diff --git a/src/util/CircularBuffer.hxx b/src/util/CircularBuffer.hxx
index 0321b91f6..da6f412a5 100644
--- a/src/util/CircularBuffer.hxx
+++ b/src/util/CircularBuffer.hxx
@@ -107,6 +107,18 @@ public:
 			: capacity - head + tail;
 	}
 
+	/**
+	 * Returns the number of elements that can be added to this
+	 * buffer.
+	 */
+	constexpr size_type GetSpace() const {
+		/* space = capacity - size - 1 */
+		return (head <= tail
+			? capacity - tail + head
+			: head - tail)
+			- 1;
+	}
+
 	/**
 	 * Prepares writing.  Returns a buffer range which may be written.
 	 * When you are finished, call Append().
diff --git a/test/TestCircularBuffer.hxx b/test/TestCircularBuffer.hxx
index 88b0a9ea8..c808d85dc 100644
--- a/test/TestCircularBuffer.hxx
+++ b/test/TestCircularBuffer.hxx
@@ -33,6 +33,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(true, buffer.IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(size_t(0), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(7), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(true, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Write().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(&data[0], buffer.Write().data);
@@ -45,6 +46,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(false, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(size_t(1), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(6), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(size_t(1), buffer.Read().size);
 		CPPUNIT_ASSERT_EQUAL(&data[0], buffer.Read().data);
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Write().IsEmpty());
@@ -58,6 +60,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(true, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(size_t(7), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(0), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(size_t(7), buffer.Read().size);
 		CPPUNIT_ASSERT_EQUAL(&data[0], buffer.Read().data);
 		CPPUNIT_ASSERT_EQUAL(true, buffer.Write().IsEmpty());
@@ -69,6 +72,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(false, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(size_t(6), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(1), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(size_t(6), buffer.Read().size);
 		CPPUNIT_ASSERT_EQUAL(&data[1], buffer.Read().data);
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Write().IsEmpty());
@@ -83,6 +87,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(true, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(size_t(7), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(0), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(size_t(7), buffer.Read().size);
 		CPPUNIT_ASSERT_EQUAL(&data[1], buffer.Read().data);
 		CPPUNIT_ASSERT_EQUAL(true, buffer.Write().IsEmpty());
@@ -94,6 +99,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(false, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(size_t(4), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(3), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(size_t(4), buffer.Read().size);
 		CPPUNIT_ASSERT_EQUAL(&data[4], buffer.Read().data);
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Write().IsEmpty());
@@ -107,6 +113,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(false, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(size_t(6), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(1), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(size_t(4), buffer.Read().size);
 		CPPUNIT_ASSERT_EQUAL(&data[4], buffer.Read().data);
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Write().IsEmpty());
@@ -120,6 +127,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(true, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(size_t(7), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(0), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(size_t(4), buffer.Read().size);
 		CPPUNIT_ASSERT_EQUAL(&data[4], buffer.Read().data);
 		CPPUNIT_ASSERT_EQUAL(true, buffer.Write().IsEmpty());
@@ -131,6 +139,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(false, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(size_t(3), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(4), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(size_t(3), buffer.Read().size);
 		CPPUNIT_ASSERT_EQUAL(&data[0], buffer.Read().data);
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Write().IsEmpty());
@@ -145,6 +154,7 @@ public:
 		CPPUNIT_ASSERT_EQUAL(true, buffer.IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.IsFull());
 		CPPUNIT_ASSERT_EQUAL(size_t(0), buffer.GetSize());
+		CPPUNIT_ASSERT_EQUAL(size_t(7), buffer.GetSpace());
 		CPPUNIT_ASSERT_EQUAL(true, buffer.Read().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(false, buffer.Write().IsEmpty());
 		CPPUNIT_ASSERT_EQUAL(&data[3], buffer.Write().data);