[fix] strip NUL bytes before tdengine history insert

[orangeCatDeveloper]

What's changed?

Fixes #1481: Windows SNMP interface descriptions carry NUL padding bytes (\^@). The TDengine history storage builds its INSERT by hand, so the NUL reached the SQL text, TDengine rejected it with error 216, and the whole batch was lost — including the healthy rows.

Fix in TdEngineDataStorage.formatStringValue, the single choke point all string values, labels json and the instance tag pass through:

• strip \u0000 before anything else (only NUL — tabs/newlines are legitimate data)
• truncate the logical value before SQL-escaping: the old escape-then-truncate order could cut an escape pair like \' in half, leaving a dangling backslash that escapes the closing quote and kills the batch the same way

Only TDengine is affected: the other history backends use typed clients / JSON / line protocol instead of hand-built SQL.

Verified against a real TDengine 3.3.5.0 with a metric whose string value ends in \u0000 (one poisoned row, one healthy row per batch):

Before — every batch rejected, both rows lost:

ERROR TdEngineDataStorage - TDengine ERROR (216): sql: INSERT INTO `rows1481_rows_null_80_v2`
USING `rows1481_rows_super_v2` TAGS ('null:80') VALUES
(1786324976558, '{}', 1.0, 'Software Loopback Interface 1', 10.0)
(1786324976559, '{}', 2.0, 'normal-item', 20.0),
desc: syntax error near ''Software Loopback Interface 1' (invalid data or symbol)

taos> select * from hertzbeat.rows1481_rows_super_v2 limit 5;
Query OK, 0 row(s) in set

After — no errors, both rows stored, NUL stripped:

taos> select * from hertzbeat.rows1481_rows_super_v2 limit 5;
           ts            | seq |             name               | value |
==========================================================================
 2026-08-10 01:25:43.746 | 1.0 | Software Loopback Interface 1  | 10.0  |
 2026-08-10 01:25:43.747 | 2.0 | normal-item                    | 20.0  |
Query OK, 4 row(s) in set

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.

[Aias00]

Review: [fix] strip NUL bytes before tdengine history insert — APPROVED

Small, well-scoped fix for issue #1481 (snmp octet strings carrying NUL padding that broke the INSERT SQL).

What's correct

• Added NUL_CHAR_PATTERN = Pattern.compile("\\u0000") and strip NUL bytes first in formatStringValue.
• Bonus correctness fix: the truncation is now done before escaping (SQL_SPECIAL_STRING_PATTERN), with the comment explaining why — the old code truncated the already-escaped string, which could split an escape sequence (e.g. cut \' into \ + bare '). The new order avoids that class of bug entirely.
• replaceAll("") result is never null, so removing the != null guard is safe.
• Test testSaveDataStripsControlCharacters pins the contract: NUL stripped, tab (legitimate data) preserved, quote still escaped, and no raw/escaped NUL reaches the SQL — solid regression guard.

Non-blocking suggestions

• Only \u0000 is stripped. Other control chars (CR/LF, vertical tab) still flow into the SQL text. SNMP octet strings can carry those too; consider a broader [\p{Cntrl}&&[^\t]] strip if you want to be thorough, but tab-preservation is a deliberate choice here so this is optional.
• Truncating the logical value before escaping means the post-escape length can slightly exceed tableStrColumnDefineMaxLength by the number of inserted backslashes. Acceptable tradeoff given the escape-split fix, just worth noting.

Verdict: APPROVED. Clean fix with a meaningful secondary improvement and a real test.