Update error test case

There were several issues with the error test case that have been
addressed in this commit.

- It is possible to specify a whitelist of calls that error handling
  should be ignored for.
- Additional support for ast.ExprStmt for cases where the error is
  implicitly ignored.

There were several other additions to the helpers and call list in order
to support this type of functionality.

Fixes #54

rules/errors.go

@@ -15,47 +15,82 @@
 package rules
 
 import (
+	gas "github.com/GoASTScanner/gas/core"
 	"go/ast"
 	"go/types"
-	"reflect"
-
-	gas "github.com/GoASTScanner/gas/core"
 )
 
 type NoErrorCheck struct {
 	gas.MetaData
+	whitelist gas.CallList
 }
 
-func (r *NoErrorCheck) Match(n ast.Node, c *gas.Context) (gi *gas.Issue, err error) {
-	if node, ok := n.(*ast.AssignStmt); ok {
-		sel := reflect.TypeOf(&ast.CallExpr{})
-		if call, ok := gas.SimpleSelect(node.Rhs[0], sel).(*ast.CallExpr); ok {
-			if t := c.Info.Types[call].Type; t != nil {
-				if typeVal, typeErr := t.(*types.Tuple); typeErr {
-					for i := 0; i < typeVal.Len(); i++ {
-						if typeVal.At(i).Type().String() == "error" { // TODO(tkelsey): is there a better way?
-							if id, ok := node.Lhs[i].(*ast.Ident); ok && id.Name == "_" {
-								return gas.NewIssue(c, n, r.What, r.Severity, r.Confidence), nil
-							}
-						}
-					}
-				} else if t.String() == "error" { // TODO(tkelsey): is there a better way?
-					if id, ok := node.Lhs[0].(*ast.Ident); ok && id.Name == "_" {
-						return gas.NewIssue(c, n, r.What, r.Severity, r.Confidence), nil
-					}
+func returnsError(callExpr *ast.CallExpr, ctx *gas.Context) int {
+	if tv := ctx.Info.TypeOf(callExpr); tv != nil {
+		switch t := tv.(type) {
+		case *types.Tuple:
+			for pos := 0; pos < t.Len(); pos += 1 {
+				variable := t.At(pos)
+				if variable != nil && variable.Type().String() == "error" {
+					return pos
 				}
 			}
+		case *types.Named:
+			if t.String() == "error" {
+				return 0
+			}
+		}
+	}
+	return -1
+}
+
+func (r *NoErrorCheck) Match(n ast.Node, ctx *gas.Context) (*gas.Issue, error) {
+	switch stmt := n.(type) {
+	case *ast.AssignStmt:
+		for _, expr := range stmt.Rhs {
+			if callExpr, ok := expr.(*ast.CallExpr); ok && !r.whitelist.ContainsCallExpr(callExpr, ctx) {
+				pos := returnsError(callExpr, ctx)
+				if pos < 0 || pos >= len(stmt.Lhs) {
+					return nil, nil
+				}
+				if id, ok := stmt.Lhs[pos].(*ast.Ident); ok && id.Name == "_" {
+					return gas.NewIssue(ctx, n, r.What, r.Severity, r.Confidence), nil
+				}
+			}
+		}
+	case *ast.ExprStmt:
+		if callExpr, ok := stmt.X.(*ast.CallExpr); ok && !r.whitelist.ContainsCallExpr(callExpr, ctx) {
+			pos := returnsError(callExpr, ctx)
+			if pos >= 0 {
+				return gas.NewIssue(ctx, n, r.What, r.Severity, r.Confidence), nil
+			}
 		}
 	}
 	return nil, nil
 }
 
 func NewNoErrorCheck(conf map[string]interface{}) (gas.Rule, []ast.Node) {
+
+	// TODO(gm) Come up with sensible defaults here. Or flip it to use a
+	// black list instead.
+	whitelist := gas.NewCallList()
+	whitelist.AddAll("bytes.Buffer", "Write", "WriteByte", "WriteRune", "WriteString")
+	whitelist.AddAll("fmt", "Print", "Printf", "Println")
+	whitelist.Add("io.PipeWriter", "CloseWithError")
+
+	if configured, ok := conf["G104"]; ok {
+		if whitelisted, ok := configured.(map[string][]string); ok {
+			for key, val := range whitelisted {
+				whitelist.AddAll(key, val...)
+			}
+		}
+	}
 	return &NoErrorCheck{
 		MetaData: gas.MetaData{
 			Severity:   gas.Low,
 			Confidence: gas.High,
 			What:       "Errors unhandled.",
 		},
-	}, []ast.Node{(*ast.AssignStmt)(nil)}
+		whitelist: whitelist,
+	}, []ast.Node{(*ast.AssignStmt)(nil), (*ast.ExprStmt)(nil)}
 }