fix: connect via all DNS IPs when first contact point is non-responsive (DRIVER-201)

[nikagra]

Problem

When RESOLVE_CONTACT_POINTS=false (the default), a contact point hostname is stored as a single unresolved InetSocketAddress. At connection time, ChannelFactory passed this address directly to Netty's bootstrap.connect(), which internally called InetAddress.getByName() — returning only the first IP for the hostname. If that IP was non-responsive, the driver raised AllNodesFailedException with no fallback to other IPs the hostname might resolve to.

This is particularly impactful in dynamic DNS environments where a hostname can map to multiple nodes and the first one may be temporarily unavailable.

Fixes DRIVER-201.

Changes

ChannelFactory.java

Refactored the connection logic using the Parameter Object pattern and SRP decomposition:

• ConnectRequest inner class: bundles all per-connection-attempt state, eliminating the unwieldy 9-parameter private connect() method.
• connect(ConnectRequest): resolves the endpoint address. For unresolved hostnames, calls InetAddress.getAllByName() to expand to all known IPs, building a candidate list. DNS resolution is still deferred to connection time — the EndPoint continues to hold the unresolved hostname — preserving the dynamic-DNS semantics of RESOLVE_CONTACT_POINTS=false.
• tryNextAddress(request, candidates, index): iterates the candidate list. On per-address failure it tries the next candidate; only when all are exhausted does it fail the overall resultFuture.
• connectToAddress(request, address): performs a single Netty bootstrap connect to one resolved IP. Protocol-version negotiation (downgrade retries) is scoped to the same IP, which is semantically correct. Returns its own CompletableFuture<DriverChannel> so tryNextAddress can distinguish a per-address failure from the final outcome.
• tryNextAddressRaw: unchanged pass-through for non-InetSocketAddress types (e.g. Unix domain sockets).

MockResolverIT.java

New integration test should_connect_when_first_dns_entry_is_non_responsive:

• 2-node CCM cluster on 127.0.1.x
• test.cluster.fake resolves to 127.0.1.11 (non-existent) first, then nodes 1 and 2
• RESOLVE_CONTACT_POINTS=false, RECONNECT_ON_INIT=false
• Asserts the session opens successfully and both nodes come up despite the first DNS entry being unreachable

[Copilot]

Pull request overview

This PR addresses DRIVER-201 by improving initial contact point connection behavior when RESOLVE_CONTACT_POINTS=false, ensuring that if a hostname resolves to multiple IPs and the first is unreachable, the driver can fall back to subsequent IPs instead of failing immediately.

Changes:

• Refactors ChannelFactory connection logic to expand unresolved hostnames to all DNS IPs at connection time and attempt them sequentially.
• Introduces a ConnectRequest parameter object and decomposes connection steps into smaller methods.
• Adds an integration test covering the “first DNS A record is unreachable” scenario.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
core/src/main/java/com/datastax/oss/driver/internal/core/channel/ChannelFactory.java	Expands unresolved hostnames to all resolved IPs at connect time and adds sequential fallback across addresses.
integration-tests/src/test/java/com/datastax/oss/driver/core/resolver/MockResolverIT.java	Adds an integration test to validate successful connection when the first DNS entry is non-responsive.

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