[Qemu-devel] [PATCH v6 04/24] qcow2: Only return status from qcow2_get_refcount

[Max Reitz <mreitz@redhat.com>]

Refcounts can theoretically be of type uint64_t; in order to be able to
represent the full range, qcow2_get_refcount() cannot use a single
variable to represent both all refcount values and also keep some values
reserved for errors.

One solution would be to add an Error pointer parameter to
qcow2_get_refcount(); however, no caller could (currently) pass that
error message, so it would have to be emitted immediately and be
passed to the next caller by returning -EIO or something similar.
Therefore, an Error parameter does not offer any advantages here.

The solution applied by this patch is simpler to use. Because no caller
would be able to pass the error message, they would have to print it and
free it, whereas with this patch the caller only needs to pass the
returned integer (which is often a no-op from the code perspective,
because that integer will be stored in a variable "ret" which will be
returned by the fail path of many callers).

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2-cluster.c  |  8 ++---
 block/qcow2-refcount.c | 86 +++++++++++++++++++++++++++-----------------------
 block/qcow2.h          |  3 +-
 3 files changed, 53 insertions(+), 44 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 183177d..ee50500 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1640,7 +1640,7 @@ static int expand_zero_clusters_in_l1(BlockDriverState *bs, uint64_t *l1_table,
     for (i = 0; i < l1_size; i++) {
         uint64_t l2_offset = l1_table[i] & L1E_OFFSET_MASK;
         bool l2_dirty = false;
-        int l2_refcount;
+        uint16_t l2_refcount;
 
         if (!l2_offset) {
             /* unallocated */
@@ -1672,9 +1672,9 @@ static int expand_zero_clusters_in_l1(BlockDriverState *bs, uint64_t *l1_table,
             goto fail;
         }
 
-        l2_refcount = qcow2_get_refcount(bs, l2_offset >> s->cluster_bits);
-        if (l2_refcount < 0) {
-            ret = l2_refcount;
+        ret = qcow2_get_refcount(bs, l2_offset >> s->cluster_bits,
+                                 &l2_refcount);
+        if (ret < 0) {
             goto fail;
         }
 
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 69c6822..9286c64 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -87,26 +87,29 @@ static int load_refcount_block(BlockDriverState *bs,
 }
 
 /*
- * 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.
+ * Retrieves the refcount of the cluster given by its index and stores it in
+ * *refcount. Returns 0 on success and -errno on failure.
  */
-int qcow2_get_refcount(BlockDriverState *bs, int64_t cluster_index)
+int qcow2_get_refcount(BlockDriverState *bs, int64_t cluster_index,
+                       uint16_t *refcount)
 {
     BDRVQcowState *s = bs->opaque;
     uint64_t refcount_table_index, block_index;
     int64_t refcount_block_offset;
     int ret;
     uint16_t *refcount_block;
-    uint16_t refcount;
 
     refcount_table_index = cluster_index >> s->refcount_block_bits;
-    if (refcount_table_index >= s->refcount_table_size)
+    if (refcount_table_index >= s->refcount_table_size) {
+        *refcount = 0;
         return 0;
+    }
     refcount_block_offset =
         s->refcount_table[refcount_table_index] & REFT_OFFSET_MASK;
-    if (!refcount_block_offset)
+    if (!refcount_block_offset) {
+        *refcount = 0;
         return 0;
+    }
 
     if (offset_into_cluster(s, refcount_block_offset)) {
         qcow2_signal_corruption(bs, true, -1, -1, "Refblock offset %#" PRIx64
@@ -122,7 +125,7 @@ int qcow2_get_refcount(BlockDriverState *bs, int64_t cluster_index)
     }
 
     block_index = cluster_index & (s->refcount_block_size - 1);
-    refcount = be16_to_cpu(refcount_block[block_index]);
+    *refcount = be16_to_cpu(refcount_block[block_index]);
 
     ret = qcow2_cache_put(bs, s->refcount_block_cache,
         (void**) &refcount_block);
@@ -130,7 +133,7 @@ int qcow2_get_refcount(BlockDriverState *bs, int64_t cluster_index)
         return ret;
     }
 
-    return refcount;
+    return 0;
 }
 
 /*
@@ -674,16 +677,17 @@ static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size)
 {
     BDRVQcowState *s = bs->opaque;
     uint64_t i, nb_clusters;
-    int refcount;
+    uint16_t refcount;
+    int ret;
 
     nb_clusters = size_to_clusters(s, size);
 retry:
     for(i = 0; i < nb_clusters; i++) {
         uint64_t next_cluster_index = s->free_cluster_index++;
-        refcount = qcow2_get_refcount(bs, next_cluster_index);
+        ret = qcow2_get_refcount(bs, next_cluster_index, &refcount);
 
-        if (refcount < 0) {
-            return refcount;
+        if (ret < 0) {
+            return ret;
         } else if (refcount != 0) {
             goto retry;
         }
@@ -733,7 +737,8 @@ int qcow2_alloc_clusters_at(BlockDriverState *bs, uint64_t offset,
     BDRVQcowState *s = bs->opaque;
     uint64_t cluster_index;
     uint64_t i;
-    int refcount, ret;
+    uint16_t refcount;
+    int ret;
 
     assert(nb_clusters >= 0);
     if (nb_clusters == 0) {
@@ -744,10 +749,9 @@ int qcow2_alloc_clusters_at(BlockDriverState *bs, uint64_t offset,
         /* Check how many clusters there are free */
         cluster_index = offset >> s->cluster_bits;
         for(i = 0; i < nb_clusters; i++) {
-            refcount = qcow2_get_refcount(bs, cluster_index++);
-
-            if (refcount < 0) {
-                return refcount;
+            ret = qcow2_get_refcount(bs, cluster_index++, &refcount);
+            if (ret < 0) {
+                return ret;
             } else if (refcount != 0) {
                 break;
             }
@@ -781,9 +785,10 @@ int64_t qcow2_alloc_bytes(BlockDriverState *bs, int size)
     offset = s->free_byte_offset;
 
     if (offset) {
-        int refcount = qcow2_get_refcount(bs, offset >> s->cluster_bits);
-        if (refcount < 0) {
-            return refcount;
+        uint16_t refcount;
+        ret = qcow2_get_refcount(bs, offset >> s->cluster_bits, &refcount);
+        if (ret < 0) {
+            return ret;
         }
 
         if (refcount == s->refcount_max) {
@@ -890,7 +895,8 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
     uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2;
     bool l1_allocated = false;
     int64_t old_offset, old_l2_offset;
-    int i, j, l1_modified = 0, nb_csectors, refcount;
+    int i, j, l1_modified = 0, nb_csectors;
+    uint16_t refcount;
     int ret;
 
     l2_table = NULL;
@@ -995,9 +1001,8 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
                             }
                         }
 
-                        refcount = qcow2_get_refcount(bs, cluster_index);
-                        if (refcount < 0) {
-                            ret = refcount;
+                        ret = qcow2_get_refcount(bs, cluster_index, &refcount);
+                        if (ret < 0) {
                             goto fail;
                         }
                         break;
@@ -1038,9 +1043,9 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
                     goto fail;
                 }
             }
-            refcount = qcow2_get_refcount(bs, l2_offset >> s->cluster_bits);
-            if (refcount < 0) {
-                ret = refcount;
+            ret = qcow2_get_refcount(bs, l2_offset >> s->cluster_bits,
+                                     &refcount);
+            if (ret < 0) {
                 goto fail;
             } else if (refcount == 1) {
                 l2_offset |= QCOW_OFLAG_COPIED;
@@ -1358,7 +1363,7 @@ static int check_oflag_copied(BlockDriverState *bs, BdrvCheckResult *res,
     BDRVQcowState *s = bs->opaque;
     uint64_t *l2_table = qemu_blockalign(bs, s->cluster_size);
     int ret;
-    int refcount;
+    uint16_t refcount;
     int i, j;
 
     for (i = 0; i < s->l1_size; i++) {
@@ -1370,8 +1375,9 @@ static int check_oflag_copied(BlockDriverState *bs, BdrvCheckResult *res,
             continue;
         }
 
-        refcount = qcow2_get_refcount(bs, l2_offset >> s->cluster_bits);
-        if (refcount < 0) {
+        ret = qcow2_get_refcount(bs, l2_offset >> s->cluster_bits,
+                                 &refcount);
+        if (ret < 0) {
             /* don't print message nor increment check_errors */
             continue;
         }
@@ -1412,9 +1418,10 @@ static int check_oflag_copied(BlockDriverState *bs, BdrvCheckResult *res,
 
             if ((cluster_type == QCOW2_CLUSTER_NORMAL) ||
                 ((cluster_type == QCOW2_CLUSTER_ZERO) && (data_offset != 0))) {
-                refcount = qcow2_get_refcount(bs,
-                                              data_offset >> s->cluster_bits);
-                if (refcount < 0) {
+                ret = qcow2_get_refcount(bs,
+                                         data_offset >> s->cluster_bits,
+                                         &refcount);
+                if (ret < 0) {
                     /* don't print message nor increment check_errors */
                     continue;
                 }
@@ -1646,13 +1653,14 @@ static void compare_refcounts(BlockDriverState *bs, BdrvCheckResult *res,
 {
     BDRVQcowState *s = bs->opaque;
     int64_t i;
-    int refcount1, refcount2, ret;
+    uint16_t refcount1, refcount2;
+    int ret;
 
     for (i = 0, *highest_cluster = 0; i < nb_clusters; i++) {
-        refcount1 = qcow2_get_refcount(bs, i);
-        if (refcount1 < 0) {
+        ret = qcow2_get_refcount(bs, i, &refcount1);
+        if (ret < 0) {
             fprintf(stderr, "Can't get refcount for cluster %" PRId64 ": %s\n",
-                i, strerror(-refcount1));
+                    i, strerror(-ret));
             res->check_errors++;
             continue;
         }
@@ -1682,7 +1690,7 @@ static void compare_refcounts(BlockDriverState *bs, BdrvCheckResult *res,
 
             if (num_fixed) {
                 ret = update_refcount(bs, i << s->cluster_bits, 1,
-                                      refcount2 - refcount1,
+                                      (int)refcount2 - (int)refcount1,
                                       QCOW2_DISCARD_ALWAYS);
                 if (ret >= 0) {
                     (*num_fixed)++;
diff --git a/block/qcow2.h b/block/qcow2.h
index 4d8c902..1e59277 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -489,7 +489,8 @@ void qcow2_signal_corruption(BlockDriverState *bs, bool fatal, int64_t offset,
 int qcow2_refcount_init(BlockDriverState *bs);
 void qcow2_refcount_close(BlockDriverState *bs);
 
-int qcow2_get_refcount(BlockDriverState *bs, int64_t cluster_index);
+int qcow2_get_refcount(BlockDriverState *bs, int64_t cluster_index,
+                       uint16_t *refcount);
 
 int qcow2_update_cluster_refcount(BlockDriverState *bs, int64_t cluster_index,
                                   int addend, enum qcow2_discard_type type);
-- 
2.1.0