diff --git a/gopls/internal/regtest/bench/bench_test.go b/gopls/internal/regtest/bench/bench_test.go
index 7f0da83..090a51c 100644
--- a/gopls/internal/regtest/bench/bench_test.go
+++ b/gopls/internal/regtest/bench/bench_test.go
@@ -33,7 +33,7 @@
 		// Skip logs as they buffer up memory unnaturally.
 		SkipLogs(),
 		// The Debug server only makes sense if running in singleton mode.
-		Modes(Singleton),
+		Modes(Default),
 		// Remove the default timeout. Individual tests should control their
 		// own graceful termination.
 		NoDefaultTimeout(),
diff --git a/gopls/internal/regtest/debug/debug_test.go b/gopls/internal/regtest/debug/debug_test.go
index d01d44e..bae1480 100644
--- a/gopls/internal/regtest/debug/debug_test.go
+++ b/gopls/internal/regtest/debug/debug_test.go
@@ -20,7 +20,7 @@
 	// Verify that a properly configured session gets notified of a bug on the
 	// server.
 	WithOptions(
-		Modes(Singleton), // must be in-process to receive the bug report below
+		Modes(Default), // must be in-process to receive the bug report below
 		Settings{"showBugReports": true},
 	).Run(t, "", func(t *testing.T, env *Env) {
 		const desc = "got a bug"
diff --git a/gopls/internal/regtest/diagnostics/diagnostics_test.go b/gopls/internal/regtest/diagnostics/diagnostics_test.go
index 9c9ad36..ac5307e 100644
--- a/gopls/internal/regtest/diagnostics/diagnostics_test.go
+++ b/gopls/internal/regtest/diagnostics/diagnostics_test.go
@@ -298,7 +298,7 @@
 
 	t.Run("without workspace module", func(t *testing.T) {
 		WithOptions(
-			Modes(Singleton),
+			Modes(Default),
 		).Run(t, noMod, func(t *testing.T, env *Env) {
 			env.Await(
 				env.DiagnosticAtRegexp("main.go", `"mod.com/bob"`),
@@ -1678,7 +1678,7 @@
 		WithOptions(
 			InGOPATH(),
 			EnvVars{"GO111MODULE": "off"},
-			Modes(Singleton),
+			Modes(Default),
 		).Run(t, mod, func(t *testing.T, env *Env) {
 			env.Await(
 				env.DiagnosticAtRegexpWithMessage("main.go", `"nosuchpkg"`, `cannot find package "nosuchpkg" in any of`),
@@ -1705,7 +1705,7 @@
 	for _, go111module := range []string{"on", "auto"} {
 		t.Run("GO111MODULE="+go111module, func(t *testing.T) {
 			WithOptions(
-				Modes(Singleton),
+				Modes(Default),
 				EnvVars{"GO111MODULE": go111module},
 			).Run(t, modules, func(t *testing.T, env *Env) {
 				env.OpenFile("a/a.go")
@@ -1722,7 +1722,7 @@
 	// Expect no warning if GO111MODULE=auto in a directory in GOPATH.
 	t.Run("GOPATH_GO111MODULE_auto", func(t *testing.T) {
 		WithOptions(
-			Modes(Singleton),
+			Modes(Default),
 			EnvVars{"GO111MODULE": "auto"},
 			InGOPATH(),
 		).Run(t, modules, func(t *testing.T, env *Env) {
@@ -1784,7 +1784,7 @@
 `
 	WithOptions(
 		ProxyFiles(proxy),
-		Modes(Singleton),
+		Modes(Default),
 	).Run(t, nested, func(t *testing.T, env *Env) {
 		// Expect a diagnostic in a nested module.
 		env.OpenFile("nested/hello/hello.go")
@@ -1996,7 +1996,7 @@
 `
 	WithOptions(
 		Settings{"experimentalUseInvalidMetadata": true},
-		Modes(Singleton),
+		Modes(Default),
 	).Run(t, mod, func(t *testing.T, env *Env) {
 		env.OpenFile("go.mod")
 		env.RegexpReplace("go.mod", "module mod.com", "modul mod.com") // break the go.mod file
@@ -2052,7 +2052,7 @@
 		Settings{"experimentalUseInvalidMetadata": true},
 		// ExperimentalWorkspaceModule has a different failure mode for this
 		// case.
-		Modes(Singleton),
+		Modes(Default),
 	).Run(t, mod, func(t *testing.T, env *Env) {
 		env.Await(
 			OnceMet(
diff --git a/gopls/internal/regtest/misc/semantictokens_test.go b/gopls/internal/regtest/misc/semantictokens_test.go
index dca2b8e..4437d40 100644
--- a/gopls/internal/regtest/misc/semantictokens_test.go
+++ b/gopls/internal/regtest/misc/semantictokens_test.go
@@ -25,7 +25,7 @@
 
 `
 	WithOptions(
-		Modes(Singleton),
+		Modes(Default),
 		Settings{"allExperiments": true},
 	).Run(t, src, func(t *testing.T, env *Env) {
 		params := &protocol.SemanticTokensParams{}
diff --git a/gopls/internal/regtest/misc/vendor_test.go b/gopls/internal/regtest/misc/vendor_test.go
index 4e02799..b0f507a 100644
--- a/gopls/internal/regtest/misc/vendor_test.go
+++ b/gopls/internal/regtest/misc/vendor_test.go
@@ -49,7 +49,7 @@
 }
 `
 	WithOptions(
-		Modes(Singleton),
+		Modes(Default),
 		ProxyFiles(basicProxy),
 	).Run(t, pkgThatUsesVendoring, func(t *testing.T, env *Env) {
 		env.OpenFile("a/a1.go")
diff --git a/gopls/internal/regtest/modfile/modfile_test.go b/gopls/internal/regtest/modfile/modfile_test.go
index c0bef83..e6f76d4d 100644
--- a/gopls/internal/regtest/modfile/modfile_test.go
+++ b/gopls/internal/regtest/modfile/modfile_test.go
@@ -742,7 +742,7 @@
 	WithOptions(
 		EnvVars{"GOFLAGS": "-mod=readonly"},
 		ProxyFiles(proxy),
-		Modes(Singleton),
+		Modes(Default),
 	).Run(t, mod, func(t *testing.T, env *Env) {
 		env.OpenFile("main.go")
 		original := env.ReadWorkspaceFile("go.mod")
@@ -922,7 +922,7 @@
 		// TODO(rFindley) this doesn't work in multi-module workspace mode, because
 		// it keeps around the last parsing modfile. Update this test to also
 		// exercise the workspace module.
-		Modes(Singleton),
+		Modes(Default),
 	).Run(t, mod, func(t *testing.T, env *Env) {
 		env.OpenFile("go.mod")
 		env.Await(env.DoneWithOpen())
@@ -1090,7 +1090,7 @@
 `
 	WithOptions(
 		ProxyFiles(workspaceProxy),
-		Modes(Singleton),
+		Modes(Default),
 	).Run(t, mod, func(t *testing.T, env *Env) {
 		env.OpenFile("go.mod")
 		params := &protocol.PublishDiagnosticsParams{}
@@ -1159,7 +1159,7 @@
 `
 	WithOptions(
 		ProxyFiles(proxy),
-		Modes(Singleton),
+		Modes(Default),
 	).Run(t, mod, func(t *testing.T, env *Env) {
 		env.OpenFile("main.go")
 		d := &protocol.PublishDiagnosticsParams{}
diff --git a/gopls/internal/regtest/workspace/workspace_test.go b/gopls/internal/regtest/workspace/workspace_test.go
index 7eafaf1..e4a1c4b 100644
--- a/gopls/internal/regtest/workspace/workspace_test.go
+++ b/gopls/internal/regtest/workspace/workspace_test.go
@@ -1205,7 +1205,7 @@
 `
 	WithOptions(
 		EnvVars{"GOPATH": filepath.FromSlash("$SANDBOX_WORKDIR/gopath")},
-		Modes(Singleton),
+		Modes(Default),
 	).Run(t, mod, func(t *testing.T, env *Env) {
 		env.Await(
 			// Confirm that the build configuration is seen as valid,
@@ -1236,7 +1236,7 @@
 func main() {}
 `
 	WithOptions(
-		Modes(Singleton),
+		Modes(Default),
 	).Run(t, nomod, func(t *testing.T, env *Env) {
 		env.OpenFile("a/main.go")
 		env.OpenFile("b/main.go")
diff --git a/gopls/internal/vulncheck/command_test.go b/gopls/internal/vulncheck/command_test.go
index e7bf708..71eaf4a 100644
--- a/gopls/internal/vulncheck/command_test.go
+++ b/gopls/internal/vulncheck/command_test.go
@@ -293,7 +293,7 @@
 		t.Fatal(err)
 	}
 
-	cache := cache.New(nil)
+	cache := cache.New(nil, nil, nil)
 	session := cache.NewSession(ctx)
 	options := source.DefaultOptions().Clone()
 	tests.DefaultOptions(options)
diff --git a/internal/lsp/cache/cache.go b/internal/lsp/cache/cache.go
index a59c890..c002850 100644
--- a/internal/lsp/cache/cache.go
+++ b/internal/lsp/cache/cache.go
@@ -28,23 +28,46 @@
 	"golang.org/x/tools/internal/span"
 )
 
-func New(options func(*source.Options)) *Cache {
+// New Creates a new cache for gopls operation results, using the given file
+// set, shared store, and session options.
+//
+// All of the fset, store and options may be nil, but if store is non-nil so
+// must be fset (and they must always be used together), otherwise it may be
+// possible to get cached data referencing token.Pos values not mapped by the
+// FileSet.
+func New(fset *token.FileSet, store *memoize.Store, options func(*source.Options)) *Cache {
 	index := atomic.AddInt64(&cacheIndex, 1)
+
+	if store != nil && fset == nil {
+		panic("non-nil store with nil fset")
+	}
+	if fset == nil {
+		fset = token.NewFileSet()
+	}
+	if store == nil {
+		store = &memoize.Store{}
+	}
+
 	c := &Cache{
 		id:          strconv.FormatInt(index, 10),
-		fset:        token.NewFileSet(),
+		fset:        fset,
 		options:     options,
+		store:       store,
 		fileContent: map[span.URI]*fileHandle{},
 	}
 	return c
 }
 
 type Cache struct {
-	id      string
-	fset    *token.FileSet
+	id   string
+	fset *token.FileSet
+
+	// TODO(rfindley): it doesn't make sense that cache accepts LSP options, just
+	// so that it can create a session: the cache does not (and should not)
+	// depend on options. Invert this relationship to remove options from Cache.
 	options func(*source.Options)
 
-	store memoize.Store
+	store *memoize.Store
 
 	fileMu      sync.Mutex
 	fileContent map[span.URI]*fileHandle
diff --git a/internal/lsp/cache/check.go b/internal/lsp/cache/check.go
index 4caf4ba..6c02d53 100644
--- a/internal/lsp/cache/check.go
+++ b/internal/lsp/cache/check.go
@@ -249,6 +249,16 @@
 	for _, file := range files {
 		b.WriteString(file.FileIdentity().String())
 	}
+	// Metadata errors are interpreted and memoized on the computed package, so
+	// we must hash them into the key here.
+	//
+	// TODO(rfindley): handle metadata diagnostics independently from
+	// type-checking diagnostics.
+	for _, err := range m.Errors {
+		b.WriteString(err.Msg)
+		b.WriteString(err.Pos)
+		b.WriteRune(rune(err.Kind))
+	}
 	return packageHandleKey(source.HashOf(b.Bytes()))
 }
 
diff --git a/internal/lsp/cache/session.go b/internal/lsp/cache/session.go
index 2374a52..a27efe3 100644
--- a/internal/lsp/cache/session.go
+++ b/internal/lsp/cache/session.go
@@ -231,7 +231,7 @@
 		backgroundCtx:        backgroundCtx,
 		cancel:               cancel,
 		initializeOnce:       &sync.Once{},
-		store:                &s.cache.store,
+		store:                s.cache.store,
 		packages:             persistent.NewMap(packageKeyLessInterface),
 		meta:                 &metadataGraph{},
 		files:                newFilesMap(),
diff --git a/internal/lsp/cmd/capabilities_test.go b/internal/lsp/cmd/capabilities_test.go
index 1d01b4b..930621b 100644
--- a/internal/lsp/cmd/capabilities_test.go
+++ b/internal/lsp/cmd/capabilities_test.go
@@ -43,7 +43,7 @@
 	params.Capabilities.Workspace.Configuration = true
 
 	// Send an initialize request to the server.
-	c.Server = lsp.NewServer(cache.New(app.options).NewSession(ctx), c.Client)
+	c.Server = lsp.NewServer(cache.New(nil, nil, app.options).NewSession(ctx), c.Client)
 	result, err := c.Server.Initialize(ctx, params)
 	if err != nil {
 		t.Fatal(err)
diff --git a/internal/lsp/cmd/cmd.go b/internal/lsp/cmd/cmd.go
index a81eb83..5911f97 100644
--- a/internal/lsp/cmd/cmd.go
+++ b/internal/lsp/cmd/cmd.go
@@ -286,7 +286,7 @@
 	switch {
 	case app.Remote == "":
 		connection := newConnection(app)
-		connection.Server = lsp.NewServer(cache.New(app.options).NewSession(ctx), connection.Client)
+		connection.Server = lsp.NewServer(cache.New(nil, nil, app.options).NewSession(ctx), connection.Client)
 		ctx = protocol.WithClient(ctx, connection.Client)
 		return connection, connection.initialize(ctx, app.options)
 	case strings.HasPrefix(app.Remote, "internal@"):
diff --git a/internal/lsp/cmd/serve.go b/internal/lsp/cmd/serve.go
index 1c229a4..10730fd 100644
--- a/internal/lsp/cmd/serve.go
+++ b/internal/lsp/cmd/serve.go
@@ -101,7 +101,7 @@
 			return fmt.Errorf("creating forwarder: %w", err)
 		}
 	} else {
-		ss = lsprpc.NewStreamServer(cache.New(s.app.options), isDaemon)
+		ss = lsprpc.NewStreamServer(cache.New(nil, nil, s.app.options), isDaemon)
 	}
 
 	var network, addr string
diff --git a/internal/lsp/cmd/test/cmdtest.go b/internal/lsp/cmd/test/cmdtest.go
index ff0461b..5342e9b 100644
--- a/internal/lsp/cmd/test/cmdtest.go
+++ b/internal/lsp/cmd/test/cmdtest.go
@@ -50,7 +50,7 @@
 
 func NewTestServer(ctx context.Context, options func(*source.Options)) *servertest.TCPServer {
 	ctx = debug.WithInstance(ctx, "", "")
-	cache := cache.New(options)
+	cache := cache.New(nil, nil, options)
 	ss := lsprpc.NewStreamServer(cache, false)
 	return servertest.NewTCPServer(ctx, ss, nil)
 }
diff --git a/internal/lsp/lsp_test.go b/internal/lsp/lsp_test.go
index 8e0e628..53890dc 100644
--- a/internal/lsp/lsp_test.go
+++ b/internal/lsp/lsp_test.go
@@ -49,7 +49,7 @@
 func testLSP(t *testing.T, datum *tests.Data) {
 	ctx := tests.Context(t)
 
-	cache := cache.New(nil)
+	cache := cache.New(nil, nil, nil)
 	session := cache.NewSession(ctx)
 	options := source.DefaultOptions().Clone()
 	tests.DefaultOptions(options)
diff --git a/internal/lsp/lsprpc/lsprpc_test.go b/internal/lsp/lsprpc/lsprpc_test.go
index cde641c..498566d 100644
--- a/internal/lsp/lsprpc/lsprpc_test.go
+++ b/internal/lsp/lsprpc/lsprpc_test.go
@@ -58,7 +58,7 @@
 	client := FakeClient{Logs: make(chan string, 10)}
 
 	ctx = debug.WithInstance(ctx, "", "")
-	ss := NewStreamServer(cache.New(nil), false)
+	ss := NewStreamServer(cache.New(nil, nil, nil), false)
 	ss.serverForTest = server
 	ts := servertest.NewPipeServer(ss, nil)
 	defer checkClose(t, ts.Close)
@@ -121,7 +121,7 @@
 func setupForwarding(ctx context.Context, t *testing.T, s protocol.Server) (direct, forwarded servertest.Connector, cleanup func()) {
 	t.Helper()
 	serveCtx := debug.WithInstance(ctx, "", "")
-	ss := NewStreamServer(cache.New(nil), false)
+	ss := NewStreamServer(cache.New(nil, nil, nil), false)
 	ss.serverForTest = s
 	tsDirect := servertest.NewTCPServer(serveCtx, ss, nil)
 
@@ -216,7 +216,7 @@
 	clientCtx := debug.WithInstance(baseCtx, "", "")
 	serverCtx := debug.WithInstance(baseCtx, "", "")
 
-	cache := cache.New(nil)
+	cache := cache.New(nil, nil, nil)
 	ss := NewStreamServer(cache, false)
 	tsBackend := servertest.NewTCPServer(serverCtx, ss, nil)
 
diff --git a/internal/lsp/mod/mod_test.go b/internal/lsp/mod/mod_test.go
index 09a182d..56af986 100644
--- a/internal/lsp/mod/mod_test.go
+++ b/internal/lsp/mod/mod_test.go
@@ -26,7 +26,7 @@
 	testenv.NeedsGo1Point(t, 14)
 
 	ctx := tests.Context(t)
-	cache := cache.New(nil)
+	cache := cache.New(nil, nil, nil)
 	session := cache.NewSession(ctx)
 	options := source.DefaultOptions().Clone()
 	tests.DefaultOptions(options)
diff --git a/internal/lsp/regtest/regtest.go b/internal/lsp/regtest/regtest.go
index 9ebc673..b3a543d 100644
--- a/internal/lsp/regtest/regtest.go
+++ b/internal/lsp/regtest/regtest.go
@@ -8,6 +8,7 @@
 	"context"
 	"flag"
 	"fmt"
+	"go/token"
 	"io/ioutil"
 	"os"
 	"runtime"
@@ -16,6 +17,7 @@
 
 	"golang.org/x/tools/internal/lsp/cmd"
 	"golang.org/x/tools/internal/lsp/source"
+	"golang.org/x/tools/internal/memoize"
 	"golang.org/x/tools/internal/testenv"
 	"golang.org/x/tools/internal/tool"
 )
@@ -87,9 +89,11 @@
 }
 
 func DefaultModes() Mode {
-	normal := Singleton | Experimental
+	// TODO(rfindley): these modes should *not* depend on GOOS. Depending on
+	// testing.Short() should be sufficient.
+	normal := Default | Experimental
 	if slowGOOS[runtime.GOOS] && testing.Short() {
-		normal = Singleton
+		normal = Default
 	}
 	if *runSubprocessTests {
 		return normal | SeparateProcess
@@ -116,6 +120,8 @@
 		PrintGoroutinesOnFailure: *printGoroutinesOnFailure,
 		SkipCleanup:              *skipCleanup,
 		OptionsHook:              hook,
+		fset:                     token.NewFileSet(),
+		store:                    memoize.NewStore(memoize.NeverEvict),
 	}
 	if *runSubprocessTests {
 		goplsPath := *goplsBinaryPath
@@ -126,13 +132,13 @@
 				panic(fmt.Sprintf("finding test binary path: %v", err))
 			}
 		}
-		runner.GoplsPath = goplsPath
+		runner.goplsPath = goplsPath
 	}
 	dir, err := ioutil.TempDir("", "gopls-regtest-")
 	if err != nil {
 		panic(fmt.Errorf("creating regtest temp directory: %v", err))
 	}
-	runner.TempDir = dir
+	runner.tempDir = dir
 
 	code := m.Run()
 	if err := runner.Close(); err != nil {
diff --git a/internal/lsp/regtest/runner.go b/internal/lsp/regtest/runner.go
index b2992e9..6c96e61 100644
--- a/internal/lsp/regtest/runner.go
+++ b/internal/lsp/regtest/runner.go
@@ -8,6 +8,7 @@
 	"bytes"
 	"context"
 	"fmt"
+	"go/token"
 	"io"
 	"io/ioutil"
 	"net"
@@ -29,24 +30,58 @@
 	"golang.org/x/tools/internal/lsp/lsprpc"
 	"golang.org/x/tools/internal/lsp/protocol"
 	"golang.org/x/tools/internal/lsp/source"
+	"golang.org/x/tools/internal/memoize"
 	"golang.org/x/tools/internal/testenv"
 	"golang.org/x/tools/internal/xcontext"
 )
 
 // Mode is a bitmask that defines for which execution modes a test should run.
+//
+// Each mode controls several aspects of gopls' configuration:
+//   - Which server options to use for gopls sessions
+//   - Whether to use a shared cache
+//   - Whether to use a shared server
+//   - Whether to run the server in-process or in a separate process
+//
+// The behavior of each mode with respect to these aspects is summarized below.
+// TODO(rfindley, cleanup): rather than using arbitrary names for these modes,
+// we can compose them explicitly out of the features described here, allowing
+// individual tests more freedom in constructing problematic execution modes.
+// For example, a test could assert on a certain behavior when running with
+// experimental options on a separate process. Moreover, we could unify 'Modes'
+// with 'Options', and use RunMultiple rather than a hard-coded loop through
+// modes.
+//
+// Mode            | Options      | Shared Cache? | Shared Server? | In-process?
+// ---------------------------------------------------------------------------
+// Default         | Default      | Y             | N              | Y
+// Forwarded       | Default      | Y             | Y              | Y
+// SeparateProcess | Default      | Y             | Y              | N
+// Experimental    | Experimental | N             | N              | Y
 type Mode int
 
 const (
-	// Singleton mode uses a separate in-process gopls instance for each test,
-	// and communicates over pipes to mimic the gopls sidecar execution mode,
-	// which communicates over stdin/stderr.
-	Singleton Mode = 1 << iota
-	// Forwarded forwards connections to a shared in-process gopls instance.
+	// Default mode runs gopls with the default options, communicating over pipes
+	// to emulate the lsp sidecar execution mode, which communicates over
+	// stdin/stdout.
+	//
+	// It uses separate servers for each test, but a shared cache, to avoid
+	// duplicating work when processing GOROOT.
+	Default Mode = 1 << iota
+
+	// Forwarded uses the default options, but forwards connections to a shared
+	// in-process gopls server.
 	Forwarded
-	// SeparateProcess forwards connection to a shared separate gopls process.
+
+	// SeparateProcess uses the default options, but forwards connection to an
+	// external gopls daemon.
 	SeparateProcess
+
 	// Experimental enables all of the experimental configurations that are
-	// being developed.
+	// being developed, and runs gopls in sidecar mode.
+	//
+	// It uses a separate cache for each test, to exercise races that may only
+	// appear with cache misses.
 	Experimental
 )
 
@@ -55,14 +90,20 @@
 // remote), any tests that execute on the same Runner will share the same
 // state.
 type Runner struct {
-	DefaultModes             Mode
-	Timeout                  time.Duration
-	GoplsPath                string
-	PrintGoroutinesOnFailure bool
-	TempDir                  string
-	SkipCleanup              bool
-	OptionsHook              func(*source.Options)
+	// Configuration
+	DefaultModes             Mode                  // modes to run for each test
+	Timeout                  time.Duration         // per-test timeout, if set
+	PrintGoroutinesOnFailure bool                  // whether to dump goroutines on test failure
+	SkipCleanup              bool                  // if set, don't delete test data directories when the test exits
+	OptionsHook              func(*source.Options) // if set, use these options when creating gopls sessions
 
+	// Immutable state shared across test invocations
+	goplsPath string         // path to the gopls executable (for SeparateProcess mode)
+	tempDir   string         // shared parent temp directory
+	fset      *token.FileSet // shared FileSet
+	store     *memoize.Store // shared store
+
+	// Lazily allocated resources
 	mu        sync.Mutex
 	ts        *servertest.TCPServer
 	socketDir string
@@ -193,7 +234,7 @@
 }
 
 // DebugAddress configures a debug server bound to addr. This option is
-// currently only supported when executing in Singleton mode. It is intended to
+// currently only supported when executing in Default mode. It is intended to
 // be used for long-running stress tests.
 func DebugAddress(addr string) RunOption {
 	return optionSetter(func(opts *runConfig) {
@@ -252,10 +293,10 @@
 		mode      Mode
 		getServer func(*testing.T, func(*source.Options)) jsonrpc2.StreamServer
 	}{
-		{"singleton", Singleton, singletonServer},
+		{"default", Default, r.defaultServer},
 		{"forwarded", Forwarded, r.forwardedServer},
 		{"separate_process", SeparateProcess, r.separateProcessServer},
-		{"experimental", Experimental, experimentalServer},
+		{"experimental", Experimental, r.experimentalServer},
 	}
 
 	for _, tc := range tests {
@@ -267,10 +308,10 @@
 		if config.modes&tc.mode == 0 {
 			continue
 		}
-		if config.debugAddr != "" && tc.mode != Singleton {
+		if config.debugAddr != "" && tc.mode != Default {
 			// Debugging is useful for running stress tests, but since the daemon has
 			// likely already been started, it would be too late to debug.
-			t.Fatalf("debugging regtest servers only works in Singleton mode, "+
+			t.Fatalf("debugging regtest servers only works in Default mode, "+
 				"got debug addr %q and mode %v", config.debugAddr, tc.mode)
 		}
 
@@ -298,7 +339,7 @@
 				di.MonitorMemory(ctx)
 			}
 
-			rootDir := filepath.Join(r.TempDir, filepath.FromSlash(t.Name()))
+			rootDir := filepath.Join(r.tempDir, filepath.FromSlash(t.Name()))
 			if err := os.MkdirAll(rootDir, 0755); err != nil {
 				t.Fatal(err)
 			}
@@ -434,11 +475,13 @@
 	fmt.Fprintf(os.Stderr, "#### End Gopls Test Logs for %q\n", testname)
 }
 
-func singletonServer(t *testing.T, optsHook func(*source.Options)) jsonrpc2.StreamServer {
-	return lsprpc.NewStreamServer(cache.New(optsHook), false)
+// defaultServer handles the Default execution mode.
+func (r *Runner) defaultServer(t *testing.T, optsHook func(*source.Options)) jsonrpc2.StreamServer {
+	return lsprpc.NewStreamServer(cache.New(r.fset, r.store, optsHook), false)
 }
 
-func experimentalServer(t *testing.T, optsHook func(*source.Options)) jsonrpc2.StreamServer {
+// experimentalServer handles the Experimental execution mode.
+func (r *Runner) experimentalServer(t *testing.T, optsHook func(*source.Options)) jsonrpc2.StreamServer {
 	options := func(o *source.Options) {
 		optsHook(o)
 		o.EnableAllExperiments()
@@ -446,28 +489,23 @@
 		// source.Options.EnableAllExperiments, but we want to test it.
 		o.ExperimentalWorkspaceModule = true
 	}
-	return lsprpc.NewStreamServer(cache.New(options), false)
+	return lsprpc.NewStreamServer(cache.New(nil, nil, options), false)
 }
 
-func (r *Runner) forwardedServer(t *testing.T, optsHook func(*source.Options)) jsonrpc2.StreamServer {
-	ts := r.getTestServer(optsHook)
-	return newForwarder("tcp", ts.Addr)
-}
-
-// getTestServer gets the shared test server instance to connect to, or creates
-// one if it doesn't exist.
-func (r *Runner) getTestServer(optsHook func(*source.Options)) *servertest.TCPServer {
-	r.mu.Lock()
-	defer r.mu.Unlock()
+// forwardedServer handles the Forwarded execution mode.
+func (r *Runner) forwardedServer(_ *testing.T, optsHook func(*source.Options)) jsonrpc2.StreamServer {
 	if r.ts == nil {
+		r.mu.Lock()
 		ctx := context.Background()
 		ctx = debug.WithInstance(ctx, "", "off")
-		ss := lsprpc.NewStreamServer(cache.New(optsHook), false)
+		ss := lsprpc.NewStreamServer(cache.New(nil, nil, optsHook), false)
 		r.ts = servertest.NewTCPServer(ctx, ss, nil)
+		r.mu.Unlock()
 	}
-	return r.ts
+	return newForwarder("tcp", r.ts.Addr)
 }
 
+// separateProcessServer handles the SeparateProcess execution mode.
 func (r *Runner) separateProcessServer(t *testing.T, optsHook func(*source.Options)) jsonrpc2.StreamServer {
 	// TODO(rfindley): can we use the autostart behavior here, instead of
 	// pre-starting the remote?
@@ -498,21 +536,22 @@
 		return filepath.Join(r.socketDir, daemonFile)
 	}
 
-	if r.GoplsPath == "" {
+	if r.goplsPath == "" {
 		t.Fatal("cannot run tests with a separate process unless a path to a gopls binary is configured")
 	}
 	var err error
-	r.socketDir, err = ioutil.TempDir(r.TempDir, "gopls-regtest-socket")
+	r.socketDir, err = ioutil.TempDir(r.tempDir, "gopls-regtest-socket")
 	if err != nil {
 		t.Fatalf("creating tempdir: %v", err)
 	}
 	socket := filepath.Join(r.socketDir, daemonFile)
 	args := []string{"serve", "-listen", "unix;" + socket, "-listen.timeout", "10s"}
-	cmd := exec.Command(r.GoplsPath, args...)
+	cmd := exec.Command(r.goplsPath, args...)
 	cmd.Env = append(os.Environ(), runTestAsGoplsEnvvar+"=true")
 	var stderr bytes.Buffer
 	cmd.Stderr = &stderr
 	go func() {
+		// TODO(rfindley): this is racy; we're returning before we know that the command is running.
 		if err := cmd.Run(); err != nil {
 			panic(fmt.Sprintf("error running external gopls: %v\nstderr:\n%s", err, stderr.String()))
 		}
@@ -537,7 +576,7 @@
 		}
 	}
 	if !r.SkipCleanup {
-		if err := os.RemoveAll(r.TempDir); err != nil {
+		if err := os.RemoveAll(r.tempDir); err != nil {
 			errmsgs = append(errmsgs, err.Error())
 		}
 	}
diff --git a/internal/lsp/source/source_test.go b/internal/lsp/source/source_test.go
index c670bde..5fdcc0f 100644
--- a/internal/lsp/source/source_test.go
+++ b/internal/lsp/source/source_test.go
@@ -49,7 +49,7 @@
 func testSource(t *testing.T, datum *tests.Data) {
 	ctx := tests.Context(t)
 
-	cache := cache.New(nil)
+	cache := cache.New(nil, nil, nil)
 	session := cache.NewSession(ctx)
 	options := source.DefaultOptions().Clone()
 	tests.DefaultOptions(options)
diff --git a/internal/memoize/memoize.go b/internal/memoize/memoize.go
index aa4d58d..150c79a 100644
--- a/internal/memoize/memoize.go
+++ b/internal/memoize/memoize.go
@@ -236,12 +236,34 @@
 	}
 }
 
+// An EvictionPolicy controls the eviction behavior of keys in a Store when
+// they no longer have any references.
+type EvictionPolicy int
+
+const (
+	// ImmediatelyEvict evicts keys as soon as they no longer have references.
+	ImmediatelyEvict EvictionPolicy = iota
+
+	// NeverEvict does not evict keys.
+	NeverEvict
+)
+
 // A Store maps arbitrary keys to reference-counted promises.
+//
+// The zero value is a valid Store, though a store may also be created via
+// NewStore if a custom EvictionPolicy is required.
 type Store struct {
+	evictionPolicy EvictionPolicy
+
 	promisesMu sync.Mutex
 	promises   map[interface{}]*Promise
 }
 
+// NewStore creates a new store with the given eviction policy.
+func NewStore(policy EvictionPolicy) *Store {
+	return &Store{evictionPolicy: policy}
+}
+
 // Promise returns a reference-counted promise for the future result of
 // calling the specified function.
 //
@@ -264,7 +286,9 @@
 	store.promisesMu.Unlock()
 
 	release := func() {
-		if atomic.AddInt32(&p.refcount, -1) == 0 {
+		// TODO(rfindley): this looks racy: it's possible that the refcount is
+		// incremented before we grab the lock.
+		if atomic.AddInt32(&p.refcount, -1) == 0 && store.evictionPolicy != NeverEvict {
 			store.promisesMu.Lock()
 			delete(store.promises, key)
 			store.promisesMu.Unlock()