[gopls-release-branch.0.15] internal/imports: fix two "nil pointer in…

… interface" bugs

CL 559635 changed newModuleResolver so that it can return
(nil, err). That means it is no longer safe to unconditionally
convert the first result to a Resolver interface, but we
forgot to check in two places, causing a crash that was
reported by telemetry.

This change adds the two checks.

Updates golang/go#66490
Updates golang/go#66730

Change-Id: I3f2b84ed792b1eea179fc0d4d5ee9843281506fc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/574136
Reviewed-by: Peter Weinberger <pjw@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit 63b3b5a)
Reviewed-on: https://go-review.googlesource.com/c/tools/+/577297
Reviewed-by: Alan Donovan <adonovan@google.com>

internal/imports/fix.go

@@ -988,8 +988,10 @@ func (e *ProcessEnv) GetResolver() (Resolver, error) {
 		// already know the view type.
 		if len(e.Env["GOMOD"]) == 0 && len(e.Env["GOWORK"]) == 0 {
 			e.resolver = newGopathResolver(e)
+		} else if r, err := newModuleResolver(e, e.ModCache); err != nil {
+			e.resolverErr = err
 		} else {
-			e.resolver, e.resolverErr = newModuleResolver(e, e.ModCache)
+			e.resolver = Resolver(r)
 		}
 	}
 

internal/imports/mod.go

@@ -313,15 +313,19 @@ func (r *ModuleResolver) ClearForNewScan() Resolver {
 // TODO(rfindley): move this to a new env.go, consolidating ProcessEnv methods.
 func (e *ProcessEnv) ClearModuleInfo() {
 	if r, ok := e.resolver.(*ModuleResolver); ok {
-		resolver, resolverErr := newModuleResolver(e, e.ModCache)
-		if resolverErr == nil {
-			<-r.scanSema // acquire (guards caches)
-			resolver.moduleCacheCache = r.moduleCacheCache
-			resolver.otherCache = r.otherCache
-			r.scanSema <- struct{}{} // release
-		}
-		e.resolver = resolver
-		e.resolverErr = resolverErr
+		resolver, err := newModuleResolver(e, e.ModCache)
+		if err != nil {
+			e.resolver = nil
+			e.resolverErr = err
+			return
+		}
+
+		<-r.scanSema // acquire (guards caches)
+		resolver.moduleCacheCache = r.moduleCacheCache
+		resolver.otherCache = r.otherCache
+		r.scanSema <- struct{}{} // release
+
+		e.UpdateResolver(resolver)
 	}
 }