responses

internal/builders/responses — maintainers' guide

Builds OAS v2 response entries (swagger:response) including the response body and any header fields. One Builder per declaration; one Walk pass per field doc-comment.

Sections

• §overview — package shape and per-file responsibilities
• §builder — Builder, Build, the build chain
• §in-discriminator — in: as body/header annotation switch
• §file-body — swagger:file and the body-only gate
• §dispatch — Walker handlers wiring for headers
• §typable — responseTypable, the refAttempted mechanism, body vs header
• §alias-handling — when to $ref vs expand
• §quirks-history — resolved quirks (Stream M)

§overview — files and responsibilities

The builder embeds *common.Builder (Ctx, Decl, PostDeclarations, diagnostic sink, ParseBlocks cache, MakeRef). See ../common for the common-builder rationale.

This package's shape closely mirrors ../parameters — the chain is structurally the same. Divergences are called out below.

§builder — the build chain

Build(responses) looks up the response by name (from r.Decl.ResponseNames()), runs applyBlockToDecl to capture the top-level description, then calls buildFromType on the declared type. buildFromType unwraps pointers, dispatches named types and aliases. Unlike parameters, anonymous types are rejected: responses_test.go documents the rationale — the top-level response-as-alias case under default mode is deferred to v2.

For each non-embedded exported field, processResponseField runs:

1. Find the AST field via resolvers.FindASTField (no AST → skip).
2. Pre-scan the doc-comment signals via scanFieldDocSignals (uses the common.Builder parse cache).
3. If swagger:ignore → skip.
4. Resolve JSON tag name. If json:"-" → skip. A name: keyword on the field overrides this derived name — it renames the response header (the Headers map key), mirroring name: on a parameters field (Block.GetString(grammar.KwName), applied before name flows into the Headers key / seen set). Harmless on a body field, which never consults name.
5. Resolve in: (default header; see §in-discriminator).
6. File-body gate: if swagger:file AND in==body → set resp.Schema = {type:"file"} and skip the field build (see §file-body).
7. Otherwise build the field's type into either resp.Schema (body) or a header value (non-body), through responseTypable.
8. Apply swagger:strfmt <name> override when set.
9. Walk the doc-comment block via applyBlockToHeader (description, validations, items, extensions — no required:).
10. If in != body, register the header on resp.Headers[name].

After all fields are processed, buildFromStruct deletes header entries for fields that were skipped (the seen map).

§in-discriminator — in: as body/header annotation switch

OAS v2 has no in field on the Response Object — the location exists at the parameter level only. This package overloads in: on response fields to tell apart "this field is the body" from "this field is a header" within a single Go struct:

• in: body → field's type populates resp.Schema
• in: header (or absent → defaults to header) → field becomes one entry in resp.Headers
• in: query | path | formData → recognised but unusual; not a response location per OAS v2. Treated as non-body (header-like) with no special handling.

Default — implicit header

Pre-Stream M the implicit case fell into the in != "body" branch by accident: an empty string is not "body" and so behaved identically to header. Q1 made this default explicit in code — inHeader is assigned when !signals.inSet. Observable behaviour is unchanged; the implicit fall-through is gone.

Inherited in: from an embed (go-swagger#2701)

An in: annotation on an embedded (anonymous) field applies to the response fields it promotes. buildFromStruct reads it via the shared common.EmbedInheritance kernel and threads it through the embed recursion with save/restore; processResponseField consults it as the fallback between a field's own in: and the header default. The common case — an embed of header fields marked // in: header (or unmarked) — promotes each field as a response header.

// in: body on an embed is special: a response has a single body, so per-field promotion is meaningless. The embed IS the body — the embedded struct drives resp.Schema (a $ref to a model, or its inline shape), exactly like a named Body Foo field. buildFromStruct detects the inherited in: body and routes the embed through buildBodyEmbed instead of the field-promotion recursion (go-swagger#1635). Response bodies also inherit required: from embeds, but through the schema builder (a body is built there), not here — OAS2 response headers carry no required. See common §embed-inheritance.

Invalid in: values

An in: line with a non-vocabulary value (e.g. in: cookie) emits a CodeInvalidAnnotation warning via Warnf and defaults to header. Author misuse surfaces in diagnostics without breaking the build.

Why line-scan instead of property

Same reason as parameters — in: may appear on either side of an annotation. See ../parameters/README.md#in-discriminator.

§file-body — swagger:file annotation

swagger:file on a response field marks the entire response body as a file payload (image, PDF, raw bytes). Per OAS v2, the allowed header types are {string, number, integer, boolean, array} — file is forbidden on a header. The annotation must therefore land on the body field; on a header it is misuse.

The Q3 gate

Pre-Q3 the file branch fired unconditionally and rewrote resp.Schema = {file, ""} even when in != body, silently corrupting the body schema from a header-positioned annotation. Q3 gates the branch on in == inBody:

useFileBody := signals.file && in == inBody

When signals.file fires under a non-body in, the dispatcher emits a CodeUnsupportedInSimpleSchema warning and falls through to the regular field build, treating the field like any other header. The body schema is untouched.

§dispatch — Walker dispatch for headers

applyBlockToHeader is the per-field entry point for header fields. Three phases:

1. Prose → header.Description.
2. Level-0 dispatch → dispatchHeaderLevel0 wires Walker callbacks via the handlers package. Shape mirrors parameters' level-0 dispatcher with one omission:
   • Number, Integer, String (Pattern + CollectionFormat), Raw (default/example/enum) → identical to parameters
   • Bool → handlers.UniqueBool(valid) only — no required: write. The OAS v2 Header object simply doesn't carry required:.
   • Extension → handlers.Extension(header) — v1 had no header-side extension support at all; the grammar migration closes that gap. User-authored Extensions: block entries land on the header.
3. Items-level dispatch → walkHeaderItemsLevel for each (level, items) pair returned by collectHeaderItemsLevels (1-indexed depths matching grammar's Property.ItemsDepth).

applyBlockToDecl is the top-level (response-level) entry point. It only writes resp.Description from prose — the v1 SectionedParser only accepted description at the top level, no taggers. Property-level keywords on the top-level decl are silently ignored.

Header errSink semantics

Unlike dispatchParamLevel0, the response Raw handler is called with a nil errSink: malformed default: / example: for a header produces a parser diagnostic but is not promoted to a build error. Headers are non-critical metadata; failing the build over a malformed example would be surprising.

§typable — responseTypable, body vs header

responseTypable adapts a header or body slot to ifaces.SwaggerTypable. Single struct, polymorphic by ht.in:

• Body (in == "body"): Schema() materialises and returns resp.Schema. Typed() writes to the header struct, but body callers use Schema() directly. Items() walks resp.Schema.Items.
• Header (in == "header" or anything non-body): Typed() writes to the embedded SimpleSchema on the header. Items() builds the items chain on the header side.

The Q2 refAttempted mechanism

OAS v2 response headers cannot carry $ref. Pre-Q2, SetRef wrote the ref onto response.Schema.Ref unconditionally — which corrupted the body schema with a header field's reference when a header field aliased to a named type. The Q2 fix:

func (ht responseTypable) SetRef(ref oaispec.Ref) {
    if ht.in == inBody {
        ht.Schema().Ref = ref
        return
    }
    if ht.refAttempted != nil {
        *ht.refAttempted = true
    }
}

Non-body SetRef calls no-op the ref write and flip a flag on a caller-owned bool. HasRef() reads the flag for SimpleSchemaProbe so the schema builder's exit validator can detect the violation, emit CodeUnsupportedInSimpleSchema, and call ResetForViolation() (which wipes the header's SimpleSchema back to empty).

The flag is caller-owned (passed by pointer) so a single responseTypable value can be shared across recursion levels without mutation through the ifaces.SwaggerTypable value-receiver methods.

SimpleSchemaProbe implementation

• SimpleSchemaShape() — returns the header's embedded SimpleSchema
• HasRef() — true if a non-body SetRef attempt was made
• ResetForViolation() — wipes the header's SimpleSchema back to {}

§alias-handling

The responses builder shares the alias-handling contract with the schema and parameters builders — annotation gates first-class alias identity at use sites; TransparentAliases overrides at use sites; mode flags only shape the alias's own definition. The full rule lives in schema/README.md §aliases; below captures the responses-specific reach contexts.

Two responses-specific use-site handlers:

Top-level alias annotated swagger:response (buildAlias). The alias is transparent re: model creation in all modes — neither the alias nor any chain link of its backing struct surfaces in definitions. The fields of the unaliased target become the response's body and headers. The implementation just forwards tpe.Rhs() to buildFromType; recursion handles chains naturally. No mode-specific behaviour at this layer.

Alias as a field type within a response struct (buildFieldAlias). Three branches:

• TransparentAliases=true — dissolve via the schema sub-builder.
• Non-body field (response header) — SimpleSchema target cannot carry $ref; always expand to the unaliased target via types.Unalias. Annotation has no effect at non-body sites.
• Body field — annotation gate decides:
  • With swagger:model: MakeRef to the alias's decl — $ref: <AliasName> preserves the alias name and the alias surfaces in definitions via the discovery loop.
  • Without swagger:model: dissolve via types.Unalias (full chain collapse in one step) and build from the unaliased target.

The body-field gate is mode-agnostic: Default and Ref produce the same $ref target — annotation alone decides. The mode flag shapes only the alias decl's own definition downstream.

§quirks-history — resolved quirks

The Stream M merge-readiness pass closed a handful of subtle quirks in this builder. Recorded for archaeology — none should resurface under the current dispatch.

• Q1: implicit header default — in: absent now resolves explicitly to inHeader. Pre-Q1 the empty string fell through in != "body" by accident. Observable behaviour unchanged.
• Q2: $ref leaking into response body — non-body SetRef calls no longer write to response.Schema.Ref. The refAttempted flag plumbs the attempt to the SimpleSchema exit validator. See §typable.
• Q3: swagger:file on a header — gated to in == inBody only; misuse emits a diagnostic and falls through to the regular field build.
• Q4: alias expansion at field-level — added the In() != inBody || !RefAliases() gate to buildFieldAlias so non-body header aliases expand instead of leaking a $ref. Aligns with parameters' gate exactly.
• Q5 (closed, no change) — the previously-suspected TODO around alias deduplication turned out to be obsolete; no code change needed.
• Q6: alignment with parameters — buildFieldAlias and processResponseField now mirror parameters' shape so the two builders behave consistently at field-level alias sites and during dispatch.

Deferred to v2

• Top-level response-as-alias under default mode: buildFromType's default branch rejects anonymous types with "anonymous types are currently not supported for responses". A top-level alias to an anonymous struct under default mode crashes here. Reproduces in fixtures/enhancements/alias-response-shapes. Out of scope for v1.