internal/lsp/cache: detach context before invalidation

In one of my previous refactoring changes, I lost the fact that the
context should be detached before invalidating a file's contents. If
this function is canceled, we will be in a bad state.

Also, small change to return ctx.Err() instead of a custom error message
from (*packageHandle).check.

Change-Id: I19e513e09e438feee105fdd89cb7364a0c3c5e7f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/212104
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

internal/lsp/cache/check.go

@@ -184,7 +184,7 @@ func (ph *packageHandle) Check(ctx context.Context) (source.Package, error) {
 func (ph *packageHandle) check(ctx context.Context) (*pkg, error) {
 	v := ph.handle.Get(ctx)
 	if v == nil {
-		return nil, errors.Errorf("no package for %s", ph.m.id)
+		return nil, ctx.Err()
 	}
 	data := v.(*packageData)
 	return data.pkg, data.err

internal/lsp/cache/view.go

@@ -26,6 +26,7 @@ import (
 	"golang.org/x/tools/internal/span"
 	"golang.org/x/tools/internal/telemetry/log"
 	"golang.org/x/tools/internal/telemetry/tag"
+	"golang.org/x/tools/internal/xcontext"
 	errors "golang.org/x/xerrors"
 )
 
@@ -349,6 +350,9 @@ func (v *view) getSnapshot() *snapshot {
 // including any position and type information that depends on it.
 // It returns true if we were already tracking the given file, false otherwise.
 func (v *view) invalidateContent(ctx context.Context, uri span.URI, kind source.FileKind, action source.FileAction) source.Snapshot {
+	// Detach the context so that content invalidation cannot be canceled.
+	ctx = xcontext.Detach(ctx)
+
 	// Cancel all still-running previous requests, since they would be
 	// operating on stale data.
 	switch action {