Handle OS pretty name on old OS without OS release (#35453)

Some very old ancient versions of Linux do not have /etc/os-release. For
example, old Red Hat-like OS. This commit adds a fallback for handling
pretty name for these OS.

qa/evil-tests/src/test/java/org/elasticsearch/monitor/os/EvilOsProbeTests.java

@@ -25,6 +25,7 @@ import org.elasticsearch.test.ESTestCase;
 
 import java.io.IOException;
 import java.nio.file.Files;
+import java.util.Collections;
 import java.util.List;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -36,7 +37,15 @@ public class EvilOsProbeTests extends ESTestCase {
     public void testOsPrettyName() throws IOException  {
         final OsInfo osInfo = OsProbe.getInstance().osInfo(randomLongBetween(1, 100), randomIntBetween(1, 8));
         if (Constants.LINUX) {
-            final List<String> lines = Files.readAllLines(PathUtils.get("/etc/os-release"));
+            final List<String> lines;
+            if (Files.exists(PathUtils.get("/etc/os-release"))) {
+                lines = Files.readAllLines(PathUtils.get("/etc/os-release"));
+            } else if (Files.exists(PathUtils.get("/usr/lib/os-release"))) {
+                lines = Files.readAllLines(PathUtils.get("/usr/lib/os-release"));
+            } else {
+                lines = Collections.singletonList(
+                        "PRETTY_NAME=\"" + Files.readAllLines(PathUtils.get("/etc/system-release")).get(0) + "\"");
+            }
             for (final String line : lines) {
                 if (line != null && line.startsWith("PRETTY_NAME=")) {
                     final Matcher matcher = Pattern.compile("PRETTY_NAME=(\"?|'?)?([^\"']+)\\1").matcher(line.trim());

server/src/main/java/org/elasticsearch/monitor/os/OsProbe.java

@@ -33,6 +33,7 @@ import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -566,22 +567,33 @@ public class OsProbe {
     }
 
     /**
-     * The lines from {@code /etc/os-release} or {@code /usr/lib/os-release} as a fallback. These file represents identification of the
-     * underlying operating system. The structure of the file is newlines of key-value pairs of shell-compatible variable assignments.
+     * The lines from {@code /etc/os-release} or {@code /usr/lib/os-release} as a fallback, with an additional fallback to
+     * {@code /etc/system-release}. These files represent identification of the underlying operating system. The structure of the file is
+     * newlines of key-value pairs of shell-compatible variable assignments.
      *
-     * @return the lines from {@code /etc/os-release} or {@code /usr/lib/os-release}
-     * @throws IOException if an I/O exception occurs reading {@code /etc/os-release} or {@code /usr/lib/os-release}
+     * @return the lines from {@code /etc/os-release} or {@code /usr/lib/os-release} or {@code /etc/system-release}
+     * @throws IOException if an I/O exception occurs reading {@code /etc/os-release} or {@code /usr/lib/os-release} or
+     *                     {@code /etc/system-release}
      */
-    @SuppressForbidden(reason = "access /etc/os-release or /usr/lib/os-release")
+    @SuppressForbidden(reason = "access /etc/os-release or /usr/lib/os-release or /etc/system-release")
     List<String> readOsRelease() throws IOException {
         final List<String> lines;
         if (Files.exists(PathUtils.get("/etc/os-release"))) {
             lines = Files.readAllLines(PathUtils.get("/etc/os-release"));
-        } else {
+            assert lines != null && lines.isEmpty() == false;
+            return lines;
+        } else if (Files.exists(PathUtils.get("/usr/lib/os-release"))) {
             lines = Files.readAllLines(PathUtils.get("/usr/lib/os-release"));
+            assert lines != null && lines.isEmpty() == false;
+            return lines;
+        } else if (Files.exists(PathUtils.get("/etc/system-release"))) {
+            // fallback for older Red Hat-like OS
+            lines = Files.readAllLines(PathUtils.get("/etc/system-release"));
+            assert lines != null && lines.size() == 1;
+            return Collections.singletonList("PRETTY_NAME=\"" + lines.get(0) + "\"");
+        } else {
+            return Collections.emptyList();
         }
-        assert lines != null && lines.isEmpty() == false;
-        return lines;
     }
 
     public OsStats osStats() {

server/src/main/resources/org/elasticsearch/bootstrap/security.policy

@@ -127,6 +127,7 @@ grant {
   // OS release on Linux
   permission java.io.FilePermission "/etc/os-release", "read";
   permission java.io.FilePermission "/usr/lib/os-release", "read";
+  permission java.io.FilePermission "/etc/system-release", "read";
 
   // io stats on Linux
   permission java.io.FilePermission "/proc/self/mountinfo", "read";