[Backport 2.x] Fix dissect ingest processor parsing empty brackets failed

CHANGELOG.md

@@ -76,6 +76,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Remove deprecated classes for Rounding ([#10956](https://github.com/opensearch-project/OpenSearch/issues/10956))
 
 ### Fixed
+- Fix failure in dissect ingest processor parsing empty brackets ([#9225](https://github.com/opensearch-project/OpenSearch/pull/9255))
 - Fix class_cast_exception when passing int to _version and other metadata fields in ingest simulate API ([#10101](https://github.com/opensearch-project/OpenSearch/pull/10101))
 - Fix passing wrong parameter when calling newConfigurationException() in DotExpanderProcessor ([#10737](https://github.com/opensearch-project/OpenSearch/pull/10737))
 - Fix per request latency last phase not tracked ([#10934](https://github.com/opensearch-project/OpenSearch/pull/10934))

libs/dissect/src/main/java/org/opensearch/dissect/DissectParser.java

@@ -231,7 +231,10 @@ public Map<String, String> parse(String inputString) {
             int lookAheadMatches;
             // start walking the input string byte by byte, look ahead for matches where needed
             // if a match is found jump forward to the end of the match
-            for (; i < input.length; i++) {
+            while (i < input.length) {
+                // start is only used to record the value of i
+                int start = i;
+
                 lookAheadMatches = 0;
                 // potential match between delimiter and input string
                 if (delimiter.length > 0 && input[i] == delimiter[0]) {
@@ -283,8 +286,14 @@ public Map<String, String> parse(String inputString) {
                         delimiter = dissectPair.getDelimiter().getBytes(StandardCharsets.UTF_8);
                         // i is always one byte after the last found delimiter, aka the start of the next value
                         valueStart = i;
+                    } else {
+                        i++;
                     }
+                } else {
+                    i++;
                 }
+                // i should change anyway
+                assert (i != start);
             }
             // the last key, grab the rest of the input (unless consecutive delimiters already grabbed the last key)
             // and there is no trailing delimiter

modules/ingest-common/src/test/java/org/opensearch/ingest/common/DissectProcessorTests.java

@@ -155,4 +155,28 @@ public void testNullValueWithOutIgnoreMissing() {
         IngestDocument ingestDocument = new IngestDocument(originalIngestDocument);
         expectThrows(IllegalArgumentException.class, () -> processor.execute(ingestDocument));
     }
+
+    public void testMatchEmptyBrackets() {
+        IngestDocument ingestDocument = new IngestDocument(
+            "_index",
+            "_id",
+            null,
+            null,
+            null,
+            Collections.singletonMap("message", "[foo],[bar],[]")
+        );
+        DissectProcessor dissectProcessor = new DissectProcessor("", null, "message", "[%{a}],[%{b}],[%{c}]", "", true);
+        dissectProcessor.execute(ingestDocument);
+        assertEquals("foo", ingestDocument.getFieldValue("a", String.class));
+        assertEquals("bar", ingestDocument.getFieldValue("b", String.class));
+        assertEquals("", ingestDocument.getFieldValue("c", String.class));
+
+        ingestDocument = new IngestDocument("_index", "_id", null, null, null, Collections.singletonMap("message", "{}{}{}{baz}"));
+        dissectProcessor = new DissectProcessor("", null, "message", "{%{a}}{%{b}}{%{c}}{%{d}}", "", true);
+        dissectProcessor.execute(ingestDocument);
+        assertEquals("", ingestDocument.getFieldValue("a", String.class));
+        assertEquals("", ingestDocument.getFieldValue("b", String.class));
+        assertEquals("", ingestDocument.getFieldValue("c", String.class));
+        assertEquals("baz", ingestDocument.getFieldValue("d", String.class));
+    }
 }

modules/ingest-common/src/yamlRestTest/resources/rest-api-spec/test/ingest/200_dissect_processor.yml

@@ -84,3 +84,38 @@ teardown:
             }
           ]
         }
+
+---
+"Test dissect processor can match empty brackets":
+  - do:
+      ingest.put_pipeline:
+        id: "my_pipeline"
+        body:  >
+          {
+            "description": "_description",
+            "processors": [
+              {
+                "dissect" : {
+                  "field" : "message",
+                  "pattern" : "[%{a}][%{b}][%{c}]"
+                }
+              }
+            ]
+          }
+  - match: { acknowledged: true }
+
+  - do:
+      index:
+        index: test
+        id: 1
+        pipeline: "my_pipeline"
+        body: {message: "[foo][bar][]"}
+
+  - do:
+      get:
+        index: test
+        id: 1
+  - match: { _source.message: "[foo][bar][]" }
+  - match: { _source.a: "foo" }
+  - match: { _source.b: "bar" }
+  - match: { _source.c: "" }