Connect unary should return unavailable instead of unimplemented for io.EOF errors from the transport

.golangci.yml

@@ -19,8 +19,6 @@ linters-settings:
   importas:
     no-unaliased: true
     alias:
-      - pkg: connectrpc.com/connect
-        alias: connect
       - pkg: connectrpc.com/connect/internal/gen/connect/ping/v1
         alias: pingv1
   varnamelen:

client_ext_test.go

@@ -31,7 +31,7 @@ import (
 	"testing"
 	"time"
 
-	connect "connectrpc.com/connect"
+	"connectrpc.com/connect"
 	"connectrpc.com/connect/internal/assert"
 	pingv1 "connectrpc.com/connect/internal/gen/connect/ping/v1"
 	"connectrpc.com/connect/internal/gen/connect/ping/v1/pingv1connect"
@@ -227,6 +227,58 @@ func TestGetNoContentHeaders(t *testing.T) {
 	assert.Equal(t, http.MethodGet, unaryReq.HTTPMethod())
 }
 
+func TestConnectionDropped(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	for _, protocol := range []string{connect.ProtocolConnect, connect.ProtocolGRPC, connect.ProtocolGRPCWeb} {
+		var opts []connect.ClientOption
+		switch protocol {
+		case connect.ProtocolGRPC:
+			opts = []connect.ClientOption{connect.WithGRPC()}
+		case connect.ProtocolGRPCWeb:
+			opts = []connect.ClientOption{connect.WithGRPCWeb()}
+		}
+		t.Run(protocol, func(t *testing.T) {
+			t.Parallel()
+			httpClient := httpClientFunc(func(_ *http.Request) (*http.Response, error) {
+				return nil, io.EOF
+			})
+			client := pingv1connect.NewPingServiceClient(
+				httpClient,
+				"http://1.2.3.4",
+				opts...,
+			)
+			t.Run("unary", func(t *testing.T) {
+				t.Parallel()
+				req := connect.NewRequest[pingv1.PingRequest](nil)
+				_, err := client.Ping(ctx, req)
+				assert.NotNil(t, err)
+				if !assert.Equal(t, connect.CodeOf(err), connect.CodeUnavailable) {
+					t.Logf("err = %v\n%#v", err, err)
+				}
+			})
+			t.Run("stream", func(t *testing.T) {
+				t.Parallel()
+				req := connect.NewRequest[pingv1.CountUpRequest](nil)
+				svrStream, err := client.CountUp(ctx, req)
+				if err == nil {
+					t.Cleanup(func() {
+						assert.Nil(t, svrStream.Close())
+					})
+					if !assert.False(t, svrStream.Receive()) {
+						return
+					}
+					err = svrStream.Err()
+				}
+				assert.NotNil(t, err)
+				if !assert.Equal(t, connect.CodeOf(err), connect.CodeUnavailable) {
+					t.Logf("err = %v\n%#v", err, err)
+				}
+			})
+		})
+	}
+}
+
 func TestSpecSchema(t *testing.T) {
 	t.Parallel()
 	mux := http.NewServeMux()
@@ -762,3 +814,9 @@ func addUnrecognizedBytes[M proto.Message](msg M, data []byte) M {
 	msg.ProtoReflect().SetUnknown(data)
 	return msg
 }
+
+type httpClientFunc func(*http.Request) (*http.Response, error)
+
+func (fn httpClientFunc) Do(req *http.Request) (*http.Response, error) {
+	return fn(req)
+}

duplex_http_call.go

@@ -306,6 +306,11 @@ func (d *duplexHTTPCall) makeRequest() {
 	// pipe. Write's check for io.ErrClosedPipe and will convert this to io.EOF.
 	response, err := d.httpClient.Do(d.request) //nolint:bodyclose
 	if err != nil {
+		if errors.Is(err, io.EOF) {
+			// We use io.EOF as a sentinel in many places and don't want this
+			// transport error to be confused for those other situations.
+			err = io.ErrUnexpectedEOF
+		}
 		err = wrapIfContextError(err)
 		err = wrapIfLikelyH2CNotConfiguredError(d.request, err)
 		err = wrapIfLikelyWithGRPCNotUsedError(err)