ensure order of jvm options from file and env vars is respected (elastic#15997)

Co-authored-by: Andrea Selva <[email protected]>

Author: jsvd

tools/jvm-options-parser/src/main/java/org/logstash/launchers/JvmOptionsParser.java

@@ -198,7 +198,9 @@ private Set<String> nettyMaxOrderDefaultTo11(Set<String> options) {
         if (maxOrderAlreadyContained) {
             return options;
         }
-        final Set<String> acc = new HashSet<>(options);
+        // Order is important because LS_JAVA_OPTS is added last and must take precedence 
+        // over settings in jvm.options
+        final Set<String> acc = new LinkedHashSet<>(options);
         acc.add("-Dio.netty.allocator.maxOrder=11");
         return acc;
     }

tools/jvm-options-parser/src/test/java/org/logstash/launchers/JvmOptionsParserTest.java

@@ -151,6 +151,26 @@ public void testNettyMaxOrderRuleDoNotAppliesIfAlreadyDefinedExplicitlyByUser()
 
     }
 
+    @Test
+    public void testEnvironmentOPTSVariableTakesPrecedenceOverOptionsFile() throws IOException {
+        String regex = "Xmx[^ ]+";
+        String expected = "Xmx25g";
+        File optionsFile = writeIntoTempOptionsFile(writer -> writer.println("-Xmx1g"));
+
+        JvmOptionsParser.handleJvmOptions(new String[] {"/path/to/ls_home", optionsFile.toString()}, expected);
+
+        final String output = outputStreamCaptor.toString();
+
+        java.util.regex.Pattern pattern = java.util.regex.Pattern.compile(regex);
+        String lastMatch = pattern.matcher(output)
+                .results()
+                .map(java.util.regex.MatchResult::group)
+                .reduce((first, second) -> second)
+                .orElse(null);
+
+        assertEquals("LS_JAVA_OPTS env must take precedence over jvm.options file", expected, lastMatch);
+    }
+
     private File writeIntoTempOptionsFile(Consumer<PrintWriter> writer) throws IOException {
         File optionsFile = temp.newFile("jvm.options");
         PrintWriter optionsWriter = new PrintWriter(new FileWriter(optionsFile));