feat(gin): add ProblemDetails middleware rendering Context errors as RFC 9457

ProblemDetails() renders errors attached via Context.Error as an RFC 9457
problem details JSON response when the handler chain finishes without
writing a response. The response status code is kept when it is an error
status, otherwise it defaults to 500 Internal Server Error.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/doc.md

@@ -1862,6 +1862,28 @@ func main() {
 }
 ```
 
+The `gin.ProblemDetails()` middleware renders errors attached with `c.Error()` as problem details responses, so handlers only need to report errors. The response status code is kept when it is an error status, otherwise it defaults to `500`. The last attached error's message is exposed to the client as the problem `detail` member, so do not attach errors whose messages must stay private.
+
+```go
+func main() {
+  r := gin.Default()
+  r.Use(gin.ProblemDetails())
+
+  r.GET("/users/:id", func(c *gin.Context) {
+    user, err := lookup(c.Param("id"))
+    if err != nil {
+      c.Status(http.StatusNotFound)
+      c.Error(err) //nolint:errcheck
+      return
+    }
+    c.JSON(http.StatusOK, user)
+  })
+
+  // listen and serve on 0.0.0.0:8080
+  r.Run(":8080")
+}
+```
+
 ### Serving static files
 
 ```go

problem.go

@@ -5,6 +5,8 @@
 package gin
 
 import (
+	"net/http"
+
 	"github.com/gin-gonic/gin/codec/json"
 )
 
@@ -58,3 +60,26 @@ func (p Problem) MarshalJSON() ([]byte, error) {
 	}
 	return json.API.Marshal(obj)
 }
+
+// ProblemDetails returns a middleware that renders errors attached to the
+// Context (see Context.Error) as an RFC 9457 problem details JSON response.
+// It does nothing if no errors were attached or if a response body or header
+// was already written. The response status code is kept when it is an error
+// status, otherwise it defaults to 500 Internal Server Error.
+// WARNING: the last attached error's message is exposed to the client as the
+// problem detail member. Do not attach errors whose messages must stay
+// private, or write the response yourself instead.
+func ProblemDetails() HandlerFunc {
+	return func(c *Context) {
+		c.Next()
+
+		if len(c.Errors) == 0 || c.Writer.Written() {
+			return
+		}
+		status := c.Writer.Status()
+		if status < http.StatusBadRequest {
+			status = http.StatusInternalServerError
+		}
+		c.ProblemJSON(status, Problem{Detail: c.Errors.Last().Error()})
+	}
+}

problem_test.go

@@ -5,6 +5,8 @@
 package gin
 
 import (
+	"errors"
+	"net/http"
 	"testing"
 
 	"github.com/gin-gonic/gin/codec/json"
@@ -42,8 +44,9 @@ func TestProblemMarshalJSONOmitsEmptyMembers(t *testing.T) {
 
 func TestProblemMarshalJSONExtensions(t *testing.T) {
 	p := Problem{
-		Status: 403,
-		Detail: "Your current balance is 30, but that costs 50.",
+		Status:   403,
+		Detail:   "Your current balance is 30, but that costs 50.",
+		Instance: "/account/12345/msgs/abc",
 		Extensions: map[string]any{
 			"balance": 30,
 			"status":  "extension members must not override standard members",
@@ -56,6 +59,65 @@ func TestProblemMarshalJSONExtensions(t *testing.T) {
 	assert.JSONEq(t, `{
 		"status": 403,
 		"detail": "Your current balance is 30, but that costs 50.",
+		"instance": "/account/12345/msgs/abc",
 		"balance": 30
 	}`, string(jsonBytes))
 }
+
+func TestProblemDetailsMiddleware(t *testing.T) {
+	router := New()
+	router.Use(ProblemDetails())
+	router.GET("/error", func(c *Context) {
+		c.Error(errors.New("boom")) //nolint:errcheck
+	})
+
+	w := PerformRequest(router, http.MethodGet, "/error")
+
+	assert.Equal(t, http.StatusInternalServerError, w.Code)
+	assert.Equal(t, "application/problem+json; charset=utf-8", w.Header().Get("Content-Type"))
+	assert.JSONEq(t, `{"title":"Internal Server Error","status":500,"detail":"boom"}`, w.Body.String())
+}
+
+func TestProblemDetailsMiddlewareKeepsErrorStatus(t *testing.T) {
+	router := New()
+	router.Use(ProblemDetails())
+	router.GET("/conflict", func(c *Context) {
+		c.Status(http.StatusConflict)
+		c.Error(errors.New("already exists")) //nolint:errcheck
+	})
+
+	w := PerformRequest(router, http.MethodGet, "/conflict")
+
+	assert.Equal(t, http.StatusConflict, w.Code)
+	assert.Equal(t, "application/problem+json; charset=utf-8", w.Header().Get("Content-Type"))
+	assert.JSONEq(t, `{"title":"Conflict","status":409,"detail":"already exists"}`, w.Body.String())
+}
+
+func TestProblemDetailsMiddlewareNoErrors(t *testing.T) {
+	router := New()
+	router.Use(ProblemDetails())
+	router.GET("/ok", func(c *Context) {
+		c.JSON(http.StatusOK, H{"foo": "bar"})
+	})
+
+	w := PerformRequest(router, http.MethodGet, "/ok")
+
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Equal(t, "application/json; charset=utf-8", w.Header().Get("Content-Type"))
+	assert.JSONEq(t, `{"foo":"bar"}`, w.Body.String())
+}
+
+func TestProblemDetailsMiddlewareResponseAlreadyWritten(t *testing.T) {
+	router := New()
+	router.Use(ProblemDetails())
+	router.GET("/written", func(c *Context) {
+		c.JSON(http.StatusBadGateway, H{"error": "custom body"})
+		c.Error(errors.New("boom")) //nolint:errcheck
+	})
+
+	w := PerformRequest(router, http.MethodGet, "/written")
+
+	assert.Equal(t, http.StatusBadGateway, w.Code)
+	assert.Equal(t, "application/json; charset=utf-8", w.Header().Get("Content-Type"))
+	assert.JSONEq(t, `{"error":"custom body"}`, w.Body.String())
+}