Create nil cgroups reader for processor, improve error handling

libbeat/processors/add_docker_metadata/add_docker_metadata.go

@@ -113,7 +113,9 @@ func buildDockerMetadataProcessor(log *logp.Logger, cfg *conf.C, watcherConstruc
 	}
 
 	reader, err := initCgroupPaths(resolve.NewTestResolver(config.HostFS), false)
-	if err != nil && !errors.Is(err, cgroup.ErrCgroupsMissing) {
+	if errors.Is(err, cgroup.ErrCgroupsMissing) {
+		reader = &processors.NilCGReader{}
+	} else if err != nil {
 		return nil, fmt.Errorf("error creating cgroup reader: %w", err)
 	}
 
@@ -284,14 +286,13 @@ func (d *addDockerMetadata) getProcessCgroups(pid int) (cgroup.PathList, error)
 		return cgroups, nil
 	}
 
-	if d.cgreader == nil {
-		return cgroups, fs.ErrNotExist
-	}
 	cgroups, err := d.cgreader.ProcessCgroupPaths(pid)
 	if err != nil {
 		return cgroups, fmt.Errorf("failed to read cgroups for pid=%v: %w", pid, err)
 	}
-
+	if len(cgroups.Flatten()) == 0 {
+		return cgroup.PathList{}, fs.ErrNotExist
+	}
 	d.cgroups.Put(pid, cgroups)
 	return cgroups, nil
 }

libbeat/processors/add_process_metadata/add_process_metadata.go

@@ -164,7 +164,9 @@ func newProcessMetadataProcessorWithProvider(config config, provider processMeta
 	}
 
 	reader, err := initCgroupPaths(resolve.NewTestResolver(config.HostPath), false)
-	if err != nil && !errors.Is(err, cgroup.ErrCgroupsMissing) {
+	if errors.Is(err, cgroup.ErrCgroupsMissing) {
+		reader = &processors.NilCGReader{}
+	} else if err != nil {
 		return nil, fmt.Errorf("error creating cgroup reader: %w", err)
 	}
 

libbeat/processors/add_process_metadata/add_process_metadata_test.go

@@ -54,6 +54,22 @@ func newCGHandlerBuilder(handler testCGRsolver) processors.InitCgroupHandler {
 	}
 }
 
+func TestNilProcessor(t *testing.T) {
+	initCgroupPaths = func(rootfsMountpoint resolve.Resolver, ignoreRootCgroups bool) (processors.CGReader, error) {
+		return &processors.NilCGReader{}, nil
+	}
+
+	proc, err := newProcessMetadataProcessorWithProvider(defaultConfig(), &procCache, false)
+	require.NoError(t, err)
+
+	// make sure a nil cgroup reader doesn't blow anything up
+	unwrapped, _ := proc.(*addProcessMetadata)
+	metadata, err := unwrapped.provider.GetProcessMetadata(os.Getpid())
+	require.NoError(t, err)
+	require.NotNil(t, metadata)
+
+}
+
 func TestDefaultProcessorStartup(t *testing.T) {
 	// set initCgroupPaths to system non-test defaults
 	initCgroupPaths = func(rootfsMountpoint resolve.Resolver, ignoreRootCgroups bool) (processors.CGReader, error) {
@@ -67,7 +83,7 @@ func TestDefaultProcessorStartup(t *testing.T) {
 	unwrapped, _ := proc.(*addProcessMetadata)
 	metadata, err := unwrapped.provider.GetProcessMetadata(os.Getpid())
 	require.NoError(t, err)
-	require.NotNil(t, metadata)
+	require.NotNil(t, metadata.fields)
 }
 
 func TestAddProcessMetadata(t *testing.T) {

libbeat/processors/add_process_metadata/gosigar_cid_provider.go

@@ -55,7 +55,7 @@ func (p gosigarCidProvider) GetCid(pid int) (result string, err error) {
 
 	cgroups, err := p.getProcessCgroups(pid)
 	if err != nil {
-		p.log.Debugf("failed to get cgroups for pid=%v: %v", pid, err)
+		return "", fmt.Errorf("failed to get cgroups for pid=%v: %w", pid, err)
 	}
 
 	cid = p.getContainerID(cgroups)
@@ -81,9 +81,6 @@ func newCidProvider(cgroupPrefixes []string, cgroupRegex *regexp.Regexp, process
 // returns an error if it failed to retrieve the cgroup info.
 func (p gosigarCidProvider) getProcessCgroups(pid int) (cgroup.PathList, error) {
 	//return nil if we aren't supporting cgroups
-	if p.processCgroupPaths == nil {
-		return cgroup.PathList{}, nil
-	}
 	pathList, err := p.processCgroupPaths.ProcessCgroupPaths(pid)
 	if err != nil {
 		return cgroup.PathList{}, fmt.Errorf("failed to read cgroups for pid=%v: %w", pid, err)

libbeat/processors/cgroups.go

@@ -18,6 +18,8 @@
 package processors
 
 import (
+	"io/fs"
+
 	"github.com/elastic/elastic-agent-system-metrics/metric/system/cgroup"
 	"github.com/elastic/elastic-agent-system-metrics/metric/system/resolve"
 )
@@ -30,3 +32,12 @@ type InitCgroupHandler = func(rootfsMountpoint resolve.Resolver, ignoreRootCgrou
 type CGReader interface {
 	ProcessCgroupPaths(pid int) (cgroup.PathList, error)
 }
+
+// NilCGReader does nothing
+type NilCGReader struct {
+}
+
+// ProcessCgroupPaths returns a blank pathLists and fs.ErrNotExist
+func (*NilCGReader) ProcessCgroupPaths(_ int) (cgroup.PathList, error) {
+	return cgroup.PathList{}, fs.ErrNotExist
+}