xz: Use non-blocking I/O for the output file.

Now both reading and writing should be without
race conditions with signals.

They might still be signal handling issues left.
Signals are blocked during many operations to avoid
EINTR but it may cause problems e.g. if writing to
stderr blocks when trying to display an error message.

src/xz/file_io.c

@@ -682,6 +682,30 @@ io_open_dest_real(file_pair *pair)
 		pair->dest_fd = STDOUT_FILENO;
 #ifdef TUKLIB_DOSLIKE
 		setmode(STDOUT_FILENO, O_BINARY);
+#else
+		// Set O_NONBLOCK if it isn't already set.
+		//
+		// NOTE: O_APPEND may be unset later in this function
+		// and it relies on stdout_flags being set here.
+		stdout_flags = fcntl(STDOUT_FILENO, F_GETFL);
+		if (stdout_flags == -1) {
+			message_error(_("Error getting the file status flags "
+					"from standard output: %s"),
+					strerror(errno));
+			return true;
+		}
+
+		if ((stdout_flags & O_NONBLOCK) == 0) {
+			if (fcntl(STDOUT_FILENO, F_SETFL,
+					stdout_flags | O_NONBLOCK) == -1) {
+				message_error(_("Error setting O_NONBLOCK "
+						"on standard output: %s"),
+						strerror(errno));
+				return true;
+			}
+
+			restore_stdout_flags = true;
+		}
 #endif
 	} else {
 		pair->dest_name = suffix_get_dest_name(pair->src_name);
@@ -719,8 +743,11 @@ io_open_dest_real(file_pair *pair)
 		}
 
 		// Open the file.
-		const int flags = O_WRONLY | O_BINARY | O_NOCTTY
+		int flags = O_WRONLY | O_BINARY | O_NOCTTY
 				| O_CREAT | O_EXCL;
+#ifndef TUKLIB_DOSLIKE
+		flags |= O_NONBLOCK;
+#endif
 		const mode_t mode = S_IRUSR | S_IWUSR;
 		pair->dest_fd = open(pair->dest_name, flags, mode);
 
@@ -762,10 +789,6 @@ io_open_dest_real(file_pair *pair)
 			if (!S_ISREG(pair->dest_st.st_mode))
 				return false;
 
-			stdout_flags = fcntl(STDOUT_FILENO, F_GETFL);
-			if (stdout_flags == -1)
-				return false;
-
 			if (stdout_flags & O_APPEND) {
 				// Creating a sparse file is not possible
 				// when O_APPEND is active (it's used by
@@ -784,14 +807,23 @@ io_open_dest_real(file_pair *pair)
 				if (lseek(STDOUT_FILENO, 0, SEEK_END) == -1)
 					return false;
 
+				// O_NONBLOCK was set earlier in this function
+				// so it must be kept here too. If this
+				// fcntl() call fails, we continue but won't
+				// try to create sparse output. The original
+				// flags will still be restored if needed (to
+				// unset O_NONBLOCK) when the file is finished.
 				if (fcntl(STDOUT_FILENO, F_SETFL,
-						stdout_flags & ~O_APPEND)
-						== -1)
+						(stdout_flags | O_NONBLOCK)
+						& ~O_APPEND) == -1)
 					return false;
 
 				// Disabling O_APPEND succeeded. Mark
 				// that the flags should be restored
-				// in io_close_dest().
+				// in io_close_dest(). This quite likely was
+				// already set when enabling O_NONBLOCK but
+				// just in case O_NONBLOCK was already set,
+				// set this again here.
 				restore_stdout_flags = true;
 
 			} else if (lseek(STDOUT_FILENO, 0, SEEK_CUR)
@@ -1040,6 +1072,15 @@ io_write_buf(file_pair *pair, const uint8_t *buf, size_t size)
 				continue;
 			}
 
+#ifndef TUKLIB_DOSLIKE
+			if (errno == EAGAIN || errno == EWOULDBLOCK) {
+				if (!io_wait(pair, false))
+					continue;
+
+				return true;
+			}
+#endif
+
 			// Handle broken pipe specially. gzip and bzip2
 			// don't print anything on SIGPIPE. In addition,
 			// gzip --quiet uses exit status 2 (warning) on