fix(tls): load system trust anchors for verified clients

[alanhoff]

Summary

• fix default verified-client TLS behavior by loading system trust anchors when no custom CA bundle is configured
• keep ownership of internally-created CA bundles in the event-loop client lifecycle and release temporary client connections on init failures
• document the default verification behavior and add unit coverage

Vulnerability

ClientConfig.skip_cert_verify already defaults to false, but before this patch Client.init left ca_bundle as null unless callers set ca_cert_path. That meant hostname verification could run without any trusted roots, so a self-signed or attacker-controlled certificate for the requested name could still be accepted.

Examples

• a client connecting to api.example.com from a hostile Wi-Fi network with the default ClientConfig could accept an attacker certificate that presents the same hostname
• an app using event_loop.Client(...).init(.{ .server_name = "localhost" }) against a local proxy or test harness would believe verification was enabled while trust-anchor validation was silently skipped unless ca_cert_path was set manually

Testing

• zig build
• zig build test
• zig build fuzz

[Copilot]

Pull request overview

This PR fixes event_loop.Client’s default TLS verification behavior by ensuring a trust store is present when certificate verification is enabled, addressing a scenario where hostname verification could proceed without any trust anchors.

Changes:

• Add buildClientTlsConfig to auto-load the system CA bundle when skip_cert_verify=false and no ca_cert_path is provided.
• Track ownership/lifecycle of internally-created CA bundles in event_loop.Client and add init-time cleanup on failures.
• Update documentation and add unit tests covering the default CA-bundle behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/test_all.zig	Imports event_loop.zig so its new unit tests run under the main test aggregator.
src/event_loop.zig	Implements system trust-anchor loading for verified clients, adds CA-bundle ownership tracking, and adds unit tests.
SPEC/RFC5280_CHAIN_VALIDATION.md	Documents default event_loop.ClientConfig trust-anchor behavior and updates caveats.
README.md	Updates ClientConfig.ca_cert_path docs to clarify system-root fallback behavior.

Comments suppressed due to low confidence (1)

src/event_loop.zig:1117

• errdefer conn.deinit(); becomes unsafe once conn is copied into conn_ptr.* later (line 1120): if any subsequent init step fails (address parse, socket creation, xev init, etc.), both conn_ptr.deinit() (via its errdefer) and this conn.deinit() errdefer will run, causing a double-deinit/double-free of the same connection internals. Restructure to move the connection directly into the heap allocation (e.g., assign conn_ptr.* = try connection.connect(...) and only errdefer on conn_ptr), or otherwise ensure the conn errdefer is canceled once ownership transfers.

            errdefer conn.deinit();
            // Heap-allocate so pointers remain stable
            const conn_ptr = try alloc.create(connection.Connection);
            errdefer {
                conn_ptr.deinit();

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[Copilot]

Pull request overview

Fixes the default “verified client” TLS behavior in the event-loop client by ensuring a trust anchor set is present when certificate verification is enabled, aligning runtime behavior with ClientConfig.skip_cert_verify = false and the documented security model.

Changes:

• Automatically load the system root CA bundle when skip_cert_verify=false and no ca_cert_path is provided (unless a full custom tls_config override is supplied).
• Correct ownership/lifetime handling for internally created CA bundles and ensure partially-initialized connections are cleaned up safely on init failure paths.
• Add documentation and unit tests covering the default verification/trust-anchor behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/test_all.zig	Ensures event_loop.zig tests are included in the overall test suite.
src/event_loop.zig	Builds a safer default client TLS config by loading system roots, tracks CA bundle ownership, and hardens init cleanup.
SPEC/RFC5280_CHAIN_VALIDATION.md	Documents the new default behavior and clarifies caveats for direct tls13.TlsConfig usage.
README.md	Updates ClientConfig.ca_cert_path documentation to note the system root store default for verified clients.

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