internal/lsp/progress: actually close over Context in WorkDoneWriter

CL 409936 eliminated cases where we close over a Context during progress
reporting, except in one instance where it wasn't possible: the
WorkDoneWriter that must implement the io.Writer interface.

Unfortunately it contained a glaring bug that the ctx field was never
set, and the regression test for progress reporting during `go generate`
was disabled due to flakiness (golang/go#49901).

Incidentally, the fundamental problem that CL 409936 addressed may also
fix the flakiness of TestGenerateProgress.

Fix the bug, and re-enable the test.

Fixes golang/go#53781

Change-Id: Ideb99a5525667e45d2e41fcc5078699ba1e0f1a3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/417115
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/internal/regtest/misc/generate_test.go

@@ -16,8 +16,6 @@ import (
 )
 
 func TestGenerateProgress(t *testing.T) {
-	t.Skipf("skipping flaky test: https://golang.org/issue/49901")
-
 	const generatedWorkspace = `
 -- go.mod --
 module fake.test

internal/lsp/command.go

@@ -404,7 +404,7 @@ func (c *commandHandler) runTests(ctx context.Context, snapshot source.Snapshot,
 	// create output
 	buf := &bytes.Buffer{}
 	ew := progress.NewEventWriter(ctx, "test")
-	out := io.MultiWriter(ew, progress.NewWorkDoneWriter(work), buf)
+	out := io.MultiWriter(ew, progress.NewWorkDoneWriter(ctx, work), buf)
 
 	// Run `go test -run Func` on each test.
 	var failedTests int
@@ -487,7 +487,7 @@ func (c *commandHandler) Generate(ctx context.Context, args command.GenerateArgs
 			Args:       []string{"-x", pattern},
 			WorkingDir: args.Dir.SpanURI().Filename(),
 		}
-		stderr := io.MultiWriter(er, progress.NewWorkDoneWriter(deps.work))
+		stderr := io.MultiWriter(er, progress.NewWorkDoneWriter(ctx, deps.work))
 		if err := deps.snapshot.RunGoCommandPiped(ctx, source.Normal, inv, er, stderr); err != nil {
 			return err
 		}

internal/lsp/progress/progress.go

@@ -260,8 +260,8 @@ type WorkDoneWriter struct {
 	wd  *WorkDone
 }
 
-func NewWorkDoneWriter(wd *WorkDone) *WorkDoneWriter {
-	return &WorkDoneWriter{wd: wd}
+func NewWorkDoneWriter(ctx context.Context, wd *WorkDone) *WorkDoneWriter {
+	return &WorkDoneWriter{ctx: ctx, wd: wd}
 }
 
 func (wdw *WorkDoneWriter) Write(p []byte) (n int, err error) {