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diff --git a/src/osd/SnapMapReaderI.h b/src/osd/SnapMapReaderI.h
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+++ b/src/osd/SnapMapReaderI.h
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+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+#pragma once
+
+/**
+ * \file
+ * \brief Defines the interface for the snap-mapper used by the scrubber.
+ */
+#include <set>
+
+#include "common/scrub_types.h"
+#include "include/expected.hpp"
+
+namespace Scrub {
+/*
+ * snaps-related aux structures:
+ * the scrub-backend scans the snaps associated with each scrubbed object, and
+ * fixes corrupted snap-sets.
+ * The actual access to the PG's snap_mapper, and the actual I/O transactions,
+ * are performed by the main PgScrubber object.
+ * the following aux structures are used to facilitate the required exchanges:
+ * - pre-fix snap-sets are accessed by the scrub-backend, and:
+ * - a list of fix-orders (either insert or replace operations) are returned
+ */
+
+struct SnapMapReaderI {
+  struct result_t {
+    enum class code_t { success, backend_error, not_found, inconsistent };
+    code_t code{code_t::success};
+    int backend_error{0};  ///< errno returned by the backend
+  };
+
+  /**
+   *  get SnapMapper's snap-set for a given object
+   *  \returns a set of snaps, or an error code
+   *  \attn: only OBJ_ DB entries are consulted
+   */
+  virtual tl::expected<std::set<snapid_t>, result_t> get_snaps(
+    const hobject_t& hoid) const = 0;
+
+  /**
+   *  get SnapMapper's snap-set for a given object.
+   *  The snaps gleaned from the OBJ_ entry are verified against the
+   *  mapping ('SNA_') entries.
+   *  A mismatch between both sets of entries will result in an error.
+   *  \returns a set of snaps, or an error code.
+   */
+  virtual tl::expected<std::set<snapid_t>, result_t>
+  get_snaps_check_consistency(const hobject_t& hoid) const = 0;
+
+  virtual ~SnapMapReaderI() = default;
+};
+
+enum class snap_mapper_op_t {
+  add,
+  update,
+  overwrite,  //< the mapper's data is internally inconsistent. Similar
+	      //<  to an 'update' operation, but the logs are different.
+};
+
+struct snap_mapper_fix_t {
+  snap_mapper_op_t op;
+  hobject_t hoid;
+  std::set<snapid_t> snaps;
+  std::set<snapid_t> wrong_snaps;  // only collected & returned for logging sake
+};
+
+}  // namespace Scrub