--- /dev/null
+/*
+ * Copyright (C) 2018 - Jérémie Galarneau <jeremie.galarneau@efficios.com>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License, version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef FD_TRACKER_H
+#define FD_TRACKER_H
+
+#include <stdint.h>
+#include <sys/types.h>
+
+struct fs_handle;
+struct fd_tracker;
+
+/*
+ * Callback which returns a file descriptor to track through the fd
+ * tracker. This callback must not make use of the fd_tracker as a deadlock
+ * may occur.
+ *
+ * The int pointer argument is an output parameter that should be used to return
+ * the advertised number of file descriptors.
+ *
+ * Must return zero on success. Negative values should map to a UNIX error code.
+ */
+typedef int (*fd_open_cb)(void *, int *out_fds);
+
+/*
+ * Callback to allow the user to close a now-untracked file descriptor. This
+ * callback must not make use of the fd_tracker as a deadlock may occur.
+ *
+ * The callback can freely modify the in_fds argument as it is copied by the
+ * fd_tracker before being used. The fd tracker assumes in_fds to be closed by
+ * the time the callback returns.
+ *
+ * Must return zero on success. Negative values should map to a UNIX error code.
+ */
+typedef int (*fd_close_cb)(void *, int *in_fds);
+
+/*
+ * Set the maximal number of fds that the process should be allowed to open at
+ * any given time. This function must be called before any other of this
+ * interface.
+ */
+struct fd_tracker *fd_tracker_create(unsigned int capacity);
+
+/* Returns an error if file descriptors are leaked. */
+int fd_tracker_destroy(struct fd_tracker *tracker);
+
+/*
+ * Open a handle to a suspendable filesystem file descriptor.
+ *
+ * See OPEN(3) for an explanation of flags and mode. NULL is returned in case of
+ * error and errno is left untouched. Note that passing NULL as mode will result
+ * in open()'s default behaviour being used (using the process' umask).
+ *
+ * A fs_handle wraps a file descriptor created by OPEN(3). It is suspendable
+ * meaning that the underlying file may be closed at any time unless the
+ * handle is marked as being in-use (see fs_handle_get_fd() and
+ * fs_handle_put_fd()).
+ *
+ * If the tracker opted to close the underlying file descriptor, it will
+ * be restored to its last known state when it is obtained through
+ * the fs_handle's fs_handle_get_fd() method.
+ *
+ * Note that a suspendable file descriptor can be closed by the fd tracker at
+ * anytime when it is not in use. This means that the user should not rely on it
+ * being safe to unlink the file. Moreover, concurent modifications to the file
+ * (e.g. truncation) may react differently than if the file descriptor was kept
+ * open.
+ */
+struct fs_handle *fd_tracker_open_fs_handle(struct fd_tracker *tracker,
+ const char *path, int flags, mode_t *mode);
+
+/*
+ * Open a tracked unsuspendable file descriptor.
+ *
+ * This function allows the fd tracker to keep track of unsuspendable
+ * file descriptors. A callback, open, is passed to allow the tracker
+ * to atomically reserve an entry for a given count of new file descriptors,
+ * suspending file descriptors as needed, and invoke the provided callback
+ * without ever exceeding the tracker's capacity.
+ *
+ * fd_count indicates the count of file descriptors that will be opened and
+ * returned by the open callback. The storage location at out_fds is assumed
+ * to be large enough to hold 'fd_count * sizeof(int)'.
+ *
+ * Names may be provided to allow easier debugging of file descriptor
+ * exhaustions.
+ *
+ * The callback's return value is returned to the user. Additionally, two
+ * negative tracker-specific codes may be returned:
+ * - ENOMEM: allocation of a new entry failed,
+ * - EMFILE: too many unsuspendable fds are opened and the tracker can't
+ * accomodate the request for a new unsuspendable entry.
+ */
+int fd_tracker_open_unsuspendable_fd(struct fd_tracker *tracker,
+ int *out_fds, const char **names, unsigned int fd_count,
+ fd_open_cb open, void *data);
+
+/*
+ * Close a tracked unsuspendable file descriptor.
+ *
+ * This function allows the fd tracker to keep track of unsuspendable
+ * file descriptors. A callback, close, is passed to allow the tracker
+ * to atomically release a file descriptor entry.
+ *
+ * Returns 0 if the close callback returned success. Returns the value returned
+ * by the close callback if it is negative. Additionally, a tracker-specific
+ * code may be returned:
+ * - EINVAL: a file descriptor was unknown to the tracker
+ *
+ * Closed fds are set to -1 in the fds array which, in the event of an error,
+ * allows the user to know which file descriptors are no longer being tracked.
+ */
+int fd_tracker_close_unsuspendable_fd(struct fd_tracker *tracker,
+ int *fds, unsigned int fd_count, fd_close_cb close,
+ void *data);
+
+/*
+ * Log the contents of the fd_tracker.
+ */
+void fd_tracker_log(struct fd_tracker *tracker);
+
+/*
+ * Marks the handle as the most recently used and marks the 'fd' as
+ * "in-use". This prevents the tracker from recycling the underlying
+ * file descriptor while it is actively being used by a thread.
+ *
+ * Don't forget that the tracker may be initiating an fd 'suspension'
+ * from another thread as the need to free an fd slot may arise from any
+ * thread within the daemon.
+ *
+ * Note that a restorable fd should never be held for longer than
+ * strictly necessary (e.g. the duration of a syscall()).
+ *
+ * Returns the fd on success, otherwise a negative value may be returned
+ * if the restoration of the fd failed.
+ */
+int fs_handle_get_fd(struct fs_handle *handle);
+
+/*
+ * Used by the application to signify that it is no longer using the
+ * underlying fd and that it may be suspended.
+ */
+void fs_handle_put_fd(struct fs_handle *handle);
+
+/*
+ * Frees the handle and discards the underlying fd.
+ */
+int fs_handle_close(struct fs_handle *handle);
+
+#endif /* FD_TRACKER_H */
projects / lttng-tools.git / blobdiff