lir: inject newline delimiter only when necessary

Our internal representation of the composite config file needs only to inject
newline delimiters if they are missing, and to avoid doing so if they are
present. This allows `PipelineConfig#sourceReferences()`, used to map back from
the composite line/column to source file/column, to correctly track an offset
using the source fragments `SourceWithMetadata#getLinesCount()`.

Fixes: elastic#12155

Author: yaauie

logstash-core/src/main/java/org/logstash/config/ir/PipelineConfig.java

@@ -62,6 +62,8 @@ boolean includeLine(int lineNumber) {
     private volatile String configString;
     private List<LineToSource> sourceRefs;
 
+    private static final String NEWLINE = "\n";
+
     @SuppressWarnings({"rawtypes", "unchecked"})
     public PipelineConfig(RubyClass source, RubySymbol pipelineId, RubyObject uncastedConfigParts, RubyObject logstashSettings) {
         IRubyObject uncasted = uncastedConfigParts.checkArrayType();
@@ -111,9 +113,16 @@ public String configString() {
         if (this.configString == null) {
             synchronized(this) {
                 if (this.configString == null) {
-                    this.configString = confParts.stream()
-                            .map(SourceWithMetadata::getText)
-                            .collect(Collectors.joining("\n"));
+                    final StringBuilder compositeConfig = new StringBuilder();
+                    for (SourceWithMetadata confPart : confParts) {
+                        // If our composite config ends without a trailing newline,
+                        // append one before appending the next config part
+                        if (compositeConfig.lastIndexOf(NEWLINE) < compositeConfig.length() - 1 ) {
+                            compositeConfig.append(NEWLINE);
+                        }
+                        compositeConfig.append(confPart.getText());
+                    }
+                    this.configString = compositeConfig.toString();
                 }
             }
         }

logstash-core/src/test/java/org/logstash/config/ir/PipelineConfigTest.java

@@ -83,8 +83,11 @@ static class SourceCollector {
 
         void appendSource(final String protocol, final String id, final int line, final int column, final String text) throws IncompleteSourceWithMetadataException {
             orderedConfigParts.add(new SourceWithMetadata(protocol, id, line, column, text));
+
+            if (compositeSource.length() > 0 && !compositeSource.toString().endsWith("\n")) {
+                compositeSource.append("\n");
+            }
             compositeSource.append(text);
-            compositeSource.append("\n");
         }
 
         String compositeSource() {
@@ -103,11 +106,11 @@ public void setUp() throws IncompleteSourceWithMetadataException {
         pipelineIdSym = RubySymbol.newSymbol(RubyUtil.RUBY, PIPELINE_ID);
 
         final SourceCollector sourceCollector = new SourceCollector();
-        sourceCollector.appendSource("file", "/tmp/1", 0, 0, "input { generator1 }");
+        sourceCollector.appendSource("file", "/tmp/1", 0, 0, "input { generator1 }\n");
         sourceCollector.appendSource("file", "/tmp/2", 0, 0, "input { generator2 }");
-        sourceCollector.appendSource("file", "/tmp/3", 0, 0, "input { generator3 }");
+        sourceCollector.appendSource("file", "/tmp/3", 0, 0, "input { generator3 }\n");
         sourceCollector.appendSource("file", "/tmp/4", 0, 0, "input { generator4 }");
-        sourceCollector.appendSource("file", "/tmp/5", 0, 0, "input { generator5 }");
+        sourceCollector.appendSource("file", "/tmp/5", 0, 0, "input { generator5 }\n");
         sourceCollector.appendSource("file", "/tmp/6", 0, 0, "input { generator6 }");
         sourceCollector.appendSource("string", "config_string", 0, 0, "input { generator1 }");