osd/osd_types: add pg_t::is_merge() method

This checks if we are a merge *source*, and if so, who the parent (target)
will be.

Signed-off-by: Sage Weil <sage@redhat.com>

src/osd/osd_types.cc

@@ -558,6 +558,23 @@ unsigned pg_t::get_split_bits(unsigned pg_num) const {
     return p - 1;
 }
 
+bool pg_t::is_merge(unsigned old_pg_num, unsigned new_pg_num,
+		    pg_t *parent) const
+{
+  if (m_seed < old_pg_num &&
+      m_seed >= new_pg_num) {
+    if (parent) {
+      pg_t t = *this;
+      while (t.m_seed >= new_pg_num) {
+	t = t.get_parent();
+      }
+      *parent = t;
+    }
+    return true;
+  }
+  return false;
+}
+
 pg_t pg_t::get_parent() const
 {
   unsigned bits = cbits(m_seed);

src/osd/osd_types.h

@@ -412,6 +412,8 @@ struct pg_t {
 
   bool is_split(unsigned old_pg_num, unsigned new_pg_num, set<pg_t> *pchildren) const;
 
+  bool is_merge(unsigned old_pg_num, unsigned new_pg_num, pg_t *parent) const;
+
   /**
    * Returns b such that for all object o:
    *   ~((~0)<<b) & o.hash) == 0 iff o is in the pg for *this
@@ -539,6 +541,15 @@ struct spg_t {
     }
     return is_split;
   }
+  bool is_merge(unsigned old_pg_num, unsigned new_pg_num, spg_t *parent) const {
+    spg_t out = *this;
+    bool r = pgid.is_merge(old_pg_num, new_pg_num, &out.pgid);
+    if (r && parent) {
+      *parent = out;
+    }
+    return r;
+  }
+
   bool is_no_shard() const {
     return shard == shard_id_t::NO_SHARD;
   }

src/test/osd/types.cc

@@ -668,6 +668,32 @@ TEST(pg_t, split)
 
 }
 
+TEST(pg_t, merge)
+{
+  pg_t pgid, parent;
+  bool b;
+
+  pgid = pg_t(7, 0);
+  b = pgid.is_merge(8, 7, &parent);
+  ASSERT_TRUE(b);
+  ASSERT_EQ(parent, pg_t(3, 0));
+
+  b = pgid.is_merge(8, 5, &parent);
+  ASSERT_TRUE(b);
+  ASSERT_EQ(parent, pg_t(3, 0));
+
+  b = pgid.is_merge(8, 4, &parent);
+  ASSERT_TRUE(b);
+  ASSERT_EQ(parent, pg_t(3, 0));
+
+  b = pgid.is_merge(8, 3, &parent);
+  ASSERT_TRUE(b);
+  ASSERT_EQ(parent, pg_t(1, 0));
+
+  b = pgid.is_merge(9, 8, &parent);
+  ASSERT_FALSE(b);
+}
+
 TEST(pg_missing_t, constructor)
 {
   pg_missing_t missing;