Handle checksum for unseekable 0-length s3 request body (#3086)

* Handle checksum for unseekable body with 0 content length

Author: Madrigal

.changelog/0e919f9c00d14acd87fcf6c0f1de614f.json

@@ -0,0 +1,8 @@
+{
+    "id": "0e919f9c-00d1-4acd-87fc-f6c0f1de614f",
+    "type": "bugfix",
+    "description": "Handle checksum for unseekable body with 0 content length",
+    "modules": [
+        "service/internal/checksum"
+    ]
+}

service/internal/checksum/middleware_compute_input_checksum.go

@@ -179,7 +179,7 @@ func (m *ComputeInputPayloadChecksum) HandleFinalize(
 
 	// Only seekable streams are supported for non-trailing checksums, because
 	// the stream needs to be rewound before the handler can continue.
-	if stream != nil && !req.IsStreamSeekable() {
+	if stream != nil && !req.IsStreamSeekable() && streamLength != 0 {
 		return out, metadata, computeInputHeaderChecksumError{
 			Msg: "unseekable stream is not supported without TLS and trailing checksum",
 		}
@@ -194,11 +194,13 @@ func (m *ComputeInputPayloadChecksum) HandleFinalize(
 			Err: err,
 		}
 	}
-
-	if err := req.RewindStream(); err != nil {
-		return out, metadata, computeInputHeaderChecksumError{
-			Msg: "failed to rewind stream",
-			Err: err,
+	// only attempt rewind if the stream length has been determined and is non-zero
+	if streamLength > 0 {
+		if err := req.RewindStream(); err != nil {
+			return out, metadata, computeInputHeaderChecksumError{
+				Msg: "failed to rewind stream",
+				Err: err,
+			}
 		}
 	}
 
@@ -414,7 +416,7 @@ func computeStreamChecksum(algorithm Algorithm, stream io.Reader, computePayload
 }
 
 func getRequestStreamLength(req *smithyhttp.Request) (int64, error) {
-	if v := req.ContentLength; v > 0 {
+	if v := req.ContentLength; v >= 0 {
 		return v, nil
 	}
 

service/internal/checksum/middleware_compute_input_checksum_test.go

@@ -4,6 +4,7 @@
 package checksum
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"fmt"
@@ -239,16 +240,16 @@ func TestComputeInputPayloadChecksum(t *testing.T) {
 						r := smithyhttp.NewStackRequest().(*smithyhttp.Request)
 						r.URL, _ = url.Parse("https://example.aws")
 						r.ContentLength = 0
-						r = requestMust(r.SetStream(&bytes.Buffer{}))
+						r = requestMust(r.SetStream(bufio.NewReader(bytes.NewBuffer([]byte{}))))
 						return r
 					}(),
 				},
 				expectHeader: http.Header{
 					"X-Amz-Checksum-Crc32": []string{"AAAAAA=="},
 				},
 				expectContentLength: 0,
-				expectPayload:       nil,
-				expectPayloadHash:   "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
+				expectPayload:       []byte{},
+				// payload hash is set via a different middleware, so not checking it here
 				expectChecksumMetadata: map[string]string{
 					"CRC32": "AAAAAA==",
 				},