Add server-side RESP protocol restrictions

[Haihan-Jiang]

Summary

• add --allowed-protocols with Both, Resp2, and Resp3 modes
• reject HELLO protocol switches that are disallowed by server configuration
• in RESP3-only mode, reject commands before the session has negotiated HELLO 3

Details

Both remains the default behavior. Resp2 keeps existing RESP2 clients working
and rejects HELLO 3. Resp3 allows HELLO 3 as the negotiation entry point,
then permits normal commands after the session protocol switches to RESP3. A
client in RESP3-only mode that sends ordinary commands before HELLO 3 receives:

ERR Protocol version not allowed by server configuration

For authenticated deployments, clients should use HELLO 3 AUTH <user> <pass>
as the first command.

Tests

/Users/haihan/.dotnet/dotnet test test/standalone/Garnet.test/Garnet.test.csproj --no-restore --framework net8.0 --filter 'FullyQualifiedName~GarnetServerConfigTests.AllowedProtocols' -v:minimal
/Users/haihan/.dotnet/dotnet test test/standalone/Garnet.test/Garnet.test.csproj --no-restore --framework net8.0 --filter 'FullyQualifiedName~RespTests.HelloRespectsResp' -v:minimal
/Users/haihan/.dotnet/dotnet test test/standalone/Garnet.test/Garnet.test.csproj --no-restore --framework net8.0 --filter 'FullyQualifiedName~RespTests.HelloTest1|FullyQualifiedName~RespTests.HelloAuthErrorTest' -v:minimal
/Users/haihan/.dotnet/dotnet test test/standalone/Garnet.test/Garnet.test.csproj --no-restore --framework net8.0 --filter 'FullyQualifiedName~RespTests.RespOutputTests' -v:minimal
git diff --check -- libs/host/Configuration/Options.cs libs/host/defaults.conf libs/server/Resp/BasicCommands.cs libs/server/Resp/CmdStrings.cs libs/server/Resp/RespServerSession.cs libs/server/Servers/GarnetServerOptions.cs test/standalone/Garnet.test/GarnetServerConfigTests.cs test/standalone/Garnet.test/RespTests.cs test/standalone/Garnet.test/TestUtils.cs
git diff --no-index --check /dev/null libs/server/Servers/RespProtocolMode.cs; test $? -eq 1

Latest local validation result, 2026-06-13 07:24 PDT / 14:24 UTC: all four
targeted dotnet test invocations passed, covering 7 tests total. Both
tracked-file git diff --check and the untracked RespProtocolMode.cs
no-index whitespace check passed.

Addresses #1755.

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds server configuration to restrict accepted RESP protocol versions and enforces that policy during command handling/HELLO negotiation.

Changes:

• Introduces RespProtocolMode and plumbs AllowedProtocols through server options/config/CLI.
• Enforces protocol policy in HELLO and command processing paths.
• Adds tests validating config parsing and runtime behavior for RESP2-only / RESP3-only modes.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/standalone/Garnet.test/TestUtils.cs	Extends test server factory to accept allowedProtocols.
test/standalone/Garnet.test/RespTests.cs	Adds runtime tests for HELLO + command behavior under protocol restrictions.
test/standalone/Garnet.test/GarnetServerConfigTests.cs	Adds config/CLI parsing tests for AllowedProtocols.
libs/server/Servers/RespProtocolMode.cs	New enum defining allowed protocol modes.
libs/server/Servers/GarnetServerOptions.cs	Adds AllowedProtocols and helper to validate protocol version.
libs/server/Resp/RespServerSession.cs	Enforces protocol policy during command execution.
libs/server/Resp/CmdStrings.cs	Adds new error message for disallowed protocol.
libs/server/Resp/BasicCommands.cs	Rejects HELLO protocol changes that violate server policy.
libs/host/defaults.conf	Documents and sets default config value for AllowedProtocols.
libs/host/Configuration/Options.cs	Adds CLI option and plumbs into server options.