cache: Update cache targetSize to always return a positive int

Noticed when ingesting SSTables as part of a cockroach snapshot apply.
reservedSize would exceed maxSize, leading to an infinite loop in
evict().

internal/cache/clockpro.go

@@ -372,7 +372,14 @@ func (c *shard) Size() int64 {
 }
 
 func (c *shard) targetSize() int64 {
-	return c.maxSize - c.reservedSize
+	target := c.maxSize - c.reservedSize
+	// Always return a positive integer for targetSize. This is so that we don't
+	// end up in an infinite loop in evict(), in cases where reservedSize is
+	// greater than or equal to maxSize.
+	if target < 1 {
+		return 1
+	}
+	return target
 }
 
 // Add the entry to the cache, returning true if the entry was added and false

internal/cache/clockpro_test.go

@@ -154,20 +154,22 @@ func TestZeroSize(t *testing.T) {
 }
 
 func TestReserve(t *testing.T) {
-	cache := newShards(2, 2)
+	cache := newShards(4, 2)
 	cache.Set(1, 0, 0, []byte("a"))
 	cache.Set(2, 0, 0, []byte("a"))
 	require.EqualValues(t, 2, cache.Size())
 	r := cache.Reserve(1)
 	require.EqualValues(t, 0, cache.Size())
 	cache.Set(1, 0, 0, []byte("a"))
 	cache.Set(2, 0, 0, []byte("a"))
-	require.EqualValues(t, 0, cache.Size())
+	cache.Set(3, 0, 0, []byte("a"))
+	cache.Set(4, 0, 0, []byte("a"))
+	require.EqualValues(t, 2, cache.Size())
 	r()
-	require.EqualValues(t, 0, cache.Size())
+	require.EqualValues(t, 2, cache.Size())
 	cache.Set(1, 0, 0, []byte("a"))
 	cache.Set(2, 0, 0, []byte("a"))
-	require.EqualValues(t, 2, cache.Size())
+	require.EqualValues(t, 4, cache.Size())
 }
 
 func TestReserveDoubleRelease(t *testing.T) {