fix(rust): map lance-namespace errors to TableNotFound / TableAlreadyExists

`LanceNamespaceDatabase::open_table` and `create_table` were squashing
`NamespaceError::TableNotFound` and `TableAlreadyExists` into generic
`Error::Runtime`, forcing callers (and downstream services like phalanx
in sophon) to fall back on fragile string matching to detect these
cases. This surfaced to users as HTTP 500 / "internal server error" on
operations that should have been 400/404 (see ENT-1235).

Walks the boxed-error chain from `lance::Error::Namespace` down to the
inner `NamespaceError` and maps its `ErrorCode` to the corresponding
`lancedb::Error` variant. Also replaces the brittle
`e.to_string().contains("already exists")` check in
`LanceNamespaceDatabase::create_table` with a downcast on the
`NamespaceError` code, so the create-after-declare-conflict path works
the same way across `dir` and REST namespace backends.

Tests pin the desired behavior at both the root namespace and inside a
child namespace, for both open_table and create_table.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

rust/lancedb/src/database/namespace.rs

@@ -11,6 +11,7 @@ use lance::io::commit::namespace_manifest::LanceNamespaceExternalManifestStore;
 use lance_io::object_store::{ObjectStoreParams, StorageOptionsAccessor};
 use lance_namespace::{
     LanceNamespace,
+    error::{ErrorCode, NamespaceError},
     models::{
         CreateNamespaceRequest, CreateNamespaceResponse, DeclareTableRequest,
         DescribeNamespaceRequest, DescribeNamespaceResponse, DescribeTableRequest,
@@ -29,7 +30,7 @@ use crate::database::listing::{
     OPT_NEW_TABLE_V2_MANIFEST_PATHS,
 };
 use crate::error::{Error, Result};
-use crate::table::NativeTable;
+use crate::table::{NativeTable, map_namespace_lance_error};
 use lance::dataset::WriteMode;
 
 use super::{
@@ -37,6 +38,19 @@ use super::{
     Database, OpenTableRequest, TableNamesRequest,
 };
 
+/// Returns true if the given `lance::Error` (anywhere in its source chain) is a
+/// `NamespaceError::TableAlreadyExists`.
+fn is_table_already_exists_namespace_error(err: &lance::Error) -> bool {
+    let mut current: Option<&(dyn std::error::Error + 'static)> = Some(err);
+    while let Some(e) = current {
+        if let Some(ns_err) = e.downcast_ref::<NamespaceError>() {
+            return ns_err.code() == ErrorCode::TableAlreadyExists;
+        }
+        current = e.source();
+    }
+    false
+}
+
 /// A database implementation that uses lance-namespace for table management
 pub struct LanceNamespaceDatabase {
     namespace: Arc<dyn LanceNamespace>,
@@ -356,13 +370,15 @@ impl Database for LanceNamespaceDatabase {
                         (loc, opts, response.managed_versioning)
                     }
                     Err(e)
-                        if matches!(request.mode, CreateTableMode::Create) && {
-                            let err_str = e.to_string();
-                            err_str.contains("already exists")
-                                || err_str.contains("TableAlreadyExists")
-                                || err_str.contains("table already exists")
-                        } =>
+                        if matches!(request.mode, CreateTableMode::Create)
+                            && is_table_already_exists_namespace_error(&e) =>
                     {
+                        // A declare conflict can either mean (a) the table was previously
+                        // *declared* but never written (in which case we should proceed and
+                        // create it), or (b) the table is fully realized (in which case the
+                        // user is creating something that already exists and we should
+                        // surface TableAlreadyExists). Disambiguate by describing the table
+                        // and checking whether it has both a version and a schema.
                         let response = self
                             .namespace
                             .describe_table(DescribeTableRequest {
@@ -370,11 +386,8 @@ impl Database for LanceNamespaceDatabase {
                                 ..Default::default()
                             })
                             .await
-                            .map_err(|describe_err| Error::Runtime {
-                                message: format!(
-                                    "Failed to describe existing declared table after declare conflict: {}",
-                                    describe_err
-                                ),
+                            .map_err(|describe_err| {
+                                map_namespace_lance_error(describe_err, &request.name)
                             })?;
 
                         if response.version.is_some() && response.schema.is_some() {
@@ -394,9 +407,7 @@ impl Database for LanceNamespaceDatabase {
                         (loc, opts, response.managed_versioning)
                     }
                     Err(e) => {
-                        return Err(Error::Runtime {
-                            message: format!("Failed to declare table: {}", e),
-                        });
+                        return Err(map_namespace_lance_error(e, &request.name));
                     }
                 }
             }
@@ -1086,8 +1097,120 @@ mod tests {
             .execute()
             .await;
 
-        // Verify: Should return an error
-        assert!(result.is_err());
+        // Verify: Should return TableNotFound — not a generic Runtime/internal error
+        // (regression test for ENT-1235: open_table on missing table previously surfaced as
+        // a generic 500/Runtime error rather than TableNotFound).
+        match result {
+            Err(Error::TableNotFound { name, .. }) => {
+                assert_eq!(name, "non_existent_table");
+            }
+            Err(other) => panic!("Expected TableNotFound, got: {:?}", other),
+            Ok(_) => panic!("Expected open_table to fail, but it succeeded"),
+        }
+    }
+
+    #[tokio::test]
+    async fn test_namespace_open_table_not_found_at_root() {
+        // Same as above, but at the root namespace (no parent namespace creation).
+        // Covers the common code path used by `db.open_table("foo")` without a namespace.
+        let tmp_dir = tempdir().unwrap();
+        let root_path = tmp_dir.path().to_str().unwrap().to_string();
+
+        let mut properties = HashMap::new();
+        properties.insert("root".to_string(), root_path);
+
+        let conn = connect_namespace("dir", properties)
+            .execute()
+            .await
+            .expect("Failed to connect to namespace");
+
+        let result = conn.open_table("missing_at_root").execute().await;
+
+        match result {
+            Err(Error::TableNotFound { name, .. }) => {
+                assert_eq!(name, "missing_at_root");
+            }
+            Err(other) => panic!("Expected TableNotFound, got: {:?}", other),
+            Ok(_) => panic!("Expected open_table to fail, but it succeeded"),
+        }
+    }
+
+    #[tokio::test]
+    async fn test_namespace_create_table_already_exists() {
+        // Regression test for ENT-1235: create_table on an existing table (in default
+        // Create mode) should return TableAlreadyExists, not a generic Runtime/500 error.
+        let tmp_dir = tempdir().unwrap();
+        let root_path = tmp_dir.path().to_str().unwrap().to_string();
+
+        let mut properties = HashMap::new();
+        properties.insert("root".to_string(), root_path);
+
+        let conn = connect_namespace("dir", properties)
+            .execute()
+            .await
+            .expect("Failed to connect to namespace");
+
+        conn.create_namespace(CreateNamespaceRequest {
+            id: Some(vec!["test_ns".into()]),
+            ..Default::default()
+        })
+        .await
+        .expect("Failed to create namespace");
+
+        // Create the table once.
+        conn.create_table("dup_table", create_test_data())
+            .namespace(vec!["test_ns".into()])
+            .execute()
+            .await
+            .expect("Failed to create table the first time");
+
+        // Try to create it again with the default Create mode.
+        let result = conn
+            .create_table("dup_table", create_test_data())
+            .namespace(vec!["test_ns".into()])
+            .execute()
+            .await;
+
+        match result {
+            Err(Error::TableAlreadyExists { name }) => {
+                assert_eq!(name, "dup_table");
+            }
+            Err(other) => panic!("Expected TableAlreadyExists, got: {:?}", other),
+            Ok(_) => panic!("Expected create_table to fail, but it succeeded"),
+        }
+    }
+
+    #[tokio::test]
+    async fn test_namespace_create_table_already_exists_at_root() {
+        // Same as above, but at the root namespace.
+        let tmp_dir = tempdir().unwrap();
+        let root_path = tmp_dir.path().to_str().unwrap().to_string();
+
+        let mut properties = HashMap::new();
+        properties.insert("root".to_string(), root_path);
+
+        let conn = connect_namespace("dir", properties)
+            .execute()
+            .await
+            .expect("Failed to connect to namespace");
+
+        conn.create_table("dup_root", create_test_data())
+            .execute()
+            .await
+            .expect("Failed to create table the first time");
+
+        let result = conn
+            .create_table("dup_root", create_test_data())
+            .execute()
+            .await;
+
+        match result {
+            Err(Error::TableAlreadyExists { name }) => {
+                assert_eq!(name, "dup_root");
+            }
+            Err(other) => panic!("Expected TableAlreadyExists, got: {:?}", other),
+            Ok(_) => panic!("Expected create_table to fail, but it succeeded"),
+        }
     }
 
     #[tokio::test]

rust/lancedb/src/table.rs

@@ -36,6 +36,7 @@ pub use query::AnyQuery;
 
 use lance::io::commit::namespace_manifest::LanceNamespaceExternalManifestStore;
 use lance_namespace::LanceNamespace;
+use lance_namespace::error::NamespaceError;
 use lance_namespace::models::DescribeTableRequest;
 use lance_table::format::Manifest;
 use lance_table::io::commit::CommitHandler;
@@ -94,6 +95,53 @@ pub use schema_evolution::{AddColumnsResult, AlterColumnsResult, DropColumnsResu
 use serde_with::skip_serializing_none;
 pub use update::{UpdateBuilder, UpdateResult};
 
+/// Walk a boxed error chain to find the innermost `NamespaceError`.
+///
+/// Callers like `DatasetBuilder::from_namespace` re-wrap their inner namespace error
+/// inside a fresh `lance::Error::Namespace`, so a single downcast at the top level
+/// won't find it. This walks `.source()` to unwrap arbitrarily nested layers.
+fn find_namespace_error<'a>(
+    err: &'a (dyn std::error::Error + 'static),
+) -> Option<&'a NamespaceError> {
+    let mut current: Option<&(dyn std::error::Error + 'static)> = Some(err);
+    while let Some(e) = current {
+        if let Some(ns_err) = e.downcast_ref::<NamespaceError>() {
+            return Some(ns_err);
+        }
+        current = e.source();
+    }
+    None
+}
+
+/// Map a `lance::Error` coming from a `lance-namespace` call into a `lancedb::Error`,
+/// preserving the fine-grained namespace error code (e.g. `TableNotFound`,
+/// `TableAlreadyExists`). Errors that aren't recognized namespace error variants fall
+/// through to a generic runtime error rather than `TableNotFound`/`TableAlreadyExists`.
+pub(crate) fn map_namespace_lance_error(err: lance::Error, table_name: &str) -> Error {
+    if let Some(code) = find_namespace_error(&err).map(NamespaceError::code) {
+        match code {
+            lance_namespace::error::ErrorCode::TableNotFound => {
+                return Error::TableNotFound {
+                    name: table_name.to_string(),
+                    source: Box::new(err),
+                };
+            }
+            lance_namespace::error::ErrorCode::TableAlreadyExists => {
+                return Error::TableAlreadyExists {
+                    name: table_name.to_string(),
+                };
+            }
+            _ => {}
+        }
+    }
+    match err {
+        lance::Error::Namespace { source, .. } => Error::Runtime {
+            message: format!("Namespace error: {}", source),
+        },
+        other => other.into(),
+    }
+}
+
 /// Defines the type of column
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum ColumnKind {
@@ -1494,12 +1542,7 @@ impl NativeTable {
         // and storage options from the namespace
         let builder = DatasetBuilder::from_namespace(namespace_client.clone(), table_id)
             .await
-            .map_err(|e| match e {
-                lance::Error::Namespace { source, .. } => Error::Runtime {
-                    message: format!("Failed to get table info from namespace: {:?}", source),
-                },
-                e => e.into(),
-            })?;
+            .map_err(|e| map_namespace_lance_error(e, name))?;
 
         let dataset = builder
             .with_read_params(params)