cmd/vulnreport: improve cgo error message

Give a distinct and helpful error message when cgo is the likely culprit.

Change-Id: I419eb670a9251ae91de815c7c7a9ba9b7d872e8c
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/461515
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tim King <taking@google.com>
Reviewed-by: Tatiana Bradley <tatiana@golang.org>

Author: timothy-king

cmd/vulnreport/main.go

@@ -937,16 +937,47 @@ func loadPackage(cfg *packages.Config, importPath string) (_ []*packages.Package
 	if err != nil {
 		return nil, err
 	}
+
+	if err := packageLoadingError(pkgs); err != nil {
+		return nil, err
+	}
+
+	return pkgs, nil
+}
+
+// packageLoadingError returns an error summarizing packages.Package.Errors if there were any.
+func packageLoadingError(pkgs []*packages.Package) error {
+	pkgError := func(pkg *packages.Package) error {
+		var msgs []string
+		for _, err := range pkg.Errors {
+			msgs = append(msgs, err.Error())
+		}
+		if len(msgs) == 0 {
+			return nil
+		}
+		// Report a more helpful error message for the package if possible.
+		for _, msg := range msgs {
+			// cgo failure?
+			if strings.Contains(msg, "could not import C (no metadata for C)") {
+				const url = `https://github.com/golang/vulndb/blob/master/doc/triage.md#vulnreport-cgo-failures`
+				return fmt.Errorf("package %s has a cgo error (install relevant C packages? %s)\nerrors:%s", pkg.PkgPath, url, strings.Join(msgs, "\n"))
+			}
+		}
+		return fmt.Errorf("package %s had %d errors: %s", pkg.PkgPath, len(msgs), strings.Join(msgs, "\n"))
+	}
+
+	var paths []string
 	var msgs []string
 	packages.Visit(pkgs, nil, func(pkg *packages.Package) {
-		for _, err := range pkg.Errors {
+		if err := pkgError(pkg); err != nil {
+			paths = append(paths, pkg.PkgPath)
 			msgs = append(msgs, err.Error())
 		}
 	})
-	if len(msgs) > 0 {
-		return nil, fmt.Errorf("packages.Visit:\n%s", strings.Join(msgs, "\n"))
+	if len(msgs) == 0 {
+		return nil // no errors
 	}
-	return pkgs, nil
+	return fmt.Errorf("packages with errors: %s\nerrors:\n%s", strings.Join(paths, " "), strings.Join(msgs, "\n"))
 }
 
 func changeToTempDir() (cleanup func(), _ error) {