fix(auth): correctly parse executable command with quoted arguments

[nbayati]

The executable command for pluggable auth was being parsed with strings.Fields, which does not handle quoted arguments containing spaces correctly. This caused errors when the executable path or its arguments contained spaces.

This change replaces strings.Fields with shellwords.Parse from the github.com/mattn/go-shellwords library. This library correctly parses shell command strings, respecting quotes.

This change adds a new dependency on github.com/mattn/go-shellwords.

This was reported in b/238143555

[gemini-code-assist]

Summary of Changes

Hello @nbayati, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical parsing flaw in the pluggable authentication mechanism where executable commands with spaces in their paths or arguments were incorrectly interpreted. By switching to a more sophisticated shellwords parser, the system can now reliably execute commands, significantly improving the stability and compatibility of the authentication process. This change ensures that complex command structures are handled as intended, preventing authentication failures due to malformed command execution.

Highlights

• Bug Fix: Corrected an issue where executable commands for pluggable authentication failed to parse correctly when paths or arguments contained spaces, leading to errors.
• Parsing Logic Update: Replaced the use of strings.Fields with shellwords.Parse from the github.com/mattn/go-shellwords library to accurately handle shell command strings, including those with quoted arguments.
• New Dependency: Introduced a new indirect dependency on github.com/mattn/go-shellwords to facilitate robust command-line argument parsing.
• Improved Robustness: Added error handling for the new parsing logic, specifically checking for parsing errors and empty commands to prevent unexpected behavior.
• Test Coverage: Added new unit tests to executable_provider_test.go to verify the correct behavior of shellwords.Parse with various scenarios involving quoted and unquoted arguments.

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[gemini-code-assist]

Code Review

This pull request correctly fixes an issue with parsing executable commands that contain quoted arguments with spaces. The change replaces strings.Fields with shellwords.Parse, which is the right approach for this problem. The introduction of the new dependency is justified. My feedback includes a suggestion to enhance the test coverage for the new error handling path that was introduced.

[gemini-code-assist]

This test is a good addition for verifying the new parsing logic. To improve test coverage for the new error handling in the run function, please consider adding test cases for empty and whitespace-only command strings. These inputs cause shellwords.Parse to return an empty slice, which triggers the new if len(splitCommand) == 0 check.

 	tests := []struct {
 		name    string
 		command string
 		want    []string
 		wantErr bool
 	}{
 		{
 			name:    "simple command",
 			command: "a b c",
 			want:    []string{"a", "b", "c"},
 		},
 		{
 			name:    "quoted argument",
 			command: `a "b c"`,
 			want:    []string{"a", "b c"},
 		},
 		{
 			name:    "quoted executable",
 			command: `"a b" c`,
 			want:    []string{"a b", "c"},
 		},
 		{
 			name:    "quoted executable and argument",
 			command: `"a b" "c d"`,
 			want:    []string{"a b", "c d"},
 		},
 		{
 			name:    "single quotes",
 			command: `'a b' 'c d'`,
 			want:    []string{"a b", "c d"},
 		},
 		{
 			name:    "empty command",
 			command: "",
 			want:    []string{},
 		},
 		{
 			name:    "whitespace only",
 			command: "   ",
 			want:    []string{},
 		},
 	}