go-fuzz-build: load reflect when using libfuzzer

libfuzzer's generated main function uses package reflect.
When attempting to build a package that doesn't depend
on reflect, such as github.com/dvyukuv/go-fuzz-corpus/{bzip2,gif,url},
package reflect wasn't getting copied to GOROOT, and the build failed.

Fix that.

We may need something similar in the future for fuzz.F; see dvyukov#223.

Updates google/oss-fuzz/#2188

go-fuzz-build/main.go

@@ -251,7 +251,15 @@ func (c *Context) loadPkg(pkg string) {
  cfg.ParseFile = func(fset *token.FileSet, filename string, src []byte) (*ast.File, error) {
  return parser.ParseFile(fset, filename, src, parser.ParseComments)
  }
- initial, err := packages.Load(cfg, pkg, "github.com/dvyukov/go-fuzz/go-fuzz-dep")
+ // We need to load:
+ // * the target package, obviously
+ // * go-fuzz-dep, since we use it for instrumentation
+ // * reflect, if we are using libfuzzer, since its generated main function requires it
+ loadpkgs := []string{pkg, "github.com/dvyukov/go-fuzz/go-fuzz-dep"}
+ if *flagLibFuzzer {
+ loadpkgs = append(loadpkgs, "reflect")
+ }
+ initial, err := packages.Load(cfg, loadpkgs...)
  if err != nil {
  c.failf("could not load packages: %v", err)
  }