Add SetDefaultHostMetadataResolverFactory

[simonfaltum]

Changes

PR #1572 added Config.HostMetadataResolver so callers could override the SDK's /.well-known/databricks-config fetch on a per-Config basis. That covers "I have one Config and I want to wrap it."

The gap: programs that construct many Configs across their command surface (e.g. the Databricks CLI) end up copying the same cfg.HostMetadataResolver = ... assignment at every construction site, in the CLI roughly 10 sites across 7 files plus a guardrail test to catch drift.

This PR adds a package-level default consulted when a Config has no explicit resolver set. Callers set a factory once during startup; every subsequent Config gets the same resolver without per-site wiring. The Config-level field still takes precedence, so PR #1572's contract is unchanged.

API

// config/host_metadata.go
var DefaultHostMetadataResolverFactory func(*Config) HostMetadataResolver

Plain public variable, set once at init. Matches the stdlib pattern for single-default hooks: http.DefaultClient, http.DefaultTransport, log.Default. Callers needing per-Config or dynamic behaviour should use Config.HostMetadataResolver instead.

Resolution order inside Config.EnsureResolved

1. If Config.HostMetadataResolver is set, use it.
2. Else, if DefaultHostMetadataResolverFactory is non-nil, invoke it with the resolving Config and use its return value. If it returns nil, fall through.
3. Else, SDK's default HTTP fetch (unchanged behavior for all existing callers).

How the Databricks CLI will use this

The canonical Go idiom for "library A registers itself with library B" is a blank import that triggers an init() in A. This is how database/sql drivers (_ "github.com/lib/pq"), image codecs (_ "image/png"), and encoding formats register themselves.

After this PR lands and is bumped into the CLI, CLI PR #5011 will collapse from ~10 wired-in hostmetadata.Attach(cfg) calls + a guardrail test down to two small pieces:

repos/cli/libs/hostmetadata/resolver.go — set the caching factory at package init:

func init() {
    config.DefaultHostMetadataResolverFactory = func(cfg *config.Config) config.HostMetadataResolver {
        return NewResolver(cfg.DefaultHostMetadataResolver())
    }
}

repos/cli/cmd/databricks/main.go — one blank import to pull the package in at startup:

import (
    // Registers a disk-cached HostMetadataResolver with the SDK so every
    // Config the CLI constructs reuses the cached /.well-known lookup.
    _ "github.com/databricks/cli/libs/hostmetadata"
)

That's the full integration. Every Config the CLI creates, now and in the future from any new command a developer adds, automatically gets caching. No per-site Attach call to remember, no guardrail test to maintain, no new developer ever has to learn this mechanism exists to benefit from it.

Experimental

Marked experimental to match the existing HostMetadataResolver field. No default behavior change for callers that never set DefaultHostMetadataResolverFactory.

Tests

Three new tests in config/config_test.go, each using a small withDefaultHostMetadataResolverFactory(t, factory) helper that captures and restores the prior value, so tests never clobber each other via the package-level default:

• Factory is invoked when Config has no resolver; back-fill works end-to-end.

• Config-level resolver takes precedence (factory not consulted).

• Factory returning nil falls through to the SDK's HTTP fetch.

• make fmt test lint clean

• go test ./config/... -count=1 -race clean

Signed-off-by: simon simon.faltum@databricks.com

[renaudhartert-db]

[optional] If users are expected to set this at init time, then you don't need the mutex. See http and log as references from the standard library.

var DefaultHostMetadataResolverFactory func(*Config) HostMetadataResolver = nil

Users who need to use different resolvers within their code should use Config.HostMetadataResolver instead.

[simonfaltum]

Switched to a plain var DefaultHostMetadataResolverFactory, dropped the mutex and the sync import. Matches the stdlib precedent you called out (http.DefaultClient, http.DefaultTransport, log.Default).

[renaudhartert-db]

LGTM, modulo comments.

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• PR number: 1636
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