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authorPedro Alves <palves@redhat.com>2016-04-12 16:49:30 +0100
committerPedro Alves <palves@redhat.com>2016-04-12 16:53:21 +0100
commit00340e1b916fa2d040439b101c220fae3c5834fa (patch)
tree2c163159d99d4c0c862716385b1b1fe7d52b82ae /gdb/serial.c
parent5f5219fc34f7557296272230123a3837960a6f09 (diff)
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Introduce a serial interface for select'able events
This patch adds a new "event" struct serial type, that is an abstraction specifically for waking up blocking waits/selects, implemented on top of a pipe on POSIX, and on top of a native Windows event (CreateEvent, etc.) on Windows. This will be used to plug signal handler / mainline code races. For example, GDB can indefinitely delay handling a quit request if the user presses Ctrl-C between the last QUIT call and the next (blocking) gdb_select call in the event loop: QUIT; <<< press ctrl-c here and end up blocked in gdb_select indefinitely. gdb_select (...); // whoops, SIGINT was already handled, no EINTR. A global alone (either the quit flag, or the "ready" flag of the async signal handlers in the event loop) is not sufficient. To plug races such as these on POSIX systems, we have to register some waitable file descriptor in the set of files gdb_select waits on, and write to it from the signal handler. This is classically a pipe, and the pattern called the self-pipe trick. On Linux, it could be a more efficient eventfd instead, but I'm sticking with a pipe for simplifity, as we need it for portability anyway. (Alternatively, we could use pselect/ppoll, and block signals until the pselect. The latter is not a design I think GDB could use, because we want the QUIT macro to be super cheap, as it is used in loops. Plus, Windows.) This is a "struct serial" because Windows's gdb_select relies on that. Windows's gdb_select, our "select" replacement, knows how to wait on all kinds of handles (regular files, pipes, sockets, console, etc.) unlike the native Windows "select" function, which can only wait on sockets. Each file descriptor for a "serial" type that is not normally waitable with WaitForMultipleObjects must have a corresponding struct serial instance. gdb_select then internally looks up the struct serial instance that wraps each file descriptor, and asks it for the corresponding Windows waitable handle. We could use serial_pipe() to create a "struct serial"-wrapped pipe that is usable everywhere, including Windows. That's what currently python/python.c uses for cross-thread posting of events. However, serial_write and serial_readchar are not designed to be async-signal-safe on POSIX hosts. It's easier to bypass those when setting/clearing the event source. And writing and a serial pipe is a bit heavy weight on Windows. gdb_select requires an extra thread to wait on the pipe and several Windows events, when a single manual-reset Windows event, with no extra thread is sufficient. The intended usage is simply: - Call make_serial_event to create a serial event object. - From the signal handler call serial_event_set to set the event. - From mainline code, have select/poll wait for serial_event_fd(), in addition to whatever other files you're about to wait for. gdb/ChangeLog: 2016-04-12 Pedro Alves <palves@redhat.com> * Makefile.in (SFILES): Add ser-event.c. (HFILES_NO_SRCDIR): Add ser-event.h. (COMMON_OBS): Add ser-event.o. * ser-event.c, ser-event.h: New files. * serial.c (new_serial): New function, factored out from (serial_fdopen_ops): ... this. (serial_open_ops_1): New function, factored out from (serial_open): ... this. (serial_open_ops): New function. * serial.h (struct serial): Forware declare. (serial_open_ops): New declaration.
Diffstat (limited to 'gdb/serial.c')
-rw-r--r--gdb/serial.c61
1 files changed, 39 insertions, 22 deletions
diff --git a/gdb/serial.c b/gdb/serial.c
index 5d242b9..a95f85e 100644
--- a/gdb/serial.c
+++ b/gdb/serial.c
@@ -179,6 +179,27 @@ serial_for_fd (int fd)
return NULL;
}
+/* Create a new serial for OPS. */
+
+static struct serial *
+new_serial (const struct serial_ops *ops)
+{
+ struct serial *scb;
+
+ scb = XCNEW (struct serial);
+
+ scb->ops = ops;
+
+ scb->bufp = scb->buf;
+ scb->error_fd = -1;
+ scb->refcnt = 1;
+
+ return scb;
+}
+
+static struct serial *serial_open_ops_1 (const struct serial_ops *ops,
+ const char *open_name);
+
/* Open up a device or a network socket, depending upon the syntax of NAME. */
struct serial *
@@ -210,14 +231,17 @@ serial_open (const char *name)
if (!ops)
return NULL;
- scb = XNEW (struct serial);
+ return serial_open_ops_1 (ops, open_name);
+}
- scb->ops = ops;
+/* Open up a serial for OPS, passing OPEN_NAME to the open method. */
- scb->bufcnt = 0;
- scb->bufp = scb->buf;
- scb->error_fd = -1;
- scb->refcnt = 1;
+static struct serial *
+serial_open_ops_1 (const struct serial_ops *ops, const char *open_name)
+{
+ struct serial *scb;
+
+ scb = new_serial (ops);
/* `...->open (...)' would get expanded by the open(2) syscall macro. */
if ((*scb->ops->open) (scb, open_name))
@@ -227,10 +251,6 @@ serial_open (const char *name)
}
scb->next = scb_base;
- scb->debug_p = 0;
- scb->async_state = 0;
- scb->async_handler = NULL;
- scb->async_context = NULL;
scb_base = scb;
if (serial_logfile != NULL)
@@ -243,6 +263,14 @@ serial_open (const char *name)
return scb;
}
+/* See serial.h. */
+
+struct serial *
+serial_open_ops (const struct serial_ops *ops)
+{
+ return serial_open_ops_1 (ops, NULL);
+}
+
/* Open a new serial stream using a file handle, using serial
interface ops OPS. */
@@ -261,20 +289,9 @@ serial_fdopen_ops (const int fd, const struct serial_ops *ops)
if (!ops)
return NULL;
- scb = XCNEW (struct serial);
-
- scb->ops = ops;
-
- scb->bufcnt = 0;
- scb->bufp = scb->buf;
- scb->error_fd = -1;
- scb->refcnt = 1;
+ scb = new_serial (ops);
scb->next = scb_base;
- scb->debug_p = 0;
- scb->async_state = 0;
- scb->async_handler = NULL;
- scb->async_context = NULL;
scb_base = scb;
if ((ops->fdopen) != NULL)