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-rw-r--r--src/xz/file_io.c56
1 files changed, 44 insertions, 12 deletions
diff --git a/src/xz/file_io.c b/src/xz/file_io.c
index fd22dc4f..21cdecb0 100644
--- a/src/xz/file_io.c
+++ b/src/xz/file_io.c
@@ -51,6 +51,10 @@ static bool restore_stdin_flags = false;
/// io_open_dest() and io_close_dest() to save and restore the flags.
static int stdout_flags;
static bool restore_stdout_flags = false;
+
+/// Self-pipe used together with the user_abort variable to avoid
+/// race conditions with signal handling.
+static int user_abort_pipe[2];
#endif
@@ -70,6 +74,14 @@ io_init(void)
// If fchown() fails setting the owner, we warn about it only if
// we are root.
warn_fchown = geteuid() == 0;
+
+ if (pipe(user_abort_pipe)
+ || fcntl(user_abort_pipe[0], F_SETFL, O_NONBLOCK)
+ == -1
+ || fcntl(user_abort_pipe[1], F_SETFL, O_NONBLOCK)
+ == -1)
+ message_fatal(_("Error creating a pipe: %s"),
+ strerror(errno));
#endif
#ifdef __DJGPP__
@@ -82,6 +94,17 @@ io_init(void)
}
+#ifndef TUKLIB_DOSLIKE
+extern void
+io_write_to_user_abort_pipe(void)
+{
+ uint8_t b = '\0';
+ (void)write(user_abort_pipe[1], &b, 1);
+ return;
+}
+#endif
+
+
extern void
io_no_sparse(void)
{
@@ -91,11 +114,20 @@ io_no_sparse(void)
#ifndef TUKLIB_DOSLIKE
-/// \brief Waits for input or output to become available
+/// \brief Waits for input or output to become available or for a signal
+///
+/// This uses the self-pipe trick to avoid a race condition that can occur
+/// if a signal is caught after user_abort has been checked but before e.g.
+/// read() has been called. In that situation read() could block unless
+/// non-blocking I/O is used. With non-blocking I/O something like select()
+/// or poll() is needed to avoid a busy-wait loop, and the same race condition
+/// 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
io_wait(file_pair *pair, bool is_reading)
{
- struct pollfd pfd[1];
+ struct pollfd pfd[2];
if (is_reading) {
pfd[0].fd = pair->src_fd;
@@ -105,18 +137,17 @@ io_wait(file_pair *pair, bool is_reading)
pfd[0].events = POLLOUT;
}
- while (true) {
- const int ret = poll(pfd, 1, -1);
+ pfd[1].fd = user_abort_pipe[0];
+ pfd[1].events = POLLIN;
- if (ret == -1) {
- if (errno == EINTR) {
- if (user_abort)
- return true;
+ while (true) {
+ const int ret = poll(pfd, 2, -1);
- continue;
- }
+ if (user_abort)
+ return true;
- if (errno == EAGAIN)
+ if (ret == -1) {
+ if (errno == EINTR || errno == EAGAIN)
continue;
message_error(_("%s: poll() failed: %s"),
@@ -125,7 +156,8 @@ io_wait(file_pair *pair, bool is_reading)
strerror(errno));
}
- return false;
+ if (pfd[0].revents != 0)
+ return false;
}
}
#endif