qcow2: More error handling fixes

index 22b0b45..ca6b373 100644
@@ -105,11 +105,17 @@ static int load_refcount_block(BlockDriverState *bs,
     return 0;
 }
 
+/*
+ * Returns the refcount of the cluster given by its index. Any non-negative
+ * return value is the refcount of the cluster, negative values are -errno
+ * and indicate an error.
+ */
 static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
 {
     BDRVQcowState *s = bs->opaque;
     int refcount_table_index, block_index;
     int64_t refcount_block_offset;
+    int ret;
 
     refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
     if (refcount_table_index >= s->refcount_table_size)

@@ -119,8 +125,10 @@ static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
         return 0;
     if (refcount_block_offset != s->refcount_block_cache_offset) {
         /* better than nothing: return allocated if read error */
-        if (load_refcount_block(bs, refcount_block_offset) < 0)
-            return 1;
+        ret = load_refcount_block(bs, refcount_block_offset);
+        if (ret < 0) {
+            return ret;
+        }
     }
     block_index = cluster_index &
         ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);

@@ -538,7 +546,13 @@ fail:
     return ret;
 }
 
-/* addend must be 1 or -1 */
+/*
+ * Increases or decreases the refcount of a given cluster by one.
+ * addend must be 1 or -1.
+ *
+ * If the return value is non-negative, it is the new refcount of the cluster.
+ * If it is negative, it is -errno and indicates an error.
+ */
 static int update_cluster_refcount(BlockDriverState *bs,
                                    int64_t cluster_index,
                                    int addend)

@@ -779,6 +793,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
                         } else {
                             refcount = get_refcount(bs, offset >> s->cluster_bits);
                         }
+
+                        if (refcount < 0) {
+                            goto fail;
+                        }
                     }
 
                     if (refcount == 1) {

@@ -801,7 +819,9 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
             } else {
                 refcount = get_refcount(bs, l2_offset >> s->cluster_bits);
             }
-            if (refcount == 1) {
+            if (refcount < 0) {
+                goto fail;
+            } else if (refcount == 1) {
                 l2_offset |= QCOW_OFLAG_COPIED;
             }
             if (l2_offset != old_l2_offset) {

@@ -934,6 +954,10 @@ static int check_refcounts_l2(BlockDriverState *bs,
                     uint64_t entry = offset;
                     offset &= ~QCOW_OFLAG_COPIED;
                     refcount = get_refcount(bs, offset >> s->cluster_bits);
+                    if (refcount < 0) {
+                        fprintf(stderr, "Can't get refcount for offset %"
+                            PRIx64 ": %s\n", entry, strerror(-refcount));
+                    }
                     if ((refcount == 1) != ((entry & QCOW_OFLAG_COPIED) != 0)) {
                         fprintf(stderr, "ERROR OFLAG_COPIED: offset=%"
                             PRIx64 " refcount=%d\n", entry, refcount);

@@ -1011,6 +1035,10 @@ static int check_refcounts_l1(BlockDriverState *bs,
             if (check_copied) {
                 refcount = get_refcount(bs, (l2_offset & ~QCOW_OFLAG_COPIED)
                     >> s->cluster_bits);
+                if (refcount < 0) {
+                    fprintf(stderr, "Can't get refcount for l2_offset %"
+                        PRIx64 ": %s\n", l2_offset, strerror(-refcount));
+                }
                 if ((refcount == 1) != ((l2_offset & QCOW_OFLAG_COPIED) != 0)) {
                     fprintf(stderr, "ERROR OFLAG_COPIED: l2_offset=%" PRIx64
                         " refcount=%d\n", l2_offset, refcount);

@@ -1118,6 +1146,11 @@ int qcow2_check_refcounts(BlockDriverState *bs)
     /* compare ref counts */
     for(i = 0; i < nb_clusters; i++) {
         refcount1 = get_refcount(bs, i);
+        if (refcount1 < 0) {
+            fprintf(stderr, "Can't get refcount for cluster %d: %s\n",
+                i, strerror(-refcount1));
+        }
+
         refcount2 = refcount_table[i];
         if (refcount1 != refcount2) {
             fprintf(stderr, "ERROR cluster %d refcount=%d reference=%d\n",

index ca6b373..51948ae 100644
@@ -228,7 +228,10 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
     }
 
     /* Allocate the refcount block itself and mark it as used */
-    uint64_t new_block = alloc_clusters_noref(bs, s->cluster_size);
+    int64_t new_block = alloc_clusters_noref(bs, s->cluster_size);
+    if (new_block < 0) {
+        return new_block;
+    }
 
 #ifdef DEBUG_ALLOC2
     fprintf(stderr, "qcow2: Allocate refcount block %d for %" PRIx64

@@ -579,14 +582,19 @@ static int update_cluster_refcount(BlockDriverState *bs,
 static int64_t alloc_clusters_noref(BlockDriverState *bs, int64_t size)
 {
     BDRVQcowState *s = bs->opaque;
-    int i, nb_clusters;
+    int i, nb_clusters, refcount;
 
     nb_clusters = size_to_clusters(s, size);
 retry:
     for(i = 0; i < nb_clusters; i++) {
         int64_t next_cluster_index = s->free_cluster_index++;
-        if (get_refcount(bs, next_cluster_index) != 0)
+        refcount = get_refcount(bs, next_cluster_index);
+
+        if (refcount < 0) {
+            return refcount;
+        } else if (refcount != 0) {
             goto retry;
+        }
     }
 #ifdef DEBUG_ALLOC2
     printf("alloc_clusters: size=%" PRId64 " -> %" PRId64 "\n",

@@ -603,6 +611,10 @@ int64_t qcow2_alloc_clusters(BlockDriverState *bs, int64_t size)
 
     BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC);
     offset = alloc_clusters_noref(bs, size);
+    if (offset < 0) {
+        return offset;
+    }
+
     ret = update_refcount(bs, offset, size, 1);
     if (ret < 0) {
         return ret;

index 51948ae..41e1da9 100644
@@ -93,14 +93,19 @@ static int load_refcount_block(BlockDriverState *bs,
     int ret;
 
     if (cache_refcount_updates) {
-        write_refcount_block(bs);
+        ret = write_refcount_block(bs);
+        if (ret < 0) {
+            return ret;
+        }
     }
 
     BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_LOAD);
     ret = bdrv_pread(bs->file, refcount_block_offset, s->refcount_block_cache,
                      s->cluster_size);
-    if (ret != s->cluster_size)
-        return -EIO;
+    if (ret < 0) {
+        return ret;
+    }
+
     s->refcount_block_cache_offset = refcount_block_offset;
     return 0;
 }