src/common/fd-tracker/fd-tracker.h

   1 /*
   2  * Copyright (C) 2018 - Jérémie Galarneau <jeremie.galarneau@efficios.com>
   3  *
   4  * This program is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License, version 2 only, as
   6  * published by the Free Software Foundation.
   7  *
   8  * This program is distributed in the hope that it will be useful, but WITHOUT
   9  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  10  * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
  11  * more details.
  12  *
  13  * You should have received a copy of the GNU General Public License along with
  14  * this program; if not, write to the Free Software Foundation, Inc., 51
  15  * Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  16  */
  17
  18 #ifndef FD_TRACKER_H
  19 #define FD_TRACKER_H
  20
  21 #include <stdint.h>
  22 #include <sys/types.h>
  23
  24 struct fs_handle;
  25 struct fd_tracker;
  26
  27 /*
  28  * Callback which returns a file descriptor to track through the fd
  29  * tracker. This callback must not make use of the fd_tracker as a deadlock
  30  * may occur.
  31  *
  32  * The int pointer argument is an output parameter that should be used to return
  33  * the advertised number of file descriptors.
  34  *
  35  * Must return zero on success. Negative values should map to a UNIX error code.
  36  */
  37 typedef int (*fd_open_cb)(void *, int *out_fds);
  38
  39 /*
  40  * Callback to allow the user to close a now-untracked file descriptor. This
  41  * callback must not make use of the fd_tracker as a deadlock may occur.
  42  *
  43  * The callback can freely modify the in_fds argument as it is copied by the
  44  * fd_tracker before being used. The fd tracker assumes in_fds to be closed by
  45  * the time the callback returns.
  46  *
  47  * Must return zero on success. Negative values should map to a UNIX error code.
  48  */
  49 typedef int (*fd_close_cb)(void *, int *in_fds);
  50
  51 /*
  52  * Set the maximal number of fds that the process should be allowed to open at
  53  * any given time. This function must be called before any other of this
  54  * interface.
  55  */
  56 struct fd_tracker *fd_tracker_create(unsigned int capacity);
  57
  58 /* Returns an error if file descriptors are leaked. */
  59 int fd_tracker_destroy(struct fd_tracker *tracker);
  60
  61 /*
  62  * Open a handle to a suspendable filesystem file descriptor.
  63  *
  64  * See OPEN(3) for an explanation of flags and mode. NULL is returned in case of
  65  * error and errno is left untouched. Note that passing NULL as mode will result
  66  * in open()'s default behaviour being used (using the process' umask).
  67  *
  68  * A fs_handle wraps a file descriptor created by OPEN(3). It is suspendable
  69  * meaning that the underlying file may be closed at any time unless the
  70  * handle is marked as being in-use (see fs_handle_get_fd() and
  71  * fs_handle_put_fd()).
  72  *
  73  * If the tracker opted to close the underlying file descriptor, it will
  74  * be restored to its last known state when it is obtained through
  75  * the fs_handle's fs_handle_get_fd() method.
  76  *
  77  * Note that a suspendable file descriptor can be closed by the fd tracker at
  78  * anytime when it is not in use. This means that the user should not rely on it
  79  * being safe to unlink the file. Moreover, concurent modifications to the file
  80  * (e.g. truncation) may react differently than if the file descriptor was kept
  81  * open.
  82  */
  83 struct fs_handle *fd_tracker_open_fs_handle(struct fd_tracker *tracker,
  84                 const char *path,
  85                 int flags,
  86                 mode_t *mode);
  87
  88 /*
  89  * Open a tracked unsuspendable file descriptor.
  90  *
  91  * This function allows the fd tracker to keep track of unsuspendable
  92  * file descriptors. A callback, open, is passed to allow the tracker
  93  * to atomically reserve an entry for a given count of new file descriptors,
  94  * suspending file descriptors as needed, and invoke the provided callback
  95  * without ever exceeding the tracker's capacity.
  96  *
  97  * fd_count indicates the count of file descriptors that will be opened and
  98  * returned by the open callback. The storage location at out_fds is assumed
  99  * to be large enough to hold 'fd_count * sizeof(int)'.
 100  *
 101  * Names may be provided to allow easier debugging of file descriptor
 102  * exhaustions.
 103  *
 104  * The callback's return value is returned to the user. Additionally, two
 105  * negative tracker-specific codes may be returned:
 106  *   - ENOMEM: allocation of a new entry failed,
 107  *   - EMFILE: too many unsuspendable fds are opened and the tracker can't
 108  *             accomodate the request for a new unsuspendable entry.
 109  */
 110 int fd_tracker_open_unsuspendable_fd(struct fd_tracker *tracker,
 111                 int *out_fds,
 112                 const char **names,
 113                 unsigned int fd_count,
 114                 fd_open_cb open,
 115                 void *data);
 116
 117 /*
 118  * Close a tracked unsuspendable file descriptor.
 119  *
 120  * This function allows the fd tracker to keep track of unsuspendable
 121  * file descriptors. A callback, close, is passed to allow the tracker
 122  * to atomically release a file descriptor entry.
 123  *
 124  * Returns 0 if the close callback returned success. Returns the value returned
 125  * by the close callback if it is negative. Additionally, a tracker-specific
 126  * code may be returned:
 127  *   - EINVAL: a file descriptor was unknown to the tracker
 128  *
 129  * Closed fds are set to -1 in the fds array which, in the event of an error,
 130  * allows the user to know which file descriptors are no longer being tracked.
 131  */
 132 int fd_tracker_close_unsuspendable_fd(struct fd_tracker *tracker,
 133                 int *fds,
 134                 unsigned int fd_count,
 135                 fd_close_cb close,
 136                 void *data);
 137
 138 /*
 139  * Log the contents of the fd_tracker.
 140  */
 141 void fd_tracker_log(struct fd_tracker *tracker);
 142
 143 /*
 144  * Marks the handle as the most recently used and marks the 'fd' as
 145  * "in-use". This prevents the tracker from recycling the underlying
 146  * file descriptor while it is actively being used by a thread.
 147  *
 148  * Don't forget that the tracker may be initiating an fd 'suspension'
 149  * from another thread as the need to free an fd slot may arise from any
 150  * thread within the daemon.
 151  *
 152  * Note that a restorable fd should never be held for longer than
 153  * strictly necessary (e.g. the duration of a syscall()).
 154  *
 155  * Returns the fd on success, otherwise a negative value may be returned
 156  * if the restoration of the fd failed.
 157  */
 158 int fs_handle_get_fd(struct fs_handle *handle);
 159
 160 /*
 161  * Used by the application to signify that it is no longer using the
 162  * underlying fd and that it may be suspended.
 163  */
 164 void fs_handle_put_fd(struct fs_handle *handle);
 165
 166 /*
 167  * Unlink the file associated to an fs_handle. Note that the unlink
 168  * operation will not be performed immediately. It will only be performed
 169  * once all references to the underlying file (through other fs_handle objects)
 170  * have been released.
 171  *
 172  * However, note that the file will be renamed so as to provide the observable
 173  * effect of an unlink(), that is removing a name from the filesystem.
 174  *
 175  * Returns 0 on success, otherwise a negative value will be returned
 176  * if the operation failed.
 177  */
 178 int fs_handle_unlink(struct fs_handle *handle);
 179
 180 /*
 181  * Frees the handle and discards the underlying fd.
 182  */
 183 int fs_handle_close(struct fs_handle *handle);
 184
 185 #endif /* FD_TRACKER_H */