go/packages: stop loading packages when context is done

The value of ld.Context.Err() is now always checked once at
the beginning of loadPackage before it begins reading files or
exportdata. loadPackage stops early if ld.Context is done.
If any packages failed to load due to stopping early, Load returns
(nil, ld.Context.Err()).

As a side-effect, this resolves logging ld.Context.Err() multiple
times per packages as unknown internal errors. (These are not
internal errors nor are they unknown.)

Change-Id: Iab8eedbe19ad07b592b3003d2934b20039e54a94
Reviewed-on: https://go-review.googlesource.com/c/tools/+/577395
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

go/packages/packages.go

@@ -129,9 +129,8 @@ type Config struct {
 	Mode LoadMode
 
 	// Context specifies the context for the load operation.
-	// If the context is cancelled, the loader may stop early
-	// and return an ErrCancelled error.
-	// If Context is nil, the load cannot be cancelled.
+	// Cancelling the context may cause [Load] to abort and
+	// return an error.
 	Context context.Context
 
 	// Logf is the logger for the config.
@@ -214,8 +213,8 @@ type Config struct {
 // Config specifies loading options;
 // nil behaves the same as an empty Config.
 //
-// Load returns an error if any of the patterns was invalid
-// as defined by the underlying build system.
+// If any of the patterns was invalid as defined by the
+// underlying build system, Load returns an error.
 // It may return an empty list of packages without an error,
 // for instance for an empty expansion of a valid wildcard.
 // Errors associated with a particular package are recorded in the
@@ -858,6 +857,12 @@ func (ld *loader) refine(response *DriverResponse) ([]*Package, error) {
 		wg.Wait()
 	}
 
+	// If the context is done, return its error and
+	// throw out [likely] incomplete packages.
+	if err := ld.Context.Err(); err != nil {
+		return nil, err
+	}
+
 	result := make([]*Package, len(initial))
 	for i, lpkg := range initial {
 		result[i] = lpkg.Package
@@ -953,6 +958,14 @@ func (ld *loader) loadPackage(lpkg *loaderPackage) {
 	lpkg.Types = types.NewPackage(lpkg.PkgPath, lpkg.Name)
 	lpkg.Fset = ld.Fset
 
+	// Start shutting down if the context is done and do not load
+	// source or export data files.
+	// Packages that import this one will have ld.Context.Err() != nil.
+	// ld.Context.Err() will be returned later by refine.
+	if ld.Context.Err() != nil {
+		return
+	}
+
 	// Subtle: we populate all Types fields with an empty Package
 	// before loading export data so that export data processing
 	// never has to create a types.Package for an indirect dependency,
@@ -1072,6 +1085,13 @@ func (ld *loader) loadPackage(lpkg *loaderPackage) {
 		return
 	}
 
+	// Start shutting down if the context is done and do not type check.
+	// Packages that import this one will have ld.Context.Err() != nil.
+	// ld.Context.Err() will be returned later by refine.
+	if ld.Context.Err() != nil {
+		return
+	}
+
 	lpkg.TypesInfo = &types.Info{
 		Types:      make(map[ast.Expr]types.TypeAndValue),
 		Defs:       make(map[*ast.Ident]types.Object),
@@ -1249,11 +1269,6 @@ func (ld *loader) parseFiles(filenames []string) ([]*ast.File, []error) {
 	parsed := make([]*ast.File, n)
 	errors := make([]error, n)
 	for i, file := range filenames {
-		if ld.Config.Context.Err() != nil {
-			parsed[i] = nil
-			errors[i] = ld.Config.Context.Err()
-			continue
-		}
 		wg.Add(1)
 		go func(i int, filename string) {
 			parsed[i], errors[i] = ld.parseFile(filename)