rbd: use weak bindings for qemu_aio_flush and qemu_aio_discard; disab…

…le the latter by default

Use weak bindings for qemu_aio_flush and qemu_aio_discard so they
can be linked against old or new versions and select what routines
are used directly. Build in prototypes so building against old headers
and linking to new libraries works. Disable qemu_aio_discard by
default (restore with -D QEMU_USE_RBD_AIO_DISCARD) because discard
is poorly tested in qemu 1.0.

Signed-off-by: Alex Bligh <alex@alex.org.uk>

block/rbd.c

@@ -49,6 +49,36 @@
 #undef LIBRBD_SUPPORTS_DISCARD
 #endif
 
+/*
+ * Treat newer librbd functions as weak symbols so we can detect
+ * whether they're supported at runtime, and disable their use
+ * if they aren't available.
+ */
+#pragma weak rbd_aio_discard
+#pragma weak rbd_aio_flush
+#pragma weak rbd_flush
+
+/* 
+ * If at compile time we don't have the right definitions, include them here
+ */
+#ifndef LIBRBD_SUPPORTS_DISCARD
+int rbd_aio_discard(rbd_image_t image, uint64_t off, uint64_t len, rbd_completion_t c);
+#endif
+
+#ifndef LIBRBD_SUPPORTS_AIO_FLUSH
+int rbd_aio_flush(rbd_image_t image, rbd_completion_t c);
+#endif
+
+/*
+ * Discard is not well tested on qemu-1.0 so disable it unless
+ * QEMU_USE_RBD_DISCARD is enabled at compile time
+ */
+#ifdef QEMU_USE_RBD_DISCARD
+static const int disable_qemu_rbd_discard = 0;
+#else
+static const int disable_qemu_rbd_discard = 1;
+#endif
+
 #define OBJ_MAX_SIZE (1UL << OBJ_DEFAULT_OBJ_ORDER)
 
 #define RBD_MAX_CONF_NAME_SIZE 128
@@ -650,21 +680,21 @@ static int rbd_aio_discard_wrapper(rbd_image_t image,
                                    uint64_t len,
                                    rbd_completion_t comp)
 {
-#ifdef LIBRBD_SUPPORTS_DISCARD
-    return rbd_aio_discard(image, off, len, comp);
-#else
-    return -ENOTSUP;
-#endif
+    if (rbd_aio_discard) {
+        return rbd_aio_discard(image, off, len, comp);
+    } else {
+        return -ENOTSUP;
+    }
 }
 
 static int rbd_aio_flush_wrapper(rbd_image_t image,
                                  rbd_completion_t comp)
 {
-#ifdef LIBRBD_SUPPORTS_AIO_FLUSH
-    return rbd_aio_flush(image, comp);
-#else
-    return -ENOTSUP;
-#endif
+    if (rbd_aio_flush) {
+        return rbd_aio_flush(image, comp);
+    } else {
+        return -ENOTSUP;
+    }
 }
 
 static BlockDriverAIOCB *rbd_start_aio(BlockDriverState *bs,
@@ -773,16 +803,13 @@ static BlockDriverAIOCB *qemu_rbd_aio_writev(BlockDriverState *bs,
                          RBD_AIO_WRITE);
 }
 
-#ifdef LIBRBD_SUPPORTS_AIO_FLUSH
 static BlockDriverAIOCB *qemu_rbd_aio_flush(BlockDriverState *bs,
                                             BlockDriverCompletionFunc *cb,
                                             void *opaque)
 {
     return rbd_start_aio(bs, 0, NULL, 0, cb, opaque, RBD_AIO_FLUSH);
 }
 
-#else
-
 static int qemu_rbd_co_flush(BlockDriverState *bs)
 {
 #if LIBRBD_VERSION_CODE >= LIBRBD_VERSION(0, 1, 1)
@@ -793,7 +820,6 @@ static int qemu_rbd_co_flush(BlockDriverState *bs)
     return 0;
 #endif
 }
-#endif
 
 static int qemu_rbd_getinfo(BlockDriverState *bs, BlockDriverInfo *bdi)
 {
@@ -931,7 +957,6 @@ static int qemu_rbd_snap_list(BlockDriverState *bs,
     return snap_count;
 }
 
-#ifdef LIBRBD_SUPPORTS_DISCARD
 static BlockDriverAIOCB* qemu_rbd_aio_discard(BlockDriverState *bs,
                                               int64_t sector_num,
                                               int nb_sectors,
@@ -941,7 +966,6 @@ static BlockDriverAIOCB* qemu_rbd_aio_discard(BlockDriverState *bs,
     return rbd_start_aio(bs, sector_num, NULL, nb_sectors, cb, opaque,
                          RBD_AIO_DISCARD);
 }
-#endif
 
 static QEMUOptionParameter qemu_rbd_create_options[] = {
     {
@@ -972,15 +996,12 @@ static BlockDriver bdrv_rbd = {
     .bdrv_aio_readv         = qemu_rbd_aio_readv,
     .bdrv_aio_writev        = qemu_rbd_aio_writev,
 
-#ifdef LIBRBD_SUPPORTS_AIO_FLUSH
+    /* select which is used at runtime */
     .bdrv_aio_flush         = qemu_rbd_aio_flush,
-#else
     .bdrv_co_flush_to_disk  = qemu_rbd_co_flush,
-#endif
 
-#ifdef LIBRBD_SUPPORTS_DISCARD
+    /* select if used at runtime */
     .bdrv_aio_discard       = qemu_rbd_aio_discard,
-#endif
 
     .bdrv_snapshot_create   = qemu_rbd_snap_create,
     .bdrv_snapshot_delete   = qemu_rbd_snap_remove,
@@ -990,6 +1011,15 @@ static BlockDriver bdrv_rbd = {
 
 static void bdrv_rbd_init(void)
 {
+    if (!rbd_flush || rbd_aio_flush) {
+        bdrv_rbd.bdrv_co_flush_to_disk = NULL;
+    }
+    if (!rbd_aio_flush) {
+        bdrv_rbd.bdrv_aio_flush = NULL;
+    }
+    if (disable_qemu_rbd_discard || !rbd_aio_discard) {
+        bdrv_rbd.bdrv_aio_discard = NULL;
+    }
     bdrv_register(&bdrv_rbd);
 }