api: postPolicy cleanup. Simplify the code and re-use. (#3890)

This change is cleanup of the postPolicyHandler code
primarily to address the flow and also converting
certain critical parts into self contained functions.

cmd/handler-utils_test.go

@@ -22,6 +22,7 @@ import (
 	"io/ioutil"
 	"net/http"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -83,6 +84,44 @@ func TestIsValidLocationContraint(t *testing.T) {
 	}
 }
 
+// Test validate form field size.
+func TestValidateFormFieldSize(t *testing.T) {
+	testCases := []struct {
+		header http.Header
+		err    error
+	}{
+		// Empty header returns error as nil,
+		{
+			header: nil,
+			err:    nil,
+		},
+		// Valid header returns error as nil.
+		{
+			header: http.Header{
+				"Content-Type": []string{"image/png"},
+			},
+			err: nil,
+		},
+		// Invalid header value > maxFormFieldSize+1
+		{
+			header: http.Header{
+				"Garbage": []string{strings.Repeat("a", int(maxFormFieldSize)+1)},
+			},
+			err: errSizeUnexpected,
+		},
+	}
+
+	// Run validate form field size check under all test cases.
+	for i, testCase := range testCases {
+		err := validateFormFieldSize(testCase.header)
+		if err != nil {
+			if errorCause(err).Error() != testCase.err.Error() {
+				t.Errorf("Test %d: Expected error %s, got %s", i+1, testCase.err, err)
+			}
+		}
+	}
+}
+
 // Tests validate metadata extraction from http headers.
 func TestExtractMetadataHeaders(t *testing.T) {
 	testCases := []struct {
@@ -115,6 +154,11 @@ func TestExtractMetadataHeaders(t *testing.T) {
 				"X-Amz-Meta-Appid":   "amz-meta",
 				"X-Minio-Meta-Appid": "minio-meta"},
 		},
+		// Empty header input returns empty metadata.
+		{
+			header:   nil,
+			metadata: nil,
+		},
 	}
 
 	// Validate if the extracting headers.