[gegl] buffer: new tile-backend-file that writes to disk in a separate thread



commit bf9d594c4c5c45b1d8fc865cd2a05e16083fbe3d
Author: Ville Sokk <ville sokk gmail com>
Date:   Sun Jul 29 22:14:52 2012 +0300

    buffer: new tile-backend-file that writes to disk in a separate thread

 configure.ac                               |    3 +
 gegl/buffer/Makefile.am                    |    9 +-
 gegl/buffer/gegl-tile-backend-file-async.c | 1199 ++++++++++++++++++++++++++++
 3 files changed, 1210 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index c107d0a..948a942 100644
--- a/configure.ac
+++ b/configure.ac
@@ -217,6 +217,7 @@ case "$target_or_host" in
     ;;
   x86_64-*-*)
     have_x86=yes
+    have_64_bit=yes
     AC_DEFINE(ARCH_X86, 1, [Define to 1 if you are compiling for ix86.])
     AC_DEFINE(ARCH_X86_64, 1, [Define to 1 if you are compiling for amd64.])
     ;;
@@ -226,6 +227,7 @@ case "$target_or_host" in
     ;;
   ppc64-*-* | powerpc64-*)
     have_ppc=yes
+    have_64_bit=yes
     AC_DEFINE(ARCH_PPC, 1, [Define to 1 if you are compiling for PowerPC.])
     AC_DEFINE(ARCH_PPC64, 1, [Define to 1 if you are compiling for PowerPC64.])
     ;;
@@ -233,6 +235,7 @@ case "$target_or_host" in
     ;;
 esac
 
+AM_CONDITIONAL(HAVE_64_BIT, test "x$have_64_bit" = "xyes")  
 
 
 ####################################################
diff --git a/gegl/buffer/Makefile.am b/gegl/buffer/Makefile.am
index 2bd884b..3480f36 100644
--- a/gegl/buffer/Makefile.am
+++ b/gegl/buffer/Makefile.am
@@ -36,7 +36,6 @@ libbuffer_la_SOURCES = \
     gegl-tile-source.c		\
     gegl-tile-storage.c		\
     gegl-tile-backend.c		\
-    gegl-tile-backend-file.c	\
     gegl-tile-backend-ram.c	\
     gegl-tile-handler.c		\
     gegl-tile-handler-private.h	\
@@ -77,3 +76,11 @@ libbuffer_la_SOURCES = \
     gegl-tile-handler-log.h	\
     gegl-tile-handler-zoom.h	\
     gegl-id-pool.h
+
+libbuffer_la_SOURCES += gegl-tile-backend-file-async.c
+#if HAVE_64_BIT
+#libbuffer_la_SOURCES += gegl-tile-backend-file.c
+#else
+#libbuffer_la_SOURCES +=	gegl-tile-backend-file-async.c
+#endif
+
diff --git a/gegl/buffer/gegl-tile-backend-file-async.c b/gegl/buffer/gegl-tile-backend-file-async.c
new file mode 100644
index 0000000..7da0f73
--- /dev/null
+++ b/gegl/buffer/gegl-tile-backend-file-async.c
@@ -0,0 +1,1199 @@
+/* This file is part of GEGL.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 3 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with GEGL; if not, see <http://www.gnu.org/licenses/>.
+ *
+ * Copyright 2006, 2007, 2008 Ãyvind KolÃs <pippin gimp org>
+ *           2012 Ville Sokk <ville sokk gmail com>
+ */
+
+/* This tile backend stores tiles of a GeglBuffer on disk. There's one
+ * thread (shared between instances of the class) that performs all file
+ * operations except reading and opening. Communication between the main
+ * gegl thread and the writer thread is performed using a queue. The
+ * writer thread sleeps if the queue is empty. If a read is performed and
+ * the tile is in the queue then its data is copied from the queue
+ * instead of read from disk.
+ */
+
+#include "config.h"
+
+#include <gio/gio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <string.h>
+#include <errno.h>
+
+#include <glib-object.h>
+#include <glib/gprintf.h>
+#include <glib/gstdio.h>
+
+#include "gegl.h"
+#include "gegl-tile-backend.h"
+#include "gegl-tile-backend-file.h"
+#include "gegl-buffer-index.h"
+#include "gegl-buffer-types.h"
+#include "gegl-debug.h"
+
+
+#ifndef HAVE_FSYNC
+
+#ifdef G_OS_WIN32
+#define fsync _commit
+#endif
+
+#endif
+
+
+struct _GeglTileBackendFile
+{
+  GeglTileBackend  parent_instance;
+
+  /* the path to our buffer */
+  gchar           *path;
+
+  /* the file exist (and we've thus been able to initialize i and o,
+   * the utility call ensure_exist() should be called before any code
+   * using i and o)
+   */
+  gboolean         exist;
+
+  /* total size of file */
+  guint            total;
+
+  /* hashtable containing all entries of buffer, the index is written
+   * to the swapfile conforming to the structures laid out in
+   * gegl-buffer-index.h
+   */
+  GHashTable      *index;
+
+  /* list of offsets to tiles that are free */
+  GSList          *free_list;
+
+  /* offset to next pre allocated tile slot */
+  guint            next_pre_alloc;
+
+  /* revision of last index sync, for cooperated sharing of a buffer
+   * file
+   */
+  guint32          rev;
+
+  /* a local copy of the header that will be written to the file, in a
+   * multiple user per buffer scenario, the flags in the header might
+   * be used for locking/signalling
+   */
+  GeglBufferHeader header;
+
+  /* cached offsets of the file handles to avoid lseek syscall if possible */
+  gint             in_offset;
+  gint             out_offset;
+
+  /* current offset, used when writing the index */
+  gint             offset;
+
+  /* when writing buffer blocks the writer keeps one block unwritten
+   * at all times to be able to keep track of the ->next offsets in
+   * the blocks.
+   */
+  GeglBufferBlock *in_holding;
+
+  /* loading buffer */
+  GList           *tiles;
+
+  /* GFile refering to our buffer */
+  GFile           *file;
+
+  GFileMonitor    *monitor;
+
+  /* number of write operations in the queue for this file */
+  gint             pending_writes;
+
+  /* used for waiting on writes to the file to be finished */
+  GCond           *cond;
+
+  /* for writing */
+  int              o;
+
+  /* for reading */
+  int              i;
+};
+
+typedef enum
+{
+  OP_WRITE,
+  OP_TRUNCATE,
+  OP_SYNC,
+  OP_CLOSE
+} ThreadOp;
+
+typedef struct
+{
+  gint                 length;    /* length of data if writing tile or
+                                     length of file if truncating */
+  guchar              *source;
+  goffset              offset;
+  GeglTileBackendFile *file;      /* the file we are operating on */
+  ThreadOp             operation; /* type of file operation, see above */
+} ThreadParams;
+
+
+static void     gegl_tile_backend_file_ensure_exist (GeglTileBackendFile *self);
+static gboolean gegl_tile_backend_file_write_block  (GeglTileBackendFile *self,
+                                                     GeglBufferBlock     *block);
+static void     gegl_tile_backend_file_dbg_alloc    (int                  size);
+static void     gegl_tile_backend_file_dbg_dealloc  (int                  size);
+
+
+G_DEFINE_TYPE (GeglTileBackendFile, gegl_tile_backend_file, GEGL_TYPE_TILE_BACKEND)
+
+static GObjectClass * parent_class = NULL;
+
+/* this debugging is across all buffers */
+static gint allocs         = 0;
+static gint file_size      = 0;
+static gint peak_allocs    = 0;
+static gint peak_file_size = 0;
+
+static GQueue  queue       = G_QUEUE_INIT;
+static GMutex *mutex = NULL;
+static GCond  *queue_cond  = NULL;
+
+
+static inline void
+gegl_tile_backend_file_finish_writing (GeglTileBackendFile *self)
+{
+  g_mutex_lock (mutex);
+  while (self->pending_writes != 0)
+    g_cond_wait (self->cond, mutex);
+  g_mutex_unlock (mutex);
+}
+
+static inline void
+gegl_tile_backend_file_push_queue (ThreadParams *params)
+{
+  gboolean was_empty;
+
+  g_mutex_lock (mutex);
+
+  was_empty = g_queue_is_empty (&queue);
+  params->file->pending_writes += 1;
+  g_queue_push_tail (&queue, params);
+  if (was_empty)
+    g_cond_signal (queue_cond);
+
+  g_mutex_unlock (mutex);
+}
+
+static inline void
+gegl_tile_backend_file_write (ThreadParams *params)
+{
+  gint    to_be_written = params->length;
+  gint    fd            = params->file->o;
+  goffset offset        = params->offset;
+
+  if (params->file->out_offset != params->offset)
+    {
+      if (lseek (fd, offset, SEEK_SET) < 0)
+        {
+          g_warning ("unable to seek to tile in buffer: %s", g_strerror (errno));
+          return;
+        }
+      params->file->out_offset = params->offset;
+    }
+
+  while (to_be_written > 0)
+    {
+      gint wrote;
+      wrote = write (fd,
+                     params->source + params->length - to_be_written,
+                     to_be_written);
+      if (wrote <= 0)
+        {
+          g_message ("unable to write tile data to self: "
+                     "%s (%d/%d bytes written)",
+                     g_strerror (errno), wrote, to_be_written);
+          break;
+        }
+
+      to_be_written            -= wrote;
+      params->file->out_offset += wrote;;
+    }
+
+  GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "writer thread wrote at %i", (gint)offset);
+}
+
+static gpointer
+gegl_tile_backend_file_writer_thread (gpointer ignored)
+{
+  while (TRUE)
+    {
+      ThreadParams *params;
+
+      g_mutex_lock (mutex);
+
+      while (g_queue_is_empty (&queue))
+        g_cond_wait (queue_cond, mutex);
+
+      params = (ThreadParams *)g_queue_peek_head (&queue);
+      g_mutex_unlock (mutex);
+
+      switch (params->operation)
+        {
+        case OP_WRITE:
+          gegl_tile_backend_file_write (params);
+          break;
+        case OP_TRUNCATE:
+          ftruncate (params->file->o, params->length);
+          break;
+        case OP_SYNC:
+          fsync (params->file->o);
+          break;
+        case OP_CLOSE:
+          if (params->file->o != -1 && params->file->i != -1)
+            {
+              close (params->file->o);
+              close (params->file->i);
+              params->file->i = -1;
+              params->file->o = -1;
+            }
+          break;
+        }
+
+      g_mutex_lock (mutex);
+      g_queue_pop_head (&queue);
+
+      params->file->pending_writes -= 1;
+      if (params->file->pending_writes == 0)
+        g_cond_signal (params->file->cond);
+
+      if (params->operation == OP_WRITE)
+        g_free (params->source);
+
+      g_free (params);
+
+      g_mutex_unlock (mutex);
+    }
+
+  return NULL;
+}
+
+static void
+gegl_tile_backend_file_read_entry (GeglTileBackendFile *self,
+                                   GeglBufferTile      *entry,
+                                   guchar              *dest)
+{
+  gint    tile_size  = gegl_tile_backend_get_tile_size (GEGL_TILE_BACKEND (self));
+  gint    to_be_read = tile_size;
+  goffset offset     = entry->offset;
+
+  gegl_tile_backend_file_ensure_exist (self);
+
+  g_mutex_lock (mutex);
+  if (!g_queue_is_empty (&queue))
+    {
+      GList *link = g_queue_peek_tail_link (&queue);
+
+      while (link)
+        {
+          ThreadParams *queued_op = (ThreadParams *)link->data;
+
+          if (queued_op->file == self && queued_op->offset == offset)
+            {
+              memcpy (dest, queued_op->source, to_be_read);
+              g_mutex_unlock (mutex);
+              GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "read entry %i,%i,%i from write queue", entry->x, entry->y, entry->z);
+              return;
+            }
+          link = link->prev;
+        }
+    }
+  g_mutex_unlock (mutex);
+
+  if (self->in_offset != offset)
+    {
+      if (lseek (self->i, offset, SEEK_SET) < 0)
+        {
+          g_warning ("unable to seek to tile in buffer: %s", g_strerror (errno));
+          return;
+        }
+      self->in_offset = offset;
+    }
+
+  while (to_be_read > 0)
+    {
+      GError *error = NULL;
+      gint    byte_read;
+
+      byte_read = read (self->i, dest + tile_size - to_be_read, to_be_read);
+      if (byte_read <= 0)
+        {
+          g_message ("unable to read tile data from self: "
+                     "%s (%d/%d bytes read) %s",
+                     g_strerror (errno), byte_read, to_be_read, error?error->message:"--");
+          return;
+        }
+      to_be_read      -= byte_read;
+      self->in_offset += byte_read;
+    }
+
+  GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "read entry %i,%i,%i at %i", entry->x, entry->y, entry->z, (gint)offset);
+}
+
+static inline void
+gegl_tile_backend_file_write_entry (GeglTileBackendFile *self,
+                                    GeglBufferTile      *entry,
+                                    guchar              *source)
+{
+  ThreadParams *params     = g_new (ThreadParams, 1);
+  gint          length     = gegl_tile_backend_get_tile_size (GEGL_TILE_BACKEND (self));
+  guchar       *new_source = g_malloc (length);
+
+  gegl_tile_backend_file_ensure_exist (self);
+
+  memcpy (new_source, source, length);
+
+  params->operation = OP_WRITE;
+  params->length    = length;
+  params->offset    = entry->offset;
+  params->file      = self;
+  params->source    = new_source;
+
+  gegl_tile_backend_file_push_queue (params);
+
+  GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "pushed entry write %i,%i,%i at %i", entry->x, entry->y, entry->z, (gint)entry->offset);
+}
+
+static inline GeglBufferTile *
+gegl_tile_backend_file_file_entry_new (GeglTileBackendFile *self)
+{
+  GeglBufferTile *entry = gegl_tile_entry_new (0,0,0);
+
+  GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "Creating new entry");
+
+  gegl_tile_backend_file_ensure_exist (self);
+
+  if (self->free_list)
+    {
+      /* XXX: losing precision ?
+       * the free list seems to operate with fixed size datums and
+       * only keep track of offsets.
+       */
+      gint offset = GPOINTER_TO_INT (self->free_list->data);
+      entry->offset = offset;
+      self->free_list = g_slist_remove (self->free_list, self->free_list->data);
+
+      GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "  set offset %i from free list", ((gint)entry->offset));
+    }
+  else
+    {
+      gint tile_size = gegl_tile_backend_get_tile_size (GEGL_TILE_BACKEND (self));
+
+      entry->offset = self->next_pre_alloc;
+      GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "  set offset %i (next allocation)", (gint)entry->offset);
+      self->next_pre_alloc += tile_size;
+
+      if (self->next_pre_alloc >= self->total) /* automatic growing ensuring that
+                                                  we have room for next allocation..
+                                                */
+        {
+          ThreadParams *params = g_new (ThreadParams, 1);
+
+          self->total       = self->total + 32 * tile_size;
+          params->operation = OP_TRUNCATE;
+          params->file      = self;
+          params->length    = self->total;
+
+          gegl_tile_backend_file_push_queue (params);
+
+          GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "pushed truncate to %i bytes", (gint)self->total);
+
+          self->in_offset = self->out_offset = -1;
+        }
+    }
+  gegl_tile_backend_file_dbg_alloc (gegl_tile_backend_get_tile_size (GEGL_TILE_BACKEND (self)));
+  return entry;
+}
+
+static inline void
+gegl_tile_backend_file_file_entry_destroy (GeglBufferTile      *entry,
+                                           GeglTileBackendFile *self)
+{
+  /* XXX: EEEk, throwing away bits */
+  guint offset = entry->offset;
+  self->free_list = g_slist_prepend (self->free_list,
+                                     GUINT_TO_POINTER (offset));
+  g_hash_table_remove (self->index, entry);
+
+  gegl_tile_backend_file_dbg_dealloc (gegl_tile_backend_get_tile_size (GEGL_TILE_BACKEND (self)));
+  g_free (entry);
+}
+
+static gboolean
+gegl_tile_backend_file_write_header (GeglTileBackendFile *self)
+{
+  ThreadParams *params     = g_new (ThreadParams, 1);
+  guchar       *new_source = g_malloc (256);
+
+  gegl_tile_backend_file_ensure_exist (self);
+
+  memcpy (new_source, &(self->header), 256);
+
+  params->operation = OP_WRITE;
+  params->source    = new_source;
+  params->offset    = 0;
+  params->length    = 256;
+  params->file      = self;
+
+  gegl_tile_backend_file_push_queue (params);
+  GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "pushed header write, next=%i", (gint)self->header.next);
+
+  params            = g_new (ThreadParams, 1);
+  params->operation = OP_SYNC;
+  params->file      = self;
+
+  gegl_tile_backend_file_push_queue (params);
+  GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "pushed sync of %s", self->path);
+
+  return TRUE;
+}
+
+static gboolean
+gegl_tile_backend_file_write_block (GeglTileBackendFile *self,
+                                    GeglBufferBlock     *block)
+{
+  gegl_tile_backend_file_ensure_exist (self);
+  if (self->in_holding)
+    {
+      guint64 next_allocation = self->offset + self->in_holding->length;
+
+      /* update the next offset pointer in the previous block */
+      if (block == NULL)
+          /* the previous block was the last block */
+          self->in_holding->next = 0;
+      else
+          self->in_holding->next = next_allocation;
+
+      if (self->out_offset != self->offset)
+      {
+        if (lseek (self->o, self->offset, G_SEEK_SET) == -1)
+          goto fail;
+
+        self->out_offset = self->offset;
+      }
+
+      /* XXX: should promiscuosuly try to compress here as well,. if revisions
+              are not matching..
+       */
+
+      {
+        ThreadParams *params     = g_new (ThreadParams, 1);
+        gint          length     = self->in_holding->length;
+        guchar       *new_source = g_malloc (length);
+
+        memcpy (new_source, self->in_holding, length);
+
+        params->operation = OP_WRITE;
+        params->length    = length;
+        params->file      = self;
+        params->offset    = self->offset;
+        params->source    = new_source;
+
+        gegl_tile_backend_file_push_queue (params);
+
+        GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "Pushed write of block: length:%i flags:%i next:%i at offset %i",
+                   self->in_holding->length,
+                   self->in_holding->flags,
+                   (gint)self->in_holding->next,
+                   (gint)self->offset);
+      }
+
+      self->offset = next_allocation;
+    }
+  else
+    {
+      /* we're setting up for the first write */
+
+      self->offset = self->next_pre_alloc; /* start writing header at end
+                                            * of file, worry about writing
+                                            * header inside free list later
+                                            */
+      if (self->out_offset != self->offset)
+      {
+        if (lseek (self->o, self->offset, G_SEEK_SET) == -1)
+          goto fail;
+
+        self->out_offset = self->offset;
+      }
+    }
+  self->in_holding = block;
+
+  return TRUE;
+fail:
+  g_warning ("gegl buffer index writing problems for %s",
+             self->path);
+  return FALSE;
+}
+
+void
+gegl_tile_backend_file_stats (void)
+{
+  g_warning ("leaked: %i chunks (%f mb)  peak: %i (%i bytes %fmb))",
+             allocs, file_size / 1024 / 1024.0,
+             peak_allocs, peak_file_size, peak_file_size / 1024 / 1024.0);
+}
+
+static void
+gegl_tile_backend_file_dbg_alloc (gint size)
+{
+  allocs++;
+  file_size += size;
+  if (allocs > peak_allocs)
+    peak_allocs = allocs;
+  if (file_size > peak_file_size)
+    peak_file_size = file_size;
+}
+
+static void
+gegl_tile_backend_file_dbg_dealloc (gint size)
+{
+  allocs--;
+  file_size -= size;
+}
+
+static inline GeglBufferTile *
+gegl_tile_backend_file_lookup_entry (GeglTileBackendFile *self,
+                                     gint                 x,
+                                     gint                 y,
+                                     gint                 z)
+{
+  GeglBufferTile *ret;
+  GeglBufferTile *key = gegl_tile_entry_new (x,y,z);
+  ret = g_hash_table_lookup (self->index, key);
+  gegl_tile_entry_destroy (key);
+  return ret;
+}
+
+/* this is the only place that actually should
+ * instantiate tiles, when the cache is large enough
+ * that should make sure we don't hit this function
+ * too often.
+ */
+static GeglTile *
+gegl_tile_backend_file_get_tile (GeglTileSource *self,
+                                 gint            x,
+                                 gint            y,
+                                 gint            z)
+{
+  GeglTileBackend     *backend;
+  GeglTileBackendFile *tile_backend_file;
+  GeglBufferTile      *entry;
+  GeglTile            *tile = NULL;
+  gint                 tile_size;
+
+  backend           = GEGL_TILE_BACKEND (self);
+  tile_backend_file = GEGL_TILE_BACKEND_FILE (backend);
+  entry             = gegl_tile_backend_file_lookup_entry (tile_backend_file, x, y, z);
+
+  if (!entry)
+    return NULL;
+
+  tile_size = gegl_tile_backend_get_tile_size (GEGL_TILE_BACKEND (self));
+  tile      = gegl_tile_new (tile_size);
+  gegl_tile_set_rev (tile, entry->rev);
+  gegl_tile_mark_as_stored (tile);
+
+  gegl_tile_backend_file_read_entry (tile_backend_file, entry, gegl_tile_get_data (tile));
+  return tile;
+}
+
+static gpointer
+gegl_tile_backend_file_set_tile (GeglTileSource *self,
+                                 GeglTile       *tile,
+                                 gint            x,
+                                 gint            y,
+                                 gint            z)
+{
+  GeglTileBackend     *backend;
+  GeglTileBackendFile *tile_backend_file;
+  GeglBufferTile      *entry;
+
+  backend           = GEGL_TILE_BACKEND (self);
+  tile_backend_file = GEGL_TILE_BACKEND_FILE (backend);
+  entry             = gegl_tile_backend_file_lookup_entry (tile_backend_file, x, y, z);
+
+  if (entry == NULL)
+    {
+      entry    = gegl_tile_backend_file_file_entry_new (tile_backend_file);
+      entry->x = x;
+      entry->y = y;
+      entry->z = z;
+      g_hash_table_insert (tile_backend_file->index, entry, entry);
+    }
+  entry->rev = gegl_tile_get_rev (tile);
+
+  gegl_tile_backend_file_write_entry (tile_backend_file, entry, gegl_tile_get_data (tile));
+  gegl_tile_mark_as_stored (tile);
+  return NULL;
+}
+
+static gpointer
+gegl_tile_backend_file_void_tile (GeglTileSource *self,
+                                  GeglTile       *tile,
+                                  gint            x,
+                                  gint            y,
+                                  gint            z)
+{
+  GeglTileBackend     *backend;
+  GeglTileBackendFile *tile_backend_file;
+  GeglBufferTile      *entry;
+
+  backend           = GEGL_TILE_BACKEND (self);
+  tile_backend_file = GEGL_TILE_BACKEND_FILE (backend);
+  entry             = gegl_tile_backend_file_lookup_entry (tile_backend_file, x, y, z);
+
+  if (entry != NULL)
+    {
+      gegl_tile_backend_file_file_entry_destroy (entry, tile_backend_file);
+    }
+
+  return NULL;
+}
+
+static gpointer
+gegl_tile_backend_file_exist_tile (GeglTileSource *self,
+                                   GeglTile       *tile,
+                                   gint            x,
+                                   gint            y,
+                                   gint            z)
+{
+  GeglTileBackend     *backend;
+  GeglTileBackendFile *tile_backend_file;
+  GeglBufferTile      *entry;
+
+  backend           = GEGL_TILE_BACKEND (self);
+  tile_backend_file = GEGL_TILE_BACKEND_FILE (backend);
+  entry             = gegl_tile_backend_file_lookup_entry (tile_backend_file, x, y, z);
+
+  return entry!=NULL?((gpointer)0x1):NULL;
+}
+
+static gpointer
+gegl_tile_backend_file_flush (GeglTileSource *source,
+                              GeglTile       *tile,
+                              gint            x,
+                              gint            y,
+                              gint            z)
+{
+  GeglTileBackend     *backend;
+  GeglTileBackendFile *self;
+  GList               *tiles;
+
+  backend  = GEGL_TILE_BACKEND (source);
+  self     = GEGL_TILE_BACKEND_FILE (backend);
+
+  gegl_tile_backend_file_ensure_exist (self);
+
+  GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "flushing %s", self->path);
+
+  self->header.rev ++;
+  self->header.next = self->next_pre_alloc; /* this is the offset
+                                               we start handing
+                                               out headers from*/
+  tiles = g_hash_table_get_keys (self->index);
+
+  if (tiles == NULL)
+    self->header.next = 0;
+  else
+    {
+      GList *iter;
+      for (iter = tiles; iter; iter = iter->next)
+        {
+          GeglBufferItem *item = iter->data;
+
+          gegl_tile_backend_file_write_block (self, &item->block);
+        }
+      gegl_tile_backend_file_write_block (self, NULL); /* terminate the index */
+      g_list_free (tiles);
+    }
+
+  gegl_tile_backend_file_write_header (self);
+
+  GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "flushed %s", self->path);
+
+  return (gpointer)0xf0f;
+}
+
+enum
+{
+  PROP_0,
+  PROP_PATH
+};
+
+static gpointer
+gegl_tile_backend_file_command (GeglTileSource  *self,
+                                GeglTileCommand  command,
+                                gint             x,
+                                gint             y,
+                                gint             z,
+                                gpointer         data)
+{
+  switch (command)
+    {
+      case GEGL_TILE_GET:
+        return gegl_tile_backend_file_get_tile (self, x, y, z);
+      case GEGL_TILE_SET:
+        return gegl_tile_backend_file_set_tile (self, data, x, y, z);
+
+      case GEGL_TILE_IDLE:
+        return NULL;       /* we could perhaps lazily be writing indexes
+                            * at some intervals, making it work as an
+                            * autosave for the buffer?
+                            */
+
+      case GEGL_TILE_VOID:
+        return gegl_tile_backend_file_void_tile (self, data, x, y, z);
+
+      case GEGL_TILE_EXIST:
+        return gegl_tile_backend_file_exist_tile (self, data, x, y, z);
+      case GEGL_TILE_FLUSH:
+        return gegl_tile_backend_file_flush (self, data, x, y, z);
+
+      default:
+        g_assert (command < GEGL_TILE_LAST_COMMAND &&
+                  command >= 0);
+    }
+  return FALSE;
+}
+
+static void
+gegl_tile_backend_file_set_property (GObject      *object,
+                                     guint         property_id,
+                                     const GValue *value,
+                                     GParamSpec   *pspec)
+{
+  GeglTileBackendFile *self = GEGL_TILE_BACKEND_FILE (object);
+
+  switch (property_id)
+    {
+      case PROP_PATH:
+        if (self->path)
+          g_free (self->path);
+        self->path = g_value_dup_string (value);
+        break;
+
+      default:
+        G_OBJECT_WARN_INVALID_PROPERTY_ID (object, property_id, pspec);
+        break;
+    }
+}
+
+static void
+gegl_tile_backend_file_get_property (GObject    *object,
+                                     guint       property_id,
+                                     GValue     *value,
+                                     GParamSpec *pspec)
+{
+  GeglTileBackendFile *self = GEGL_TILE_BACKEND_FILE (object);
+
+  switch (property_id)
+    {
+      case PROP_PATH:
+        g_value_set_string (value, self->path);
+        break;
+
+      default:
+        G_OBJECT_WARN_INVALID_PROPERTY_ID (object, property_id, pspec);
+        break;
+    }
+}
+
+static void
+gegl_tile_backend_file_finalize (GObject *object)
+{
+  GeglTileBackendFile *self = (GeglTileBackendFile *) object;
+
+  if (self->index)
+    g_hash_table_unref (self->index);
+
+  if (self->exist)
+    {
+      ThreadParams *params = g_new (ThreadParams, 1);
+
+      params->operation = OP_CLOSE;
+      params->file = (GeglTileBackendFile *) self;
+
+      gegl_tile_backend_file_push_queue (params);
+
+      GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "pushed close of %s", self->path);
+    }
+
+  gegl_tile_backend_file_finish_writing (self);
+
+  if (self->path)
+    g_free (self->path);
+
+  if (self->monitor)
+    g_object_unref (self->monitor);
+
+  if (self->file)
+    g_object_unref (self->file);
+
+  (*G_OBJECT_CLASS (parent_class)->finalize)(object);
+}
+
+static guint
+gegl_tile_backend_file_hashfunc (gconstpointer key)
+{
+  const GeglBufferTile *e    = key;
+  guint                 hash;
+  gint                  i;
+  gint                  srcA = e->x;
+  gint                  srcB = e->y;
+  gint                  srcC = e->z;
+
+  /* interleave the 10 least significant bits of all coordinates,
+   * this gives us Z-order / morton order of the space and should
+   * work well as a hash
+   */
+  hash = 0;
+  for (i = 9; i >= 0; i--)
+    {
+#define ADD_BIT(bit)    do { hash |= (((bit) != 0) ? 1 : 0); hash <<= 1; } while (0)
+      ADD_BIT (srcA & (1 << i));
+      ADD_BIT (srcB & (1 << i));
+      ADD_BIT (srcC & (1 << i));
+#undef ADD_BIT
+    }
+  return hash;
+}
+
+static gboolean
+gegl_tile_backend_file_equalfunc (gconstpointer a,
+                                  gconstpointer b)
+{
+  const GeglBufferTile *ea = a;
+  const GeglBufferTile *eb = b;
+
+  if (ea->x == eb->x &&
+      ea->y == eb->y &&
+      ea->z == eb->z)
+    return TRUE;
+
+  return FALSE;
+}
+
+
+static void
+gegl_tile_backend_file_load_index (GeglTileBackendFile *self,
+                                   gboolean             block)
+{
+  GeglBufferHeader  new_header;
+  GList            *iter;
+  GeglTileBackend  *backend;
+  goffset           offset = 0;
+  goffset           max    = 0;
+  gint              tile_size;
+
+  /* compute total from and next pre alloc by monitoring tiles as they
+   * are added here
+   */
+  /* reload header */
+  new_header = gegl_buffer_read_header (self->i, &offset)->header;
+
+  while (new_header.flags & GEGL_FLAG_LOCKED)
+    {
+      g_usleep (50000);
+      new_header = gegl_buffer_read_header (self->i, &offset)->header;
+    }
+
+  if (new_header.rev == self->header.rev)
+    {
+      GEGL_NOTE(GEGL_DEBUG_TILE_BACKEND, "header not changed: %s", self->path);
+      return;
+    }
+  else
+    {
+      self->header = new_header;
+      GEGL_NOTE(GEGL_DEBUG_TILE_BACKEND, "loading index: %s", self->path);
+    }
+
+  tile_size       = gegl_tile_backend_get_tile_size (GEGL_TILE_BACKEND (self));
+  offset          = self->header.next;
+  self->tiles     = gegl_buffer_read_index (self->i, &offset);
+  self->in_offset = self->out_offset = -1;
+  backend         = GEGL_TILE_BACKEND (self);
+
+  for (iter = self->tiles; iter; iter=iter->next)
+    {
+      GeglBufferItem *item = iter->data;
+      GeglBufferItem *existing = g_hash_table_lookup (self->index, item);
+
+      if (item->tile.offset > max)
+        max = item->tile.offset + tile_size;
+
+      if (existing)
+        {
+          if (existing->tile.rev == item->tile.rev)
+            {
+              g_assert (existing->tile.offset == item->tile.offset);
+              existing->tile = item->tile;
+              g_free (item);
+              continue;
+            }
+          else
+            {
+              GeglTileStorage *storage =
+                (void*)gegl_tile_backend_peek_storage (backend);
+              GeglRectangle rect;
+              g_hash_table_remove (self->index, existing);
+
+              gegl_tile_source_refetch (GEGL_TILE_SOURCE (storage),
+                                        existing->tile.x,
+                                        existing->tile.y,
+                                        existing->tile.z);
+
+              if (existing->tile.z == 0)
+                {
+                  rect.width = self->header.tile_width;
+                  rect.height = self->header.tile_height;
+                  rect.x = existing->tile.x * self->header.tile_width;
+                  rect.y = existing->tile.y * self->header.tile_height;
+                }
+              g_free (existing);
+
+              g_signal_emit_by_name (storage, "changed", &rect, NULL);
+            }
+        }
+      g_hash_table_insert (self->index, iter->data, iter->data);
+    }
+  g_list_free (self->tiles);
+  g_slist_free (self->free_list);
+  self->free_list      = NULL;
+  self->next_pre_alloc = max; /* if bigger than own? */
+  self->total          = max;
+  self->tiles          = NULL;
+}
+
+static void
+gegl_tile_backend_file_file_changed (GFileMonitor        *monitor,
+                                     GFile               *file,
+                                     GFile               *other_file,
+                                     GFileMonitorEvent    event_type,
+                                     GeglTileBackendFile *self)
+{
+  if (event_type == G_FILE_MONITOR_EVENT_CHANGED)
+    {
+      gegl_tile_backend_file_load_index (self, TRUE);
+      self->in_offset = self->out_offset = -1;
+    }
+}
+
+static GObject *
+gegl_tile_backend_file_constructor (GType                  type,
+                                    guint                  n_params,
+                                    GObjectConstructParam *params)
+{
+  GObject             *object;
+  GeglTileBackendFile *self;
+  GeglTileBackend     *backend;
+
+  object  = G_OBJECT_CLASS (parent_class)->constructor (type, n_params, params);
+  self    = GEGL_TILE_BACKEND_FILE (object);
+  backend = GEGL_TILE_BACKEND (object);
+
+  GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "constructing file backend: %s", self->path);
+
+  self->file           = g_file_new_for_commandline_arg (self->path);
+  self->i              = self->o = -1;
+  self->index          = g_hash_table_new (gegl_tile_backend_file_hashfunc,
+                                           gegl_tile_backend_file_equalfunc);
+  self->pending_writes = 0;
+  self->cond           = g_cond_new ();
+
+  /* If the file already exists open it, assuming it is a GeglBuffer. */
+  if (g_access (self->path, F_OK) != -1)
+    {
+      goffset offset = 0;
+
+      /* Install a monitor for changes to the file in case other applications
+       * might be writing to the buffer
+       */
+      self->monitor = g_file_monitor_file (self->file, G_FILE_MONITOR_NONE,
+                                           NULL, NULL);
+      g_signal_connect (self->monitor, "changed",
+                        G_CALLBACK (gegl_tile_backend_file_file_changed),
+                        self);
+
+      self->o = g_open (self->path, O_RDWR|O_CREAT, 0770);
+      if (self->o == -1)
+        {
+          /* Try again but this time with only read access. This is
+           * a quick-fix for make distcheck, where img_cmp fails
+           * when it opens a GeglBuffer file in the source tree
+           * (which is read-only).
+           */
+          self->o = g_open (self->path, O_RDONLY, 0770);
+
+          if (self->o == -1)
+            g_warning ("%s: Could not open '%s': %s", G_STRFUNC, self->path, g_strerror (errno));
+        }
+      self->i = g_open (self->path, O_RDONLY);
+
+      self->header     = gegl_buffer_read_header (self->i, &offset)->header;
+      self->header.rev = self->header.rev -1;
+
+      /* we are overriding all of the work of the actual constructor here,
+       * a really evil hack :d
+       */
+      backend->priv->tile_width  = self->header.tile_width;
+      backend->priv->tile_height = self->header.tile_height;
+      backend->priv->format      = babl_format (self->header.description);
+      backend->priv->px_size     = babl_format_get_bytes_per_pixel (backend->priv->format);
+      backend->priv->tile_size   = backend->priv->tile_width *
+                                    backend->priv->tile_height *
+                                    backend->priv->px_size;
+
+      /* insert each of the entries into the hash table */
+      gegl_tile_backend_file_load_index (self, TRUE);
+      self->exist = TRUE;
+      g_assert (self->i != -1);
+      g_assert (self->o != -1);
+
+      /* to autoflush gegl_buffer_set */
+
+      /* XXX: poking at internals, icky */
+      backend->priv->shared = TRUE;
+    }
+  else
+    {
+      self->exist = FALSE; /* this is also the default, the file will be created on demand */
+    }
+
+  g_assert (self->file);
+
+  backend->priv->header = &self->header;
+
+  return object;
+}
+
+static void
+gegl_tile_backend_file_ensure_exist (GeglTileBackendFile *self)
+{
+  if (!self->exist)
+    {
+      GeglTileBackend *backend;
+      self->exist = TRUE;
+
+      backend = GEGL_TILE_BACKEND (self);
+
+      GEGL_NOTE (GEGL_DEBUG_TILE_BACKEND, "creating swapfile  %s", self->path);
+
+      self->o = g_open (self->path, O_RDWR|O_CREAT, 0770);
+      if (self->o == -1)
+        g_warning ("%s: Could not open '%s': %s", G_STRFUNC, self->path, g_strerror (errno));
+
+      self->next_pre_alloc = 256; /* reserved space for header */
+      self->total          = 256; /* reserved space for header */
+      self->in_offset      = self->out_offset = 0;
+      self->pending_writes = 0;
+      gegl_buffer_header_init (&self->header,
+                               backend->priv->tile_width,
+                               backend->priv->tile_height,
+                               backend->priv->px_size,
+                               backend->priv->format);
+      gegl_tile_backend_file_write_header (self);
+      self->i = g_open (self->path, O_RDONLY);
+
+      g_assert (self->i != -1);
+      g_assert (self->o != -1);
+    }
+}
+
+static void
+gegl_tile_backend_file_class_init (GeglTileBackendFileClass *klass)
+{
+  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
+
+  parent_class = g_type_class_peek_parent (klass);
+
+  gobject_class->get_property = gegl_tile_backend_file_get_property;
+  gobject_class->set_property = gegl_tile_backend_file_set_property;
+  gobject_class->constructor  = gegl_tile_backend_file_constructor;
+  gobject_class->finalize     = gegl_tile_backend_file_finalize;
+
+  queue_cond = g_cond_new ();
+  mutex      = g_mutex_new ();
+  g_thread_create_full (gegl_tile_backend_file_writer_thread,
+                        NULL, 0, TRUE, TRUE, G_THREAD_PRIORITY_NORMAL, NULL);
+
+  GEGL_BUFFER_STRUCT_CHECK_PADDING;
+
+  g_object_class_install_property (gobject_class, PROP_PATH,
+                                   g_param_spec_string ("path",
+                                                        "path",
+                                                        "The base path for this backing file for a buffer",
+                                                        NULL,
+                                                        G_PARAM_CONSTRUCT |
+                                                        G_PARAM_READWRITE));
+}
+
+static void
+gegl_tile_backend_file_init (GeglTileBackendFile *self)
+{
+  ((GeglTileSource*)self)->command = gegl_tile_backend_file_command;
+  self->path                       = NULL;
+  self->file                       = NULL;
+  self->i                          = -1;
+  self->o                          = -1;
+  self->index                      = NULL;
+  self->free_list                  = NULL;
+  self->next_pre_alloc             = 256; /* reserved space for header */
+  self->total                      = 256; /* reserved space for header */
+  self->pending_writes             = 0;
+}
+
+gboolean
+gegl_tile_backend_file_try_lock (GeglTileBackendFile *self)
+{
+  GeglBufferHeader new_header;
+
+  new_header = gegl_buffer_read_header (self->i, NULL)->header;
+  if (new_header.flags & GEGL_FLAG_LOCKED)
+    {
+      return FALSE;
+    }
+  self->header.flags += GEGL_FLAG_LOCKED;
+  gegl_tile_backend_file_write_header (self);
+
+  return TRUE;
+}
+
+gboolean
+gegl_tile_backend_file_unlock (GeglTileBackendFile *self)
+{
+  if (!(self->header.flags & GEGL_FLAG_LOCKED))
+    {
+      g_warning ("tried to unlock unlocked buffer");
+      return FALSE;
+    }
+  self->header.flags -= GEGL_FLAG_LOCKED;
+  gegl_tile_backend_file_write_header (self);
+
+  /* wait until all writes to this file are finished before handing it over
+     to another process */
+  gegl_tile_backend_file_finish_writing (self);
+
+  return TRUE;
+}



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