parse commands passed to -shellcheck and -pyflakes persistently

process.go

@@ -111,7 +111,7 @@ func (proc *concurrentProcess) wait() {
 // is resolved in this function.
 func (proc *concurrentProcess) newCommandRunner(exe string, combineOutput bool) (*externalCommand, error) {
 	var args []string
-	p, args, err := findExe(exe)
+	p, args, err := parseExternalCommand(exe)
 	if err != nil {
 		return nil, err
 	}
@@ -124,20 +124,16 @@ func (proc *concurrentProcess) newCommandRunner(exe string, combineOutput bool)
 	return cmd, nil
 }
 
-func findExe(exe string) (string, []string, error) {
-	p, err := execabs.LookPath(exe)
-	if err == nil {
-		return p, nil, nil
+func parseExternalCommand(cmd string) (string, []string, error) {
+	a, err := shellwords.Parse(cmd)
+	if err != nil || len(a) == 0 {
+		return "", nil, fmt.Errorf("broken command line %q: %w", cmd, err)
 	}
-	// See if the command string contains args. As it is best effort, we do not
-	// handle parse errors.
-	if exeArgs, _ := shellwords.Parse(exe); len(exeArgs) > 0 {
-		if p, err := execabs.LookPath(exeArgs[0]); err == nil {
-			return p, exeArgs[1:], nil
-		}
+	c, err := execabs.LookPath(a[0])
+	if err != nil {
+		return "", nil, err
 	}
-
-	return "", nil, err
+	return c, a[1:], nil
 }
 
 // externalCommand is struct to run specific command concurrently with concurrentProcess bounding

process_test.go

@@ -343,3 +343,33 @@ func TestProcessCommandExitStatusNonZero(t *testing.T) {
 		t.Fatalf("Unexpected error happened: %q", msg)
 	}
 }
+
+func TestProcessCommandlineParseError(t *testing.T) {
+	tests := []struct {
+		what string
+		cmd  string
+		want string
+	}{
+		{
+			cmd:  "'broken' 'arg",
+			want: "broken command line",
+		},
+		{
+			cmd:  "this-command-does-not-exist",
+			want: "executable file not found",
+		},
+	}
+
+	p := newConcurrentProcess(1)
+	for _, tc := range tests {
+		t.Run(tc.want, func(t *testing.T) {
+			_, err := p.newCommandRunner(tc.cmd, true)
+			if err == nil {
+				t.Fatalf("Command %q caused no error", tc)
+			}
+			if msg := err.Error(); !strings.Contains(msg, tc.want) {
+				t.Fatalf("Wanted error message %q to contain %q", msg, tc.want)
+			}
+		})
+	}
+}