libbeat/idxmgmt/ilm: fix alias creation

When creating the initial index/write alias fails, don't
check the status code, just check if the alias exists
regardless of the error. This fixes a bug where alias
creation fails when the alias exists but points to another
index, and the initial index does not exist (e.g. due to
ILM deletion.)

CHANGELOG.next.asciidoc

@@ -238,6 +238,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix 'make setup' instructions for a new beat {pull}24944[24944]
 - Fix out of date FreeBSD vagrantbox. {pull}25652[25652]
 - Fix handling of `file_selectors` in aws-s3 input. {pull}25792[25792]
+- Fix ILM alias creation when write alias exists and initial index does not exist {pull}26143[26143]
 
 *Auditbeat*
 

libbeat/idxmgmt/ilm/client_handler.go

@@ -191,19 +191,18 @@ func (h *ESClientHandler) CreateAlias(alias Alias) error {
 	}
 
 	// Note: actual aliases are accessible via the index
-	status, res, err := h.client.Request("PUT", "/"+firstIndex, "", nil, body)
-	if status == 400 {
-		// HasAlias fails if there is an index with the same name, that is
-		// what we want to check here.
-		_, err := h.HasAlias(alias.Name)
-		if err != nil {
+	if _, res, err := h.client.Request("PUT", "/"+firstIndex, "", nil, body); err != nil {
+		// Creating the index may fail for multiple reasons, e.g. because
+		// the index exists, or because the write alias exists and points
+		// to another index.
+		if ok, err := h.HasAlias(alias.Name); err != nil {
+			// HasAlias fails if there is an index with the same name.
 			return err
+		} else if ok {
+			return errOf(ErrAliasAlreadyExists)
 		}
-		return errOf(ErrAliasAlreadyExists)
-	} else if err != nil {
 		return wrapErrf(err, ErrAliasCreateFailed, "failed to create alias: %s", res)
 	}
-
 	return nil
 }
 

libbeat/idxmgmt/ilm/client_handler_integration_test.go

@@ -136,13 +136,44 @@ func TestESClientHandler_Alias(t *testing.T) {
 		assert.True(t, b)
 	})
 
-	t.Run("second create", func(t *testing.T) {
+	t.Run("create index exists", func(t *testing.T) {
 		alias := makeAlias("esch-alias-2create")
 		h := newESClientHandler(t)
 
 		err := h.CreateAlias(alias)
 		assert.NoError(t, err)
 
+		// Second time around creating the alias, ErrAliasAlreadyExists is returned:
+		// the initial index already exists and the write alias points to it.
+		err = h.CreateAlias(alias)
+		require.Error(t, err)
+		assert.Equal(t, ilm.ErrAliasAlreadyExists, ilm.ErrReason(err))
+
+		b, err := h.HasAlias(alias.Name)
+		assert.NoError(t, err)
+		assert.True(t, b)
+	})
+
+	t.Run("create alias exists", func(t *testing.T) {
+		alias := makeAlias("esch-alias-2create")
+		alias.Pattern = "000001" // no date math, so we get predictable index names
+		h := newESClientHandler(t)
+
+		err := h.CreateAlias(alias)
+		assert.NoError(t, err)
+
+		// Rollover, so write alias points at -000002.
+		es := newRawESClient(t)
+		_, _, err = es.Request("POST", "/"+alias.Name+"/_rollover", "", nil, nil)
+		require.NoError(t, err)
+
+		// Delete -000001, simulating ILM delete.
+		_, _, err = es.Request("DELETE", "/"+alias.Name+"-"+alias.Pattern, "", nil, nil)
+		require.NoError(t, err)
+
+		// Second time around creating the alias, ErrAliasAlreadyExists is returned:
+		// initial index does not exist, but the write alias exists and points to
+		// another index.
 		err = h.CreateAlias(alias)
 		require.Error(t, err)
 		assert.Equal(t, ilm.ErrAliasAlreadyExists, ilm.ErrReason(err))