Fix issue with log_writer panic on line with all spaces (#4910) (#4915)

* Fix #4907.

* Add changelog.

* Apply suggestions from code review

Co-authored-by: Julien Lind <julien.lind@elastic.co>

---------

Co-authored-by: Julien Lind <julien.lind@elastic.co>
(cherry picked from commit 3df9f8b)

Co-authored-by: Blake Rouse <blake.rouse@elastic.co>

Author: mergify[bot]

changelog/fragments/1718138001-Fix-possible-crash-in-reading-component-logs.yaml

@@ -0,0 +1,32 @@
+# Kind can be one of:
+# - breaking-change: a change to previously-documented behavior
+# - deprecation: functionality that is being removed in a later release
+# - bug-fix: fixes a problem in a previous version
+# - enhancement: extends functionality but does not break or fix existing behavior
+# - feature: new functionality
+# - known-issue: problems that we are aware of in a given version
+# - security: impacts on the security of a product or a user’s deployment.
+# - upgrade: important information for someone upgrading from a prior version
+# - other: does not fit into any of the other categories
+kind: bug-fix
+
+# Change summary; a 80ish characters long description of the change.
+summary: Fix possible crash in reading component logs
+
+# Long description; in case the summary is not enough to describe the change
+# this field accommodate a description without length limits.
+# NOTE: This field will be rendered only for breaking-change and known-issue kinds at the moment.
+#description:
+
+# Affected component; usually one of "elastic-agent", "fleet-server", "filebeat", "metricbeat", "auditbeat", "all", etc.
+component: elastic-agent
+
+# PR URL; optional; the PR number that added the changeset.
+# If not present is automatically filled by the tooling finding the PR where this changelog fragment has been added.
+# NOTE: the tooling supports backports, so it's able to fill the original PR number instead of the backport PR number.
+# Please provide it if you are adding a fragment for a different PR.
+pr: https://github.com/elastic/elastic-agent/pull/4910
+
+# Issue URL; optional; the GitHub issue related to this changeset (either closes or is part of).
+# If not present is automatically filled by the tooling with the issue linked to the PR number.
+issue: https://github.com/elastic/elastic-agent/issues/4907

pkg/component/runtime/log_writer.go

@@ -106,6 +106,10 @@ func (r *logWriter) Write(p []byte) (int, error) {
 			continue
 		}
 		str := strings.TrimSpace(string(line))
+		if len(str) == 0 {
+			// empty line after trim
+			continue
+		}
 		// try to parse line as JSON
 		if str[0] == '{' && r.handleJSON(str) {
 			// handled as JSON

pkg/component/runtime/log_writer_test.go

@@ -122,6 +122,33 @@ func TestLogWriter(t *testing.T) {
 				},
 			},
 		},
+		{
+			Name:      "multi empty text line",
+			LogLevel:  zapcore.InfoLevel,
+			LogSource: logSourceStdout,
+			Lines: []string{
+				"simple written line\r\n",
+				"\r\n",  // empty line
+				" \r\n", // empty line with space
+				"another line\n",
+			},
+			Wrote: []wrote{
+				{
+					entry: zapcore.Entry{
+						Level:   zapcore.InfoLevel,
+						Time:    time.Time{},
+						Message: "simple written line",
+					},
+				},
+				{
+					entry: zapcore.Entry{
+						Level:   zapcore.InfoLevel,
+						Time:    time.Time{},
+						Message: "another line",
+					},
+				},
+			},
+		},
 		{
 			Name:      "json log line split",
 			LogLevel:  zapcore.DebugLevel,