Drop errgroup.WithContext and add withCause (#1380)

We are sometimes unable to expand an APK into our cache due to an opaque
"context canceled" error. This change attempts to address that in two
ways.

First, we drop our use of errgroup.WithContext when installing packages.
If one package fails to install, we'll let the rest run to completion so
that we make more forward progress towards populating the cache (if
there is one). Even though the overall operation will still fail, in
cases where that failure was transient, we avoid throwing away in-flight
work towards the other packages.

Second, we'll try to extract a cause for the context cancellation and
annotate the errgroup's error with it so that we know why the overall
operation was canceled.

At least one of these changes should help determine what's going on.

Signed-off-by: Jon Johnson <[email protected]>

pkg/apk/apk/implementation.go

@@ -518,7 +518,7 @@ func (a *APK) CalculateWorld(ctx context.Context, allpkgs []*RepositoryPackage)
 	// TODO: Consider making this configurable option.
 	jobs := runtime.GOMAXPROCS(0)
 
-	g, gctx := errgroup.WithContext(ctx)
+	var g errgroup.Group
 	g.SetLimit(jobs + 1)
 
 	resolved := make([]*APKResolved, len(allpkgs))
@@ -535,11 +535,11 @@ func (a *APK) CalculateWorld(ctx context.Context, allpkgs []*RepositoryPackage)
 		i, pkg := i, pkg
 
 		g.Go(func() error {
-			r, err := a.FetchPackage(gctx, pkg)
+			r, err := a.FetchPackage(ctx, pkg)
 			if err != nil {
 				return fmt.Errorf("fetching %s: %w", pkg.Name, err)
 			}
-			res, err := ResolveApk(gctx, r)
+			res, err := ResolveApk(ctx, r)
 			if err != nil {
 				return fmt.Errorf("resolving %s: %w", pkg.Name, err)
 			}
@@ -554,12 +554,22 @@ func (a *APK) CalculateWorld(ctx context.Context, allpkgs []*RepositoryPackage)
 	}
 
 	if err := g.Wait(); err != nil {
-		return nil, fmt.Errorf("installing packages: %w", err)
+		return nil, fmt.Errorf("calculating world: %w", withCause(ctx, err))
 	}
 
 	return resolved, nil
 }
 
+// Sometimes we get an opaque error about context cancellation, and it's unclear what caused it.
+// If we get something useful from ctx via context.Cause, we'll annotate err with it.
+func withCause(ctx context.Context, err error) error {
+	if cause := context.Cause(ctx); cause != nil {
+		return fmt.Errorf("%w: %w", err, cause)
+	}
+
+	return err
+}
+
 func (a *APK) ResolveAndCalculateWorld(ctx context.Context) ([]*APKResolved, error) {
 	log := clog.FromContext(ctx)
 	log.Debug("resolving and calculating 'world' (packages to install)")
@@ -625,7 +635,7 @@ func (a *APK) InstallPackages(ctx context.Context, sourceDateEpoch *time.Time, a
 	// TODO: Consider making this configurable option.
 	jobs := runtime.GOMAXPROCS(0)
 
-	g, gctx := errgroup.WithContext(ctx)
+	var g errgroup.Group
 	g.SetLimit(jobs + 1)
 
 	expanded := make([]*expandapk.APKExpanded, len(allpkgs))
@@ -648,8 +658,8 @@ func (a *APK) InstallPackages(ctx context.Context, sourceDateEpoch *time.Time, a
 	g.Go(func() error {
 		for i, ch := range done {
 			select {
-			case <-gctx.Done():
-				return gctx.Err()
+			case <-ctx.Done():
+				return ctx.Err()
 			case <-ch:
 				exp := expanded[i]
 				pkg := allpkgs[i]
@@ -670,7 +680,7 @@ func (a *APK) InstallPackages(ctx context.Context, sourceDateEpoch *time.Time, a
 				}
 				infos[i] = pkgInfo
 
-				installedFiles, err := a.installPackage(gctx, pkgInfo, exp, sourceDateEpoch)
+				installedFiles, err := a.installPackage(ctx, pkgInfo, exp, sourceDateEpoch)
 				if err != nil {
 					return fmt.Errorf("installing %s: %w", pkg, err)
 				}
@@ -688,7 +698,7 @@ func (a *APK) InstallPackages(ctx context.Context, sourceDateEpoch *time.Time, a
 		i, pkg := i, pkg
 
 		g.Go(func() error {
-			exp, err := a.expandPackage(gctx, pkg)
+			exp, err := a.expandPackage(ctx, pkg)
 			if err != nil {
 				return fmt.Errorf("expanding %s: %w", pkg, err)
 			}
@@ -701,7 +711,7 @@ func (a *APK) InstallPackages(ctx context.Context, sourceDateEpoch *time.Time, a
 	}
 
 	if err := g.Wait(); err != nil {
-		return fmt.Errorf("installing packages: %w", err)
+		return fmt.Errorf("installing packages: %w", withCause(ctx, err))
 	}
 
 	// update the installed file