MGMT-15243: Skip any zero size manifests when applying (#5355)

When applying manifests when installation is started, we should skip any
manifests that are of zero size to prevent them from causing bootstrap
problems later.

We can't validate zero size at the point of file upload as the UI is
presently using an approach where it uploads an empty manifest file to
track that the user wishes to add user defined manifests.

Discussions are underway to change this but for now we need to take this
alternative approach.

This PR implements this.

internal/ignition/ignition.go

@@ -1574,6 +1574,12 @@ func (g *installerGenerator) downloadManifest(ctx context.Context, manifest stri
 	if err != nil {
 		return err
 	}
+
+	if len(content) == 0 {
+		// Ignore any empty files.
+		return nil
+	}
+
 	// manifest has full path as object-key on s3: clusterID/manifests/[manifests|openshift]/filename
 	// clusterID/manifests should be trimmed
 	prefix := manifests.GetManifestObjectName(*g.cluster.ID, "")

internal/ignition/ignition_test.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"strings"
@@ -921,15 +922,29 @@ var _ = Describe("downloadManifest", func() {
 	It("writes the correct file", func() {
 		ctx := context.Background()
 		manifestName := fmt.Sprintf("%s/manifests/openshift/masters-chrony-configuration.yaml", cluster.ID)
-		mockS3Client.EXPECT().Download(ctx, manifestName).Return(io.NopCloser(strings.NewReader("chronyconf")), int64(10), nil)
+		mockS3Client.EXPECT().Download(ctx, manifestName).Return(io.NopCloser(strings.NewReader("content:entry")), int64(10), nil)
 		Expect(os.Mkdir(filepath.Join(workDir, "/openshift"), 0755)).To(Succeed())
 		Expect(os.Mkdir(filepath.Join(workDir, "/manifests"), 0755)).To(Succeed())
 
 		Expect(generator.downloadManifest(ctx, manifestName)).To(Succeed())
 
 		content, err := os.ReadFile(filepath.Join(workDir, "/openshift/masters-chrony-configuration.yaml"))
 		Expect(err).NotTo(HaveOccurred())
-		Expect(content).To(Equal([]byte("chronyconf")))
+		Expect(content).To(Equal([]byte("content:entry")))
+	})
+
+	It("If a file has empty content, it will not be written", func() {
+		ctx := context.Background()
+		manifestName := fmt.Sprintf("%s/manifests/openshift/masters-chrony-configuration.yaml", cluster.ID)
+		mockS3Client.EXPECT().Download(ctx, manifestName).Return(io.NopCloser(strings.NewReader("")), int64(0), nil)
+		Expect(os.Mkdir(filepath.Join(workDir, "/openshift"), 0755)).To(Succeed())
+		Expect(os.Mkdir(filepath.Join(workDir, "/manifests"), 0755)).To(Succeed())
+
+		Expect(generator.downloadManifest(ctx, manifestName)).To(Succeed())
+
+		_, err := os.Stat(filepath.Join(workDir, "/openshift/masters-chrony-configuration.yaml"))
+		Expect(err).To(HaveOccurred())
+		Expect(errors.Is(err, fs.ErrNotExist)).To(BeTrue())
 	})
 })