[Analytics Backend / DataFusion] Onboard PPL mvfind via custom Rust UDF

PPL `mvfind(arr, regex)` finds the 0-based index of the first array element
matching a regex pattern (Java `Matcher.find` substring-match semantics), or
NULL if no match. DataFusion has no stdlib equivalent, and rewriting in terms
of array_position requires per-element regex evaluation that's only
expressible with substrait lambda support — out of scope here. Onboards a
custom Rust ScalarUDF on the analytics-backend-datafusion plugin's session
context, mirroring the mvzip/convert_tz pattern.

Templated shape:

  Rust side:
    udf::mvfind::MvfindUdf — Signature::user_defined; coerce_types pins arg 0
      to a list type and arg 1 to Utf8; invoke_with_args walks each row and
      finds the first non-null element whose stringified form matches the
      regex via Rust's `regex` crate (`Regex::is_match` is unanchored, same
      as Java's `Matcher.find`). Scalar pattern operands compile once up
      front and surface invalid-regex errors at plan time (mirrors the SQL
      plugin's plan-time `tryCompileLiteralPattern`); column-valued patterns
      compile per row and yield NULL for invalid patterns. Supports list
      element types Utf8 / Int{8,16,32,64} / UInt{8,16,32,64} / Float{32,64}
      / Boolean / Null. 7 unit tests cover the basic-match / no-match /
      null-array / empty-array / null-element / numeric-array / unanchored
      shapes.
    Registered on each session context via udf::register_all alongside
    convert_tz and mvzip.

  Java side:
    ScalarFunction.MVFIND enum entry (SqlKind.OTHER_FUNCTION; resolves
      through identifier-name valueOf("MVFIND") since PPL's
      MVFindFunctionImpl registers under the function name "mvfind").
    MvfindAdapter — locally-declared SqlFunction("mvfind") +
      ADDITIONAL_SCALAR_SIGS bridge so isthmus emits a Substrait scalar
      function call with the exact name the Rust UDF is registered under.
    DataFusionAnalyticsBackendPlugin: STANDARD_PROJECT_OPS membership
      (returns INTEGER, registered against the existing scalar
      SUPPORTED_FIELD_TYPES); adapter registration in
      scalarFunctionAdapters().
    opensearch_array_functions.yaml: arity-2 impl returning `i32?`.

  * Before: 34/60.
  * After:  42/60.

  Newly passing — 8 of 9 testMvfind* variants:
    testMvfindWithMatch, testMvfindWithFirstMatch, testMvfindWithMultipleMatches,
    testMvfindWithNoMatch, testMvfindWithEmptyArray, testMvfindWithNumericArray,
    testMvfindWithCaseInsensitive, testMvfindWithComplexRegex.

  Remaining mvfind failure:
    testMvfindWithDynamicRegex — fails with "Unable to convert call
    CONCAT(string, string)" because the test computes the pattern via
    `concat('ban', '.*')` and substrait can't bind the CONCAT call. This is a
    separate analytics-engine CONCAT type-conversion issue, not mvfind-specific.

Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

sandbox/libs/analytics-framework/src/main/java/org/opensearch/analytics/spi/ScalarFunction.java

@@ -166,7 +166,15 @@ public enum ScalarFunction {
      * No DataFusion stdlib equivalent — the analytics-backend-datafusion plugin ships
      * a custom Rust UDF (`udf::mvzip`) registered on its session context.
      */
-    MVZIP(Category.SCALAR, SqlKind.OTHER_FUNCTION);
+    MVZIP(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+    /**
+     * PPL {@code mvfind(arr, regex)} — find the 0-based index of the first array
+     * element matching a regex, or NULL if no match. Resolves through the SQL
+     * plugin's {@code MVFindFunctionImpl} UDF named {@code "mvfind"}. No
+     * DataFusion stdlib equivalent — the analytics-backend-datafusion plugin
+     * ships a custom Rust UDF (`udf::mvfind`) registered on its session context.
+     */
+    MVFIND(Category.SCALAR, SqlKind.OTHER_FUNCTION);
 
     /**
      * Category of scalar function.

sandbox/plugins/analytics-backend-datafusion/rust/Cargo.toml

@@ -49,6 +49,9 @@ chrono-tz = "0.10"
 
 tokio-metrics = { workspace = true }
 
+# mvfind UDF — regex matching against stringified array elements
+regex = "1.10"
+
 [dev-dependencies]
 criterion = { workspace = true }
 tempfile = { workspace = true }

sandbox/plugins/analytics-backend-datafusion/rust/src/udf/mod.rs

@@ -113,12 +113,14 @@ pub(crate) fn coerce_args(
 }
 
 pub mod convert_tz;
+pub mod mvfind;
 pub mod mvzip;
 
 pub fn register_all(ctx: &SessionContext) {
     convert_tz::register_all(ctx);
     mvzip::register_all(ctx);
-    log::info!("OpenSearch UDF register_all: convert_tz, mvzip registered");
+    mvfind::register_all(ctx);
+    log::info!("OpenSearch UDF register_all: convert_tz, mvzip, mvfind registered");
 }
 
 #[cfg(test)]