feat: Support Accept: application/json on /config

[darrenjaneczek]

What this PR does / why we need it:

/config returns YAML by default; this adds Accept: application/json content negotiation on top, for both the base response and the q-scoped response (#24230). A caller that sends Accept: application/json gets the same data as JSON instead of YAML; everything else is unchanged.

Which issue(s) this PR fixes:

Special notes for your reviewer:

• Mirrors the content-negotiation pattern already used by dskit's ring status handler (vendor/github.com/grafana/dskit/ring/ring_http.go).
• The response is normalized through a YAML round-trip before JSON-encoding, so JSON keys match the existing YAML field names (config structs carry yaml tags, not json ones).
• No behavior change when Accept doesn't ask for JSON — this is purely additive.

Checklist

• Reviewed the CONTRIBUTING.md guide
• Documentation added — happy to add a short mention to docs/sources/reference/loki-http-api.md if desired
• Tests updated
• Title matches the required conventional commits format
• N/A — no upgrade-impacting change
• N/A — no config option deprecated or removed

[Copilot]

🟡 Changes recommended

Address the content-negotiation issues and update the API documentation before approval.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Adds JSON content negotiation for /config while preserving YAML by default.

Changes:

• Supports JSON for base and q-scoped responses.
• Preserves YAML field names through normalization.
• Adds negotiation tests.

File summaries

File	Summary
pkg/loki/config_handler.go	Adds YAML/JSON response selection. Findings: set Vary: Accept (moderate, 1 vote); properly parse media ranges and q values (moderate, 3 votes); update API documentation (nit, 2 votes).
pkg/loki/config_handler_test.go	Adds tests for JSON responses and Accept headers.

Review details

Suppressed comments (1)

pkg/loki/config_handler.go:198

• Because the representation now varies on the request's Accept header, this handler needs to send Vary: Accept. Without it, an intermediary may cache a YAML response and serve it to a JSON client (or the reverse), since the header is not part of the cache key; set Vary before both branches.

	if !strings.Contains(r.Header.Get("Accept"), "application/json") {

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

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[github-actions]

💻 Deploy preview available (feat: Support Accept: application/json on /config):

• https://deploy-preview-loki-24557-zb444pucvq-vp.a.run.app/docs/loki/next/

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[Copilot]

🟡 Changes recommended

Address the Vary header, invalid q-value handling, regression coverage, and contradictory documentation.

Get a fresh assessment by requesting another Copilot review.

Review details

Suppressed comments (2)

pkg/loki/config_handler.go:200

• Because this handler now selects the representation from Accept, it must emit Vary: Accept. Without that response metadata, an intermediary can cache the first YAML or JSON response and serve it to requests with the other Accept value. Add the Vary header before this branch while preserving any existing Vary values.

func writeConfigResponse(w http.ResponseWriter, r *http.Request, v any) {

pkg/loki/config_handler.go:230

• When q is present but malformed or outside the RFC q-value range, this code silently keeps the default q = 1 and selects JSON (for example, Accept: application/json;q=bogus or q=2). A media range with an invalid q-value should not be treated as an explicit positive preference; reject/skip the range on parse or range errors and add a regression test.

		if raw, ok := params["q"]; ok {
			if parsed, err := strconv.ParseFloat(raw, 64); err == nil {
				q = parsed
			}

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

Address invalid q-values, repeated Accept headers, and missing Vary: Accept handling.

Review details

Suppressed comments (3)

Previously missed (2) — in code that hasn't changed since the last review.

pkg/loki/config_handler.go:202

• Header.Get returns only the first value, but HTTP permits repeated Accept fields. A request with Accept: text/html followed by Accept: application/json therefore falls back to YAML even though JSON was requested; join Header.Values("Accept") before parsing rather than dropping later values.
  pkg/loki/config_handler.go:231
• A malformed or out-of-range q value is treated as acceptable here: parse errors leave q at 1, while values such as q=2 remain positive. Thus Accept: application/json;q=bogus or q=2 selects JSON even though those are not valid qvalues; reject invalid values instead of defaulting them to 1.

pkg/loki/config_handler.go:202

• This response now varies by the Accept request header, but it never emits Vary: Accept. A shared HTTP cache can consequently reuse a JSON response for a YAML request (or the reverse); set Vary before selecting the representation.

func writeConfigResponse(w http.ResponseWriter, r *http.Request, v any) {
	// Only the first Accept header occurrence is honored; subsequent Accept headers are ignored.
	if !acceptsJSON(r.Header.Get("Accept")) {

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

Resolve the two outstanding Accept-header parsing issues before approval.

Review details

Suppressed comments (2)

Previously missed (1) — in code that hasn't changed since the last review.

pkg/loki/config_handler.go:206

• Header.Get returns only the first field value, but Accept is a list-valued header and may legally be sent on multiple field lines. For example, separate Accept: text/plain and Accept: application/json lines currently fall back to YAML even though JSON is explicitly accepted; combine/iterate over r.Header.Values("Accept") before parsing.

pkg/loki/config_handler.go:227

• Splitting the header with strings.Split does not correctly parse all valid Accept media ranges: commas inside a quoted parameter value are not separators. For example, application/json;profile="a,b" is split into invalid fragments and incorrectly falls back to YAML. Use quote-aware list parsing (or a dedicated Accept parser) before calling mime.ParseMediaType.

	for _, part := range strings.Split(accept, ",") {
		mediaType, params, err := mime.ParseMediaType(part)

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Lite