GH-50852: [C++][FlightRPC][ODBC] Decode wide connection/descriptor string attributes with the wide decoder

[vikrantpuppala]

Rationale for this change

Two string attributes in the ODBC driver are decoded with the byte-wise
decoder even when the value arrives through a wide (Unicode / *W) entry point,
so the wide buffer is misread and corrupted.

ODBCConnection::SetConnectAttr(SQL_ATTR_CURRENT_CATALOG) had its two decode
branches swapped:

if (is_unicode) {
  SetAttributeUTF8(value, string_length, catalog);      // byte-wise
} else {
  SetAttributeSQLWCHAR(value, string_length, catalog);  // wide
}

is_unicode selects the buffer width: a unicode (*W) call passes a wide
SQLWCHAR buffer that must be decoded with SetAttributeSQLWCHAR; a non-unicode
call passes a byte string decoded with SetAttributeUTF8. With the branches
swapped, a wide catalog name (e.g. UTF-16 "my_catalog") is reinterpreted
byte-wise and stored with the wide encoding's embedded NULs
("m\0y\0_\0c\0a\0t\0a\0l\0o\0g\0"), so catalog scoping operates on a corrupt
name. This is the inverse of the mapping the getter already uses:
GetStringAttribute maps is_unicode -> GetAttributeSQLWCHAR.

ODBCDescriptor::SetField(SQL_DESC_NAME) is the same class of bug: it
unconditionally used the byte-wise SetAttributeUTF8, even though the matching
getter (GetField / SQL_DESC_NAME) reads the field back with
GetAttributeSQLWCHAR.

How this happened (history):

• The SQL_ATTR_CURRENT_CATALOG branches were swapped from the start, in the
  original driver import ([C++][FlightRPC] Accept donation of ODBC driver #46522, GH-46522: [C++][FlightRPC] Add Arrow Flight SQL ODBC driver #40939). Note the getter side was written
  against the shared GetStringAttribute helper (which decides width in one
  place and got it right), while the setter open-coded the branch inline and
  inverted the polarity — there is no matching SetStringAttribute helper.
• The descriptor SQL_DESC_NAME case was originally consistent (getter and
  setter both byte-wise). [C++][FlightRPC][ODBC] SQLColAttribute implementation #47721 (GH-47721: [C++][FlightRPC] Return ODBC Column Attribute from result set #48050) later migrated the descriptor
  string getters to the wide GetAttributeSQLWCHAR for correct Unicode column
  attributes, but left the SQL_DESC_NAME setter on the byte-wise decoder,
  creating the asymmetry.

What changes are included in this PR?

• odbc_connection.cc: swap the SQL_ATTR_CURRENT_CATALOG decode branches so a
  unicode call uses SetAttributeSQLWCHAR and a non-unicode call uses
  SetAttributeUTF8, matching GetStringAttribute.
• odbc_descriptor.cc: decode SQL_DESC_NAME with SetAttributeSQLWCHAR to
  match its getter.
• connection_attr_test.cc: add TestSQLSetGetConnectAttrCurrentCatalogWide, a
  TYPED_TEST (mock + remote fixtures) that sets a multi-character catalog
  through the wide entry point and asserts the full name round-trips back.

Are these changes tested?

Yes. The new test is a set-then-get round-trip through the real driver stack
(SQLSetConnectAttr / SQLGetConnectAttr), placed next to the other connection
attribute tests, and it distinguishes the fixed and unfixed driver:

check	result
new test, with fix (mock fixture)	PASS — out_catalog == "my_catalog"
new test, without fix (mock fixture)	FAIL — out_catalog == "m\0y\0_\0c\0a\0t\0a\0l\0o\0g\0"
ConnectionAttributeTest/0.* (full mock suite)	25/25 pass
clang-format	clean on all three files

The mock fixture (FlightSQLODBCMockTestBase) runs against an in-process SQLite
Flight SQL server; the remote variant runs when ARROW_FLIGHT_SQL_ODBC_CONN is
set. Verified locally against the mock fixture.

Are there any user-facing changes?

Yes. A catalog name (and descriptor SQL_DESC_NAME) set through the wide entry
point is now decoded correctly instead of being corrupted. Applications that set
a multi-character catalog through SQLSetConnectAttrW now scope to the intended
catalog.

AI usage

Per the AI-generated code guidance:
the diagnosis, the fix, and the test were produced with the assistance of an AI
coding agent and reviewed and verified by me. Correctness was checked by
(1) confirming the getter path (GetStringAttribute) maps is_unicode to the
wide decoder, establishing the intended mapping the setters violated, and
(2) running the new round-trip test against both the fixed and unfixed driver to
confirm it distinguishes them (clean name vs. embedded-NUL corruption).

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🤖 Drafted by Claude Code (an AI agent) and reviewed & verified by vikrantpuppala.

• GitHub Issue: [C++][FlightRPC][ODBC] SQL_ATTR_CURRENT_CATALOG (and SQL_DESC_NAME) decode a wide string with the byte-wise decoder #50852

[github-actions]

⚠️ GitHub issue #50852 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

Corrects wide-string decoding for ODBC catalog and descriptor attributes.

Changes:

• Uses the wide decoder for Unicode catalog and descriptor names.
• Preserves UTF-8 decoding for non-Unicode catalog values.
• Adds catalog round-trip coverage.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
connection_attr_test.cc	Tests wide catalog round-tripping.
odbc_descriptor.cc	Corrects descriptor-name decoding.
odbc_connection.cc	Corrects catalog decoder selection.

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