feat: compile-time feature-gated Rust backtrace for JNI error diagnostics

java/lance-jni/Cargo.toml

@@ -14,6 +14,7 @@ crate-type = ["cdylib"]
 
 [features]
 default = []
+backtrace = ["lance/backtrace", "lance-core/backtrace"]
 
 [dependencies]
 lance = { path = "../../rust/lance", features = ["substrait"] }

java/lance-jni/src/error.rs

@@ -175,28 +175,35 @@ impl std::fmt::Display for Error {
 
 impl From<LanceError> for Error {
     fn from(err: LanceError) -> Self {
+        let backtrace_suffix = err
+            .backtrace()
+            .map(|bt| format!("\n\nRust backtrace:\n{}", bt))
+            .unwrap_or_default();
+        let message = format!("{}{}", err, backtrace_suffix);
+
         match &err {
             LanceError::DatasetNotFound { .. }
             | LanceError::DatasetAlreadyExists { .. }
             | LanceError::CommitConflict { .. }
-            | LanceError::InvalidInput { .. } => Self::input_error(err.to_string()),
-            LanceError::IO { .. } => Self::io_error(err.to_string()),
-            LanceError::NotSupported { .. } => Self::unsupported_error(err.to_string()),
-            LanceError::NotFound { .. } => Self::io_error(err.to_string()),
+            | LanceError::InvalidInput { .. } => Self::input_error(message),
+            LanceError::IO { .. } => Self::io_error(message),
+            LanceError::NotSupported { .. } => Self::unsupported_error(message),
+            LanceError::NotFound { .. } => Self::io_error(message),
             LanceError::Namespace { source, .. } => {
                 // Try to downcast to NamespaceError and get the error code
                 if let Some(ns_err) = source.downcast_ref::<NamespaceError>() {
-                    Self::namespace_error(ns_err.code().as_u32(), ns_err.to_string())
+                    let ns_message = format!("{}{}", ns_err, backtrace_suffix);
+                    Self::namespace_error(ns_err.code().as_u32(), ns_message)
                 } else {
                     log::warn!(
                         "Failed to downcast NamespaceError source, falling back to runtime error. \
                          This may indicate a version mismatch. Source type: {:?}",
                         source
                     );
-                    Self::runtime_error(err.to_string())
+                    Self::runtime_error(message)
                 }
             }
-            _ => Self::runtime_error(err.to_string()),
+            _ => Self::runtime_error(message),
         }
     }
 }
@@ -234,3 +241,104 @@ impl From<Utf8Error> for Error {
         Self::input_error(err.to_string())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // Helper: extract the java_class from an Error via Display output
+    fn java_class(err: &Error) -> &JavaExceptionClass {
+        &err.java_class
+    }
+
+    #[test]
+    fn test_invalid_input_maps_to_illegal_argument() {
+        let lance_err = LanceError::invalid_input("bad input");
+        let jni_err: Error = lance_err.into();
+        assert_eq!(
+            *java_class(&jni_err),
+            JavaExceptionClass::IllegalArgumentException
+        );
+        assert!(jni_err.message.contains("bad input"));
+    }
+
+    #[test]
+    fn test_dataset_not_found_maps_to_illegal_argument() {
+        let lance_err = LanceError::dataset_not_found("my_dataset", "not found".to_string().into());
+        let jni_err: Error = lance_err.into();
+        assert_eq!(
+            *java_class(&jni_err),
+            JavaExceptionClass::IllegalArgumentException
+        );
+        assert!(jni_err.message.contains("my_dataset"));
+    }
+
+    #[test]
+    fn test_dataset_already_exists_maps_to_illegal_argument() {
+        let lance_err = LanceError::dataset_already_exists("my_dataset");
+        let jni_err: Error = lance_err.into();
+        assert_eq!(
+            *java_class(&jni_err),
+            JavaExceptionClass::IllegalArgumentException
+        );
+        assert!(jni_err.message.contains("my_dataset"));
+    }
+
+    #[test]
+    fn test_commit_conflict_maps_to_illegal_argument() {
+        let lance_err = LanceError::commit_conflict_source(42, "conflict".to_string().into());
+        let jni_err: Error = lance_err.into();
+        assert_eq!(
+            *java_class(&jni_err),
+            JavaExceptionClass::IllegalArgumentException
+        );
+    }
+
+    #[test]
+    fn test_io_maps_to_ioexception() {
+        let lance_err = LanceError::io("disk failure");
+        let jni_err: Error = lance_err.into();
+        assert_eq!(*java_class(&jni_err), JavaExceptionClass::IOException);
+        assert!(jni_err.message.contains("disk failure"));
+    }
+
+    #[test]
+    fn test_not_supported_maps_to_unsupported() {
+        let lance_err = LanceError::not_supported("nope");
+        let jni_err: Error = lance_err.into();
+        assert_eq!(
+            *java_class(&jni_err),
+            JavaExceptionClass::UnsupportedOperationException
+        );
+        assert!(jni_err.message.contains("nope"));
+    }
+
+    #[test]
+    fn test_not_found_maps_to_ioexception() {
+        let lance_err = LanceError::not_found("missing_uri");
+        let jni_err: Error = lance_err.into();
+        assert_eq!(*java_class(&jni_err), JavaExceptionClass::IOException);
+        assert!(jni_err.message.contains("missing_uri"));
+    }
+
+    #[test]
+    fn test_fallthrough_maps_to_runtime() {
+        let lance_err = LanceError::internal("internal oops");
+        let jni_err: Error = lance_err.into();
+        assert_eq!(*java_class(&jni_err), JavaExceptionClass::RuntimeException);
+        assert!(jni_err.message.contains("internal oops"));
+    }
+
+    #[test]
+    fn test_no_backtrace_suffix_when_backtrace_is_none() {
+        // Without the backtrace feature enabled in lance-core default tests,
+        // backtrace() returns None, so no suffix should be appended.
+        let lance_err = LanceError::io("clean message");
+        let jni_err: Error = lance_err.into();
+        assert!(
+            !jni_err.message.contains("Rust backtrace:"),
+            "Expected no backtrace suffix, got: {}",
+            jni_err.message
+        );
+    }
+}

java/pom.xml

@@ -39,6 +39,7 @@
         <spotless.scala.scalafmt.version>3.7.5</spotless.scala.scalafmt.version>
         <spotless.delimiter>package</spotless.delimiter>
         <rust.release.build>false</rust.release.build>
+        <rust.features></rust.features>
         <skip.build.jni>false</skip.build.jni>
         <shade.base>org.lance.shaded</shade.base>
         <spotless.license.header>
@@ -396,6 +397,9 @@
                                 <configuration>
                                     <path>lance-jni</path>
                                     <release>${rust.release.build}</release>
+                                    <features>
+                                        <feature>${rust.features}</feature>
+                                    </features>
                                     <!-- Copy native libraries to target/classes for runtime access -->
                                     <copyTo>${project.build.directory}/classes/nativelib</copyTo>
                                     <copyWithPlatformDir>true</copyWithPlatformDir>
@@ -409,6 +413,9 @@
                                 <configuration>
                                     <path>lance-jni</path>
                                     <release>${rust.release.build}</release>
+                                    <features>
+                                        <feature>${rust.features}</feature>
+                                    </features>
                                     <verbosity>-v</verbosity>
                                 </configuration>
                             </execution>

rust/lance-core/Cargo.toml

@@ -55,6 +55,10 @@ proptest.workspace = true
 rstest.workspace = true
 
 [features]
+# Capture Rust backtraces in error types. When disabled (the default),
+# the backtrace field is zero-sized with no overhead. At runtime, capture
+# is still gated by RUST_BACKTRACE=1.
+backtrace = []
 datafusion = ["dep:datafusion-common", "dep:datafusion-sql"]
 
 [lints]