Add MultiValueMappingReadView, used by readers to get limited read access

to a MultiValueMapping.

2 files changed, 11 insertions, 1 deletions

diff --git a/vespalib/src/vespa/vespalib/util/arrayref.h b/vespalib/src/vespa/vespalib/util/arrayref.h
index db3b39f400d..bc1fc540a6c 100644
--- a/vespalib/src/vespa/vespalib/util/arrayref.h
+++ b/vespalib/src/vespa/vespalib/util/arrayref.h
@@ -42,7 +42,7 @@ public:
     ConstArrayRef(const SmallVector<T, N> &v) noexcept :  _v(&v[0]), _sz(v.size()) { }
     ConstArrayRef(const ArrayRef<T> & v) noexcept : _v(&v[0]), _sz(v.size()) { }
     ConstArrayRef(const Array<T> &v) noexcept : _v(&v[0]), _sz(v.size()) { }
-    ConstArrayRef() noexcept : _v(nullptr), _sz(0) {}
+    constexpr ConstArrayRef() noexcept : _v(nullptr), _sz(0) {}
     const T & operator [] (size_t i) const { return _v[i]; }
     size_t size() const { return _sz; }
     bool empty() const { return _sz == 0; }
diff --git a/vespalib/src/vespa/vespalib/util/rcuvector.h b/vespalib/src/vespa/vespalib/util/rcuvector.h
index 09957d14aaf..e3e17d8edf0 100644
--- a/vespalib/src/vespa/vespalib/util/rcuvector.h
+++ b/vespalib/src/vespa/vespalib/util/rcuvector.h
@@ -4,6 +4,7 @@
 
 #include "alloc.h"
 #include "array.h"
+#include "arrayref.h"
 #include "generationholder.h"
 #include "growstrategy.h"
 #include "memoryusage.h"
@@ -148,6 +149,15 @@ public:
 
     const T& get_elem_ref(size_t i) const noexcept { return _data[i]; } // Called from writer only
 
+    /*
+     * Readers holding a generation guard can call get_read_view() to
+     * get a read view to the rcu vector. Array bound (read_size) must
+     * be specified by reader, cf. committed docid limit in attribute vectors.
+     */
+    ConstArrayRef<T> get_read_view(size_t read_size) const noexcept {
+        return ConstArrayRef<T>(&acquire_elem_ref(0), read_size);
+    }
+
     void reset();
     void shrink(size_t newSize) __attribute__((noinline));
     void replaceVector(ArrayType replacement);