internal/lsp/cache: set GOWORK=off when mutating modfiles

Commands will fail with -mod=mod when GOWORK is set. Explicitly set it
to off.

Fixes golang/go#48941

Change-Id: I39f9d95526ca6f3b0414ff273cecd443d69afa61
Reviewed-on: https://go-review.googlesource.com/c/tools/+/391518
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

gopls/internal/regtest/modfile/modfile_test.go

@@ -239,6 +239,62 @@ require random.org v1.2.3
 	})
 }
 
+// Tests that multiple missing dependencies gives good single fixes.
+func TestMissingDependencyFixesWithGoWork(t *testing.T) {
+	testenv.NeedsGo1Point(t, 18)
+	const mod = `
+-- go.work --
+go 1.18
+
+use (
+	./a
+)
+-- a/go.mod --
+module mod.com
+
+go 1.12
+
+-- a/main.go --
+package main
+
+import "example.com/blah"
+import "random.org/blah"
+
+var _, _ = blah.Name, hello.Name
+`
+
+	const want = `module mod.com
+
+go 1.12
+
+require random.org v1.2.3
+`
+
+	RunMultiple{
+		{"default", WithOptions(ProxyFiles(proxy), WorkspaceFolders("a"))},
+		{"nested", WithOptions(ProxyFiles(proxy))},
+	}.Run(t, mod, func(t *testing.T, env *Env) {
+		env.OpenFile("a/main.go")
+		var d protocol.PublishDiagnosticsParams
+		env.Await(
+			OnceMet(
+				env.DiagnosticAtRegexp("a/main.go", `"random.org/blah"`),
+				ReadDiagnostics("a/main.go", &d),
+			),
+		)
+		var randomDiag protocol.Diagnostic
+		for _, diag := range d.Diagnostics {
+			if strings.Contains(diag.Message, "random.org") {
+				randomDiag = diag
+			}
+		}
+		env.ApplyQuickFixes("a/main.go", []protocol.Diagnostic{randomDiag})
+		if got := env.ReadWorkspaceFile("a/go.mod"); got != want {
+			t.Fatalf("unexpected go.mod content:\n%s", tests.Diff(t, want, got))
+		}
+	})
+}
+
 func TestIndirectDependencyFix(t *testing.T) {
 	testenv.NeedsGo1Point(t, 14)
 

internal/lsp/cache/snapshot.go

@@ -413,6 +413,8 @@ func (s *snapshot) goCommandInvocation(ctx context.Context, flags source.Invocat
 			}
 		case source.WriteTemporaryModFile:
 			inv.ModFlag = mutableModFlag
+			// -mod must be readonly when using go.work files - see issue #48941
+			inv.Env = append(inv.Env, "GOWORK=off")
 		}
 	}