osd: Fix decode for some extent cache reads.

The extent cache in EC can cause the backend to perform some surprising reads. Some
of the patterns were discovered in test that caused the decode to attempt to
decode more data than was anticipated during the read planning, leading to an
assert. This simple fix reduces the scope of the decode to the minimum.

Signed-off-by: Alex Ainscow <[email protected]>

Author: aainscow

src/osd/ECUtil.cc

@@ -674,7 +674,11 @@ int shard_extent_map_t::decode(const ErasureCodeInterfaceRef &ec_impl,
        */
       decode_set.insert(decode_for_parity_shards);
       need_set.insert(decode_for_parity_shards);
-      extent_set decode_for_parity = get_extent_superset();
+      extent_set decode_for_parity;
+
+      for (auto shard : encode_set) {
+        decode_for_parity.insert(want.at(shard));
+      }
 
       for (auto shard : decode_for_parity_shards) {
         extent_set parity_pad;

src/test/osd/TestECBackend.cc

@@ -1481,3 +1481,24 @@ TEST(ECCommon, decode7) {
 
   test_decode(k, m, chunk_size, object_size, want, acting_set);
 }
+
+TEST(ECCommon, decode8) {
+  const unsigned int k = 3;
+  const unsigned int m = 2;
+  const uint64_t chunk_size = 64 * 1024;
+  const uint64_t object_size = 672 * 1024;
+
+
+  ECUtil::shard_extent_set_t want(k+m);
+  shard_id_set acting_set;
+  want[shard_id_t(0)].insert(64 * 1024, 64 * 1024);
+  want[shard_id_t(2)].insert(32 * 1024, 32 * 1024);
+  want[shard_id_t(3)].insert(32 * 1024, 64 * 1024);
+  want[shard_id_t(4)].insert(32 * 1024, 64 * 1024);
+
+
+  acting_set.insert(shard_id_t(0));
+  acting_set.insert_range(shard_id_t(2), 2);
+
+  test_decode(k, m, chunk_size, object_size, want, acting_set);
+}