would block and at least
TIMEOUT milliseconds has elapsed since the previous flush.

This can be useful in realtime-like use cases where the
data is simultanously decompressed by another process
(possibly on a different computer). If new uncompressed
input data is produced slowly, without this option xz could
buffer the data for a long time until it would become
decompressible from the output.

If TIMEOUT is 0, the feature is disabled. This is the default.

This commit affects the compression side. Using xz for
the decompression side for the above purpose doesn't work
yet so well because there is quite a bit of input and
output buffering when decompressing.

The --long-help or man page were not updated yet.
The details of this feature may change.

src/xz/args.c

@@ -140,6 +140,7 @@ parse_real(args_info *args, int argc, char **argv)
 		OPT_NO_ADJUST,
 		OPT_INFO_MEMORY,
 		OPT_ROBOT,
+		OPT_FLUSH_TIMEOUT,
 	};
 
 	static const char short_opts[]
@@ -176,6 +177,7 @@ parse_real(args_info *args, int argc, char **argv)
 		{ "memory",       required_argument, NULL,  'M' }, // Old alias
 		{ "no-adjust",    no_argument,       NULL,  OPT_NO_ADJUST },
 		{ "threads",      required_argument, NULL,  'T' },
+		{ "flush-timeout", required_argument, NULL, OPT_FLUSH_TIMEOUT },
 
 		{ "extreme",      no_argument,       NULL,  'e' },
 		{ "fast",         no_argument,       NULL,  '0' },
@@ -483,6 +485,11 @@ parse_real(args_info *args, int argc, char **argv)
 			opt_auto_adjust = false;
 			break;
 
+		case OPT_FLUSH_TIMEOUT:
+			opt_flush_timeout = str_to_uint64("flush-timeout",
+					optarg, 0, UINT64_MAX);
+			break;
+
 		default:
 			message_try_help();
 			tuklib_exit(E_ERROR, E_ERROR, false);

src/xz/coder.c

@@ -586,6 +586,9 @@ coder_normal(file_pair *pair)
 				if (block_remaining == 0)
 					action = LZMA_FULL_FLUSH;
 			}
+
+			if (action == LZMA_RUN && flush_needed)
+				action = LZMA_SYNC_FLUSH;
 		}
 
 		// Let liblzma do the actual work.
@@ -601,10 +604,22 @@ coder_normal(file_pair *pair)
 			strm.avail_out = IO_BUFFER_SIZE;
 		}
 
-		if (ret == LZMA_STREAM_END && action == LZMA_FULL_FLUSH) {
+		if (ret == LZMA_STREAM_END && (action == LZMA_SYNC_FLUSH
-			// Start a new Block.
+				|| action == LZMA_FULL_FLUSH)) {
-			action = LZMA_RUN;
+			// Flushing completed. Write the pending data out
+			// immediatelly so that the reading side can
+			// decompress everything compressed so far. Do this
+			// also with LZMA_FULL_FLUSH because if it is combined
+			// with timed LZMA_SYNC_FLUSH the same flushing
+			// timer can be used.
+			if (io_write(pair, &out_buf, IO_BUFFER_SIZE
+					- strm.avail_out))
+				break;
 
+			strm.next_out = out_buf.u8;
+			strm.avail_out = IO_BUFFER_SIZE;
+
+			if (action == LZMA_FULL_FLUSH) {
 				if (opt_block_list == NULL) {
 					block_remaining = opt_block_size;
 				} else {
@@ -613,8 +628,17 @@ coder_normal(file_pair *pair)
 					if (opt_block_list[list_pos + 1] != 0)
 						++list_pos;
 
-				block_remaining = opt_block_list[list_pos];
+					block_remaining
+						= opt_block_list[list_pos];
 				}
+			}
+
+			// Set the time of the most recent flushing.
+			mytime_set_flush_time();
+
+			// Start a new Block after LZMA_FULL_FLUSH or continue
+			// the same block after LZMA_SYNC_FLUSH.
+			action = LZMA_RUN;
 
 		} else if (ret != LZMA_OK) {
 			// Determine if the return value indicates that we

src/xz/file_io.c

@@ -38,6 +38,13 @@ static bool warn_fchown;
 #endif
 
 
+typedef enum {
+	IO_WAIT_MORE,    // Reading or writing is possible.
+	IO_WAIT_ERROR,   // Error or user_abort
+	IO_WAIT_TIMEOUT, // poll() timed out
+} io_wait_ret;
+
+
 /// If true, try to create sparse files when decompressing.
 static bool try_sparse = true;
 
@@ -130,8 +137,8 @@ io_no_sparse(void)
 /// pops up again. There are pselect() (POSIX-1.2001) and ppoll() (not in
 /// POSIX) but neither is portable enough in 2013. The self-pipe trick is
 /// old and very portable.
-static bool
+static io_wait_ret
-io_wait(file_pair *pair, bool is_reading)
+io_wait(file_pair *pair, int timeout, bool is_reading)
 {
 	struct pollfd pfd[2];
 
@@ -147,10 +154,10 @@ io_wait(file_pair *pair, bool is_reading)
 	pfd[1].events = POLLIN;
 
 	while (true) {
-		const int ret = poll(pfd, 2, -1);
+		const int ret = poll(pfd, 2, timeout);
 
 		if (user_abort)
-			return true;
+			return IO_WAIT_ERROR;
 
 		if (ret == -1) {
 			if (errno == EINTR || errno == EAGAIN)
@@ -160,10 +167,17 @@ io_wait(file_pair *pair, bool is_reading)
 					is_reading ? pair->src_name
 						: pair->dest_name,
 					strerror(errno));
+			return IO_WAIT_ERROR;
+		}
+
+		if (ret == 0) {
+			assert(opt_flush_timeout != 0);
+			flush_needed = true;
+			return IO_WAIT_TIMEOUT;
 		}
 
 		if (pfd[0].revents != 0)
-			return false;
+			return IO_WAIT_MORE;
 	}
 }
 #endif
@@ -583,10 +597,10 @@ io_open_src_real(file_pair *pair)
 	// will work when open() is used with O_NONBLOCK.
 	if (!S_ISREG(pair->src_st.st_mode)) {
 		signals_unblock();
-		const bool ret = io_wait(pair, true);
+		const io_wait_ret ret = io_wait(pair, -1, true);
 		signals_block();
 
-		if (ret)
+		if (ret != IO_WAIT_MORE)
 			goto error;
 	}
 #endif
@@ -1001,10 +1015,22 @@ io_read(file_pair *pair, io_buf *buf_union, size_t size)
 
 #ifndef TUKLIB_DOSLIKE
 			if (errno == EAGAIN || errno == EWOULDBLOCK) {
-				if (!io_wait(pair, true))
+				const io_wait_ret ret = io_wait(pair,
+						mytime_get_flush_timeout(),
+						true);
+				switch (ret) {
+				case IO_WAIT_MORE:
 					continue;
 
+				case IO_WAIT_ERROR:
 					return SIZE_MAX;
+
+				case IO_WAIT_TIMEOUT:
+					return size - left;
+
+				default:
+					message_bug();
+				}
 			}
 #endif
 
@@ -1077,7 +1103,7 @@ io_write_buf(file_pair *pair, const uint8_t *buf, size_t size)
 
 #ifndef TUKLIB_DOSLIKE
 			if (errno == EAGAIN || errno == EWOULDBLOCK) {
-				if (!io_wait(pair, false))
+				if (io_wait(pair, -1, false) == IO_WAIT_MORE)
 					continue;
 
 				return true;