Compensate for invalid log files created by Delta Live Tables

It would appear that in some cases Delta Live Tables will create a Delta table
which does not adhere to the Delta Table protocol.

The metaData action as a **required** `schemaString` property which simply
doesn't exist. Since it appears that this only exists at version zero of the
transaction log, and the _actual_ schema exists in the following versions of the
table (e.g. 1), this change introduces a default deserializer on the MetaData
action which provides a simple empty schema.

This is an alternative implementation to delta-io#1305 which is a bit more invasive and
makes our schema_string struct member `Option<String>` which I do not believe is
worth it for this unfortunate compatibility issue

Closes delta-io#1305, delta-io#1302, delta-io#1357

Sponsored-by: Databricks Inc

rust/src/action/mod.rs

@@ -506,6 +506,12 @@ impl Default for Format {
     }
 }
 
+/// Return a default empty schema to be used for edge-cases when a schema is missing
+fn default_schema() -> String {
+    warn!("A `metaData` action was missing a `schemaString` and has been given an empty schema");
+    r#"{"type":"struct",  "fields": []}"#.into()
+}
+
 /// Action that describes the metadata of the table.
 /// This is a top-level action in Delta log entries.
 #[derive(Serialize, Deserialize, Debug, Default, Clone)]
@@ -520,6 +526,7 @@ pub struct MetaData {
     /// Specification of the encoding for the files stored in the table
     pub format: Format,
     /// Schema of the table
+    #[serde(default="default_schema")]
     pub schema_string: String,
     /// An array containing the names of columns by which the data should be partitioned
     pub partition_columns: Vec<String>,

rust/tests/data/delta-live-table/_delta_log/00000000000000000000.json

@@ -0,0 +1,3 @@
+{"commitInfo":{"timestamp":1681798578373,"operation":"DLT REFRESH","operationParameters":{"pipelineId":"5c7b47e9-12d6-4986-a601-6716734281ce","updateId":"adf780f9-789f-4857-8d41-e4d9938b61d5"},"isolationLevel":"WriteSerializable","isBlindAppend":false,"operationMetrics":{},"engineInfo":"Databricks-Runtime/dlt:11.3-delta-pipelines-801d604-ff1aff6-f0f113d-custom-local","txnId":"a2ad05c8-4559-41bc-b7b1-a94773bd2286"}}
+{"protocol":{"minReaderVersion":1,"minWriterVersion":2}}
+{"metaData":{"id":"ac0a0120-970e-4d8c-ae92-b5244b055d6e","format":{"provider":"parquet","options":{}},"partitionColumns":[],"configuration":{},"createdTime":1681798577757}}

rust/tests/data/delta-live-table/_delta_log/00000000000000000001.json

@@ -0,0 +1,3 @@
+{"commitInfo":{"timestamp":1681798612850,"operation":"DLT SETUP","operationParameters":{"pipelineId":"5c7b47e9-12d6-4986-a601-6716734281ce","updateId":"adf780f9-789f-4857-8d41-e4d9938b61d5"},"readVersion":0,"isolationLevel":"WriteSerializable","isBlindAppend":false,"operationMetrics":{},"engineInfo":"Databricks-Runtime/dlt:11.3-delta-pipelines-801d604-ff1aff6-f0f113d-custom-local","txnId":"c1e3c149-d6b1-4a51-b7a6-89c328d14833"}}
+{"protocol":{"minReaderVersion":2,"minWriterVersion":5}}
+{"metaData":{"id":"ac0a0120-970e-4d8c-ae92-b5244b055d6e","name":"SnowflakeTest_Snowflake_DTL_SHERPA_USER_TABLE","format":{"provider":"parquet","options":{}},"schemaString":"{\"type\":\"struct\",\"fields\":[{\"name\":\"sherpa_user_id\",\"type\":\"decimal(38,0)\",\"nullable\":true,\"metadata\":{}},{\"name\":\"enabled\",\"type\":\"boolean\",\"nullable\":true,\"metadata\":{}},{\"name\":\"last_login\",\"type\":\"timestamp\",\"nullable\":true,\"metadata\":{}},{\"name\":\"first_name\",\"type\":\"string\",\"nullable\":true,\"metadata\":{}},{\"name\":\"last_name\",\"type\":\"string\",\"nullable\":true,\"metadata\":{}},{\"name\":\"full_name\",\"type\":\"string\",\"nullable\":true,\"metadata\":{}},{\"name\":\"email\",\"type\":\"string\",\"nullable\":true,\"metadata\":{}},{\"name\":\"job_title\",\"type\":\"string\",\"nullable\":true,\"metadata\":{}},{\"name\":\"hire_date\",\"type\":\"date\",\"nullable\":true,\"metadata\":{}},{\"name\":\"skypoint_delta_index\",\"type\":\"long\",\"nullable\":true,\"metadata\":{}}]}","partitionColumns":[],"configuration":{"pipelines.pipelineId":"5c7b47e9-12d6-4986-a601-6716734281ce","pipelines.autoOptimize.managed":"false","pipelines.metastore.tableName":"automation_retailsandbox.SnowflakeTest_Snowflake_DTL_SHERPA_USER_TABLE"},"createdTime":1681798577757}}

rust/tests/read_delta_log_test.rs

@@ -149,3 +149,11 @@ async fn test_read_liquid_table() -> DeltaResult<()> {
     let _table = deltalake::open_table(&path).await?;
     Ok(())
 }
+
+// test for: https://github.com/delta-io/delta-rs/issues/1302
+#[tokio::test]
+async fn read_delta_table_from_dlt() {
+    let table = deltalake::open_table("./tests/data/delta-live-table").await.unwrap();
+    assert_eq!(table.version(), 1);
+    assert!(table.schema().is_some());
+}