xz: Use posix_fadvise() if it is available.

configure.ac

@@ -517,6 +517,9 @@ gl_GETOPT
 # Find the best function to set timestamps.
 AC_CHECK_FUNCS([futimens futimes futimesat utimes utime], [break])
 
+# This is nice to have but not mandatory.
+AC_CHECK_FUNCS([posix_fadvise])
+
 TUKLIB_PROGNAME
 TUKLIB_INTEGER
 TUKLIB_PHYSMEM

src/xz/file_io.c

@@ -293,6 +293,10 @@ io_open_src_real(file_pair *pair)
 		pair->src_fd = STDIN_FILENO;
 #ifdef TUKLIB_DOSLIKE
 		setmode(STDIN_FILENO, O_BINARY);
+#endif
+#ifdef HAVE_POSIX_FADVISE
+		// It will fail if stdin is a pipe and that's fine.
+		(void)posix_fadvise(STDIN_FILENO, 0, 0, POSIX_FADV_SEQUENTIAL);
 #endif
 		return false;
 	}
@@ -497,6 +501,17 @@ io_open_src_real(file_pair *pair)
 	}
 #endif
 
+#ifdef HAVE_POSIX_FADVISE
+	const int fadvise_ret = posix_fadvise(
+			pair->src_fd, 0, 0, POSIX_FADV_SEQUENTIAL);
+
+	// It shouldn't fail, but if it does anyway, it doesn't matter.
+	// Check it with an assertion so that if something gets messed
+	// up in the future, it will get caught when debugging is enabled.
+	assert(fadvise_ret == 0);
+	(void)fadvise_ret;
+#endif
+
 	return false;
 
 error_msg: