fix: Ensure tags are never null (#25680)

* fix: Ensure tags are never null

This injects empty strings into tags for any rows in the buffer where the tag value is null. This is required because the tags are what make up the series key, which must have all non-null values.

There is an ongoing discussion about what the real behavior should be here, but for now this will get our users running that break without this behavior. Discussion is in #25674.

Fixes #25648

* fix: clippy failures

influxdb3/tests/server/query.rs

@@ -1330,3 +1330,49 @@ async fn api_v3_query_sql_meta_cache() {
         resp
     );
 }
+
+/// Test that if we write in a row of LP that is missing a tag value, we're still able to query.
+#[tokio::test]
+async fn api_v3_query_null_tag_values() {
+    let server = TestServer::spawn().await;
+
+    server
+        .write_lp_to_db(
+            "foo",
+            "cpu,host=a,region=us-east usage=0.9 1
+            cpu,host=b usage=0.80 4",
+            Precision::Second,
+        )
+        .await
+        .unwrap();
+
+    let client = reqwest::Client::new();
+    let url = format!("{base}/api/v3/query_sql", base = server.client_addr());
+
+    let resp = client
+        .get(&url)
+        .query(&[
+            ("db", "foo"),
+            (
+                "q",
+                "SELECT host, region, time, usage FROM cpu ORDER BY time",
+            ),
+            ("format", "pretty"),
+        ])
+        .send()
+        .await
+        .unwrap()
+        .text()
+        .await
+        .unwrap();
+
+    assert_eq!(
+        "+------+---------+---------------------+-------+\n\
+            | host | region  | time                | usage |\n\
+            +------+---------+---------------------+-------+\n\
+            | a    | us-east | 1970-01-01T00:00:01 | 0.9   |\n\
+            | b    |         | 1970-01-01T00:00:04 | 0.8   |\n\
+            +------+---------+---------------------+-------+",
+        resp
+    );
+}

influxdb3_write/src/write_buffer/table_buffer.rs

@@ -282,7 +282,7 @@ impl MutableTableChunk {
                             let mut tag_builder = StringDictionaryBuilder::new();
                             // append nulls for all previous rows
                             for _ in 0..(row_index + self.row_count) {
-                                tag_builder.append_null();
+                                tag_builder.append_value("");
                             }
                             e.insert(Builder::Tag(tag_builder));
                         }
@@ -390,8 +390,8 @@ impl MutableTableChunk {
                         Builder::I64(b) => b.append_null(),
                         Builder::U64(b) => b.append_null(),
                         Builder::String(b) => b.append_null(),
-                        Builder::Tag(b) => b.append_null(),
-                        Builder::Key(b) => b.append_null(),
+                        Builder::Tag(b) => b.append_value(""),
+                        Builder::Key(b) => b.append_value(""),
                         Builder::Time(b) => b.append_null(),
                     }
                 }
@@ -467,7 +467,10 @@ impl MutableTableChunk {
                 schema_builder.influx_column(col_name.as_ref(), InfluxColumnType::Tag);
                 let mut tag_builder: StringDictionaryBuilder<Int32Type> =
                     StringDictionaryBuilder::new();
-                tag_builder.append_nulls(self.row_count);
+                for _ in 0..self.row_count {
+                    tag_builder.append_value("");
+                }
+
                 cols.push(Arc::new(tag_builder.finish()));
             }
         }