analyzer: move generated file filtering to ast.Walk phase not in package building

The prior code had filtering out of generated Go files at the wrong
phase of building packages which unfortunately caused a panic.
Instead move the filtering to the phase before walking the AST
for the file, which is the correct place.

Updates #30

Author: odeke-em

Diff for: analyzer.go

@@ -165,6 +165,21 @@ var reGeneratedGoFile = regexp.MustCompile(`^// Code generated .* DO NOT EDIT\.`
 // of the files in fullPaths for the presence of generated Go headers to avoid
 // reporting on generated code, per https://github.com/cosmos/gosec/issues/30
 func filterOutGeneratedGoFiles(fullPaths []string) (filtered []string) {
+	if len(fullPaths) == 0 {
+		return nil
+	}
+
+	if len(fullPaths) == 1 {
+		blob, err := os.ReadFile(fullPaths[0])
+		if err != nil {
+			panic(err)
+		}
+		if !reGeneratedGoFile.Match(blob) {
+			filtered = append(filtered, fullPaths[0])
+		}
+		return
+	}
+
 	// position stores the order "pos" which will later be
 	// used to sort the paths to maintain original order
 	// despite the concurrent filtering that'll take place.
@@ -248,9 +263,9 @@ func (gosec *Analyzer) load(pkgPath string, conf *packages.Config) ([]*packages.
 		}
 	}
 
-	// step 1/4 create build context.
+	// step 1/3: create build context.
 	buildD := build.Default
-	// step 2/4: add build tags to get env dependent files into basePackage.
+	// step 2/3: add build tags to get env dependent files into basePackage.
 	buildD.BuildTags = conf.BuildFlags
 	buildD.Dir = absGoModPath
 	basePackage, err := buildD.ImportDir(abspath, build.ImportComment)
@@ -276,12 +291,7 @@ func (gosec *Analyzer) load(pkgPath string, conf *packages.Config) ([]*packages.
 		}
 	}
 
-	// step 3/4: now filter out generated go files as we definitely don't
-	// want to report on generated code, which is out of our direct control.
-	// Please see: https://github.com/cosmos/gosec/issues/30
-	packageFiles = filterOutGeneratedGoFiles(packageFiles)
-
-	// step 4/4: remove build tags from conf to proceed build correctly.
+	// step 3/3: remove build tags from conf to proceed build correctly.
 	conf.BuildFlags = nil
 	conf.Dir = absGoModPath
 	pkgs, err := packages.Load(conf, packageFiles...)
@@ -294,6 +304,7 @@ func (gosec *Analyzer) load(pkgPath string, conf *packages.Config) ([]*packages.
 // Check runs analysis on the given package
 func (gosec *Analyzer) Check(pkg *packages.Package) {
 	gosec.logger.Println("Checking package:", pkg.Name)
+
 	for _, file := range pkg.Syntax {
 		checkedFile := pkg.Fset.File(file.Pos()).Name()
 		// Skip the no-Go file from analysis (e.g. a Cgo files is expanded in 3 different files
@@ -312,7 +323,13 @@ func (gosec *Analyzer) Check(pkg *packages.Package) {
 		gosec.context.Imports = NewImportTracker()
 		gosec.context.Imports.TrackFile(file)
 		gosec.context.PassedValues = make(map[string]interface{})
-		ast.Walk(gosec, file)
+
+		// Only walk non-generated Go files as we definitely don't
+		// want to report on generated code, which is out of our direct control.
+		// Please see: https://github.com/cosmos/gosec/issues/30
+		if filtered := filterOutGeneratedGoFiles([]string{checkedFile}); len(filtered) > 0 {
+			ast.Walk(gosec, file)
+		}
 		gosec.stats.NumFiles++
 		gosec.stats.NumLines += pkg.Fset.File(file.Pos()).LineCount()
 	}