session: allow large filesync pattern lists in go1.27

client/client_local_source_test.go

@@ -433,3 +433,62 @@ func testParallelLocalBuilds(t *testing.T, sb integration.Sandbox) {
 	err = eg.Wait()
 	require.NoError(t, err)
 }
+
+// testLocalSourceManyPatterns checks that filesync patterns are not limited by
+// the HTTP/2 header value count. Every pattern is sent as a separate gRPC
+// metadata value and Go 1.27 net/http rejects requests with more than 500 of
+// them by default.
+func testLocalSourceManyPatterns(t *testing.T, sb integration.Sandbox) {
+	c, err := New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	dir := integration.Tmpdir(
+		t,
+		fstest.CreateFile("foo", []byte("foo"), 0600),
+		fstest.CreateFile("ignored-1", []byte("ignored"), 0600),
+	)
+
+	const n = 1000
+	excludes := make([]string, 0, n)
+	includes := make([]string, 0, n+1)
+	for i := 1; i <= n; i++ {
+		excludes = append(excludes, fmt.Sprintf("ignored-%d", i))
+		includes = append(includes, fmt.Sprintf("included-%d", i))
+	}
+	includes = append(includes, "foo")
+
+	for _, tc := range []struct {
+		name string
+		opt  llb.LocalOption
+	}{
+		{name: "exclude", opt: llb.ExcludePatterns(excludes)},
+		{name: "include", opt: llb.IncludePatterns(includes)},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			def, err := llb.Local("mylocal", tc.opt).Marshal(sb.Context())
+			require.NoError(t, err)
+
+			destDir := t.TempDir()
+			_, err = c.Solve(sb.Context(), def, SolveOpt{
+				Exports: []ExportEntry{
+					{
+						Type:      ExporterLocal,
+						OutputDir: destDir,
+					},
+				},
+				LocalMounts: map[string]fsutil.FS{
+					"mylocal": dir,
+				},
+			}, nil)
+			require.NoError(t, err)
+
+			dt, err := os.ReadFile(filepath.Join(destDir, "foo"))
+			require.NoError(t, err)
+			require.Equal(t, "foo", string(dt))
+
+			_, err = os.Stat(filepath.Join(destDir, "ignored-1"))
+			require.ErrorIs(t, err, os.ErrNotExist)
+		})
+	}
+}

client/client_test.go

@@ -192,6 +192,7 @@ var allTests = []func(t *testing.T, sb integration.Sandbox){
 	testLocalSymlinkEscape,
 	testMetadataOnlyLocal,
 	testParallelLocalBuilds,
+	testLocalSourceManyPatterns,
 
 	// client_mergeop_test.go
 	testMergeOp,

session/grpc.go

@@ -63,7 +63,7 @@ func serve(ctx context.Context, grpcServer *grpc.Server, conn net.Conn) {
 		conn.Close()
 	}()
 	bklog.G(ctx).Debugf("serving grpc connection")
-	(&http2.Server{}).ServeConn(conn, &http2.ServeConnOpts{Handler: grpcServer}) //nolint:staticcheck // This existing HTTP/2 connection is already established.
+	newHTTP2Server(ctx).ServeConn(conn, &http2.ServeConnOpts{Handler: grpcServer}) //nolint:staticcheck // This existing HTTP/2 connection is already established.
 }
 
 func grpcClientConn(ctx context.Context, conn net.Conn, opts map[string][]string) (context.Context, *grpc.ClientConn, error) {

session/http2server_go127.go

@@ -0,0 +1,29 @@
+//go:build go1.27
+
+package session
+
+import (
+	"context"
+	"math"
+	"net/http"
+
+	"github.com/moby/buildkit/util/bklog"
+	"golang.org/x/net/http2"
+)
+
+// newHTTP2Server returns the HTTP/2 server for session connections.
+//
+// Since Go 1.27, x/net/http2 is backed by net/http, which limits requests to
+// http.DefaultMaxHeaderValueCount (500) header values. Session metadata, such
+// as filesync include/exclude patterns, uses one value per item and can exceed
+// that. The total header size is still bounded by http.DefaultMaxHeaderBytes.
+func newHTTP2Server(ctx context.Context) *http2.Server { //nolint:staticcheck // ServeConn on an established connection needs x/net/http2.
+	h2 := &http2.Server{} //nolint:staticcheck // ServeConn on an established connection needs x/net/http2.
+	// h1 only carries config for h2 connections and never serves on its own.
+	h1 := &http.Server{MaxHeaderValueCount: math.MaxInt}  //nolint:gosec // G112: session streams are long-lived, no read timeouts.
+	if err := http2.ConfigureServer(h1, h2); err != nil { //nolint:staticcheck // h1 is never served, Protocols does not apply.
+		bklog.G(ctx).WithError(err).Warn("failed to configure session http2 server")
+		return &http2.Server{} //nolint:staticcheck // ServeConn on an established connection needs x/net/http2.
+	}
+	return h2
+}

session/http2server_legacy.go

@@ -0,0 +1,15 @@
+//go:build !go1.27
+
+package session
+
+import (
+	"context"
+
+	"golang.org/x/net/http2"
+)
+
+// newHTTP2Server returns the HTTP/2 server for session connections. Before Go
+// 1.27 x/net/http2 has no header value count limit, so no extra setup is needed.
+func newHTTP2Server(_ context.Context) *http2.Server { //nolint:staticcheck // ServeConn on an established connection needs x/net/http2.
+	return &http2.Server{} //nolint:staticcheck // ServeConn on an established connection needs x/net/http2.
+}