Fix GETRANGE DivideByZeroException on empty-string value with negative start

[hexonal]

Summary

GETRANGE on a key holding an empty string, with a negative start such that start <= end (e.g. GETRANGE key -1 -1), crashes the connection with a DivideByZeroException instead of returning an empty string.

Root cause

NormalizeRange(int start, int end, int len) in libs/server/Storage/Functions/MainStore/PrivateMethods.cs does start %= len unconditionally inside its start < 0 branch. When len == 0 (empty string value), this is a modulo by zero.

Fix

Add an early guard returning (0, 0) when len == 0 — the same sentinel NormalizeRange already returns from its other "no valid range" branches (e.g. if (start > end) return (0, 0);), so it's consistent with the function's existing convention rather than a new special case. NormalizeRange has exactly one caller (GETRANGE's handling in CopyRespToWithInput), and (0, 0) correctly resolves to an empty RESP reply there.

Tests

Added GetRangeEmptyValueTest to test/standalone/Garnet.test/RespTests.cs, covering the crashing case (GETRANGE key -1 -1, GETRANGE key -1 0) alongside non-crashing empty-value cases (GETRANGE key 0 0, GETRANGE key 0 -1) to confirm all correctly return empty string. Confirmed the test reproduces a connection-level failure against the pre-fix code and passes with the fix.

* Fix `GETRANGE` on an empty-string value throwing `DivideByZeroException` (and dropping the connection) for a negative start offset.

[Copilot]

Pull request overview

This PR fixes a crash in Garnet’s GETRANGE handling for keys holding an empty string when start is negative (e.g., GETRANGE key -1 -1). The root cause is a modulo-by-zero in NormalizeRange(start, end, len) when len == 0, which previously could drop the client connection with a DivideByZeroException.

Changes:

• Add an early len == 0 guard in NormalizeRange to return the existing (0, 0) “empty range” sentinel and avoid modulo-by-zero.
• Add a regression test that exercises the previously-crashing cases (negative start) as well as other empty-value GETRANGE calls.

PR Title & Description Alignment

• Title accurately reflects the implemented fix and the failure mode.
• Description matches the actual code change (early guard in NormalizeRange) and includes concrete repro + test coverage details.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
libs/server/Storage/Functions/MainStore/PrivateMethods.cs	Adds a len == 0 early-return in NormalizeRange to prevent modulo-by-zero for empty string values in GETRANGE.
test/standalone/Garnet.test/RespTests.cs	Adds GetRangeEmptyValueTest covering GETRANGE on an empty string with negative start indices and validating empty-string replies.