Report to the RuntimeAPI any errors that occur when reading from a handler's returned reader

lambda/runtime_api_client.go

@@ -6,6 +6,7 @@ package lambda
 
 import (
 	"bytes"
+	"encoding/base64"
 	"fmt"
 	"io"
 	"io/ioutil" //nolint: staticcheck
@@ -21,6 +22,8 @@ const (
 	headerCognitoIdentity    = "Lambda-Runtime-Cognito-Identity"
 	headerClientContext      = "Lambda-Runtime-Client-Context"
 	headerInvokedFunctionARN = "Lambda-Runtime-Invoked-Function-Arn"
+	trailerLambdaErrorType   = "Lambda-Runtime-Function-Error-Type"
+	trailerLambdaErrorBody   = "Lambda-Runtime-Function-Error-Body"
 	contentTypeJSON          = "application/json"
 	contentTypeBytes         = "application/octet-stream"
 	apiVersion               = "2018-06-01"
@@ -106,10 +109,12 @@ func (c *runtimeAPIClient) next() (*invoke, error) {
 }
 
 func (c *runtimeAPIClient) post(url string, body io.Reader, contentType string) error {
-	req, err := http.NewRequest(http.MethodPost, url, body)
+	b := newErrorCapturingReader(body)
+	req, err := http.NewRequest(http.MethodPost, url, b)
 	if err != nil {
 		return fmt.Errorf("failed to construct POST request to %s: %v", url, err)
 	}
+	req.Trailer = b.Trailer
 	req.Header.Set("User-Agent", c.userAgent)
 	req.Header.Set("Content-Type", contentType)
 
@@ -122,7 +127,6 @@ func (c *runtimeAPIClient) post(url string, body io.Reader, contentType string)
 			log.Printf("runtime API client failed to close %s response body: %v", url, err)
 		}
 	}()
-
 	if resp.StatusCode != http.StatusAccepted {
 		return fmt.Errorf("failed to POST to %s: got unexpected status code: %d", url, resp.StatusCode)
 	}
@@ -134,3 +138,30 @@ func (c *runtimeAPIClient) post(url string, body io.Reader, contentType string)
 
 	return nil
 }
+
+func newErrorCapturingReader(r io.Reader) *errorCapturingReader {
+	trailer := http.Header{
+		trailerLambdaErrorType: nil,
+		trailerLambdaErrorBody: nil,
+	}
+	return &errorCapturingReader{r, trailer}
+}
+
+type errorCapturingReader struct {
+	reader  io.Reader
+	Trailer http.Header
+}
+
+func (r *errorCapturingReader) Read(p []byte) (int, error) {
+	if r.reader == nil {
+		return 0, io.EOF
+	}
+	n, err := r.reader.Read(p)
+	if err != nil && err != io.EOF {
+		lambdaErr := lambdaErrorResponse(err)
+		r.Trailer.Set(trailerLambdaErrorType, lambdaErr.Type)
+		r.Trailer.Set(trailerLambdaErrorBody, base64.StdEncoding.EncodeToString(safeMarshal(lambdaErr)))
+		return 0, io.EOF
+	}
+	return n, err
+}