fix panic when os.Stat returns an error besides ErrNotExists

cmd/cosign/cli/generate/generate_key_pair.go

@@ -85,7 +85,12 @@ func GenerateKeyPairCmd(ctx context.Context, kmsVal string, args []string) error
 		return err
 	}
 
-	if cosign.FileExists("cosign.key") {
+	fileExists, err := cosign.FileExists("cosign.key")
+	if err != nil {
+		return err
+	}
+
+	if fileExists {
 		var overwrite string
 		fmt.Fprint(os.Stderr, "File cosign.key already exists. Overwrite (y/n)? ")
 		fmt.Scanf("%s", &overwrite)

cmd/cosign/cli/importkeypair/import_key_pair.go

@@ -36,7 +36,12 @@ func ImportKeyPairCmd(ctx context.Context, keyVal string, args []string) error {
 		return err
 	}
 
-	if cosign.FileExists("import-cosign.key") {
+	fileExists, err := cosign.FileExists("import-cosign.key\000x")
+	if err != nil {
+		return err
+	}
+
+	if fileExists {
 		var overwrite string
 		fmt.Fprint(os.Stderr, "File import-cosign.key already exists. Overwrite (y/n)? ")
 		fmt.Scanf("%s", &overwrite)

pkg/cosign/common.go

@@ -27,12 +27,15 @@ import (
 )
 
 // TODO(jason): Move this to an internal package.
-func FileExists(filename string) bool {
+func FileExists(filename string) (bool, error) {
 	info, err := os.Stat(filename)
 	if os.IsNotExist(err) {
-		return false
+		return false, nil
 	}
-	return !info.IsDir()
+	if err != nil {
+		return false, err
+	}
+	return !info.IsDir(), nil
 }
 
 // ConfirmPrompt prompts the user for confirmation for an action. Supports skipping

pkg/cosign/common_test.go

@@ -0,0 +1,51 @@
+//
+// Copyright 2022 The Sigstore Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cosign
+
+import (
+	"os"
+	"testing"
+)
+
+func Test_FileExists(t *testing.T) {
+	tmpFile, err := os.CreateTemp(os.TempDir(), "cosign_test.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tests := []struct {
+		name    string
+		path    string
+		exists  bool
+		wantErr bool
+	}{
+		{"file exists", tmpFile.Name(), true, false},
+		{"file does not exist", "testt.txt", false, false},
+		{"other error e.g cannot access file", "\000x", false, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := FileExists(tt.path)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("FileExists() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if got != tt.exists {
+				t.Errorf("FileExists() = %v, want %v", got, tt.exists)
+			}
+		})
+	}
+}