diff options
Diffstat (limited to 'src/xz/file_io.c')
| -rw-r--r-- | src/xz/file_io.c | 56 |
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 |