internal/lsp: fix some cases of stuck files

If typeCheck() returned an error, we could get into a state where a
package had an entry in the pcache, but the package's files had an
empty "pkgs" map. When we got a DidChange event for one of the files,
no packages would get invalidated since the file's "pkgs" was
empty. This resulted in the cached typeCheck() error persisting
indefinitely. Fix by never caching pcache entries on error.

An easy way to reproduce the problem was to delete the package name
from a file. For example, edit "package foo" to be just
"package". This caused the package to get stuck with an "AST for %s
has an invalid position" error.

Change-Id: I330bf9e419852dffa0f2dee94b56226367488dd1
GitHub-Last-Rev: 18be7078521b942694c76f799a2d520eee47167d
GitHub-Pull-Request: golang/tools#135
Reviewed-on: https://go-review.googlesource.com/c/tools/+/185839
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
diff --git a/internal/lsp/cache/check.go b/internal/lsp/cache/check.go
index 37e612d..9b78773 100644
--- a/internal/lsp/cache/check.go
+++ b/internal/lsp/cache/check.go
@@ -68,18 +68,22 @@
 		// This goroutine becomes responsible for populating
 		// the entry and broadcasting its readiness.
 		e.pkg, e.err = imp.typeCheck(ctx, id)
+		if e.err != nil {
+			// Don't cache failed packages. If we didn't successfully cache the package
+			// in each file, then this pcache entry won't get invalidated as those files
+			// change.
+			imp.view.pcache.mu.Lock()
+			if imp.view.pcache.packages[id] == e {
+				delete(imp.view.pcache.packages, id)
+			}
+			imp.view.pcache.mu.Unlock()
+		}
 		close(e.ready)
 	}
 
 	if e.err != nil {
 		// If the import had been previously canceled, and that error cached, try again.
 		if e.err == context.Canceled && ctx.Err() == nil {
-			imp.view.pcache.mu.Lock()
-			// Clear out canceled cache entry if it is still there.
-			if imp.view.pcache.packages[id] == e {
-				delete(imp.view.pcache.packages, id)
-			}
-			imp.view.pcache.mu.Unlock()
 			return imp.getPkg(ctx, id)
 		}
 		return nil, e.err