[ch-language-clients-rust]

Summary

Fixes #501.

Since #455, a query with at least one Query::param() sends its SQL as the query field of a multipart/form-data body. ClickHouse rejects any form field longer than http_max_field_value_size (128 KiB by default) with HTML Form Exception: Field value too long, so a parameterized query with more than 128 KiB of SQL failed. In 0.15.2 the SQL was the raw request body and had no such limit.

For a multipart body, the server joins the values of all query fields in order, without a separator (DynamicQueryHandler::getQuery). The client now sends the SQL as query fields of at most 128 KiB each, split on character boundaries. A query of up to 128 KiB still produces exactly one field, so its request does not change. ClickHouse/clickhouse-cs#701 takes the same approach.

The server has joined query fields this way since at least 19.14, and the loop is in every LTS release since. I checked it against 23.8, 26.7, 26.10 and head (26.11). The new integration test runs against head in CI, so a change on the server side would show up there.

Not changed: a single parameter value over 128 KiB still fails. That is not a regression, because the server applies the same limit to URL query-string values, so 0.15.2 failed on it too. A value can't be split across repeated param_<name> fields either. The Query::param docs now state the limit, and that only the server's default settings profile can raise it. A per-query or per-user http_max_field_value_size has no effect, because the server parses the form with its global settings.

Checklist

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided so that we can include it in CHANGELOG later

Candidate CHANGELOG entry

 * Parameterized queries (`Query::param()`) whose SQL text is larger than 128 KiB
   no longer fail with `HTML Form Exception: Field value too long`. The SQL is now
   sent as several `query` form fields of at most 128 KiB each, which the server
   joins. A single parameter value is still limited by the server's
   `http_max_field_value_size` (128 KiB by default).

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