apcssh: add shell cmd timeout

It was possible for scanner.Scan() to block indefinitely if the UPS never returned the expected prompt regex pattern. This could occur with a UPS using a prompt format I'm not aware of, or if the UPS responds in a non-standard way.

This change ensures that Scan() is aborted after a fixed amount of blocking time and the shell cmd function accordingly returns an error.

Some error messages, comments, and var names are also updated for clarity.

pkg/apcssh/shell_helpers.go

@@ -1,31 +1,31 @@
 package apcssh
 
 import (
+	"io"
 	"regexp"
 )
 
 // scanAPCShell is a SplitFunc to capture shell output after each interactive
 // shell command is run
 func scanAPCShell(data []byte, atEOF bool) (advance int, token []byte, err error) {
-	if atEOF && len(data) == 0 {
+	// EOF is not an expected response and should error (e.g., when the output pipe
+	// gets closed by timeout)
+	if atEOF {
+		return len(data), dropCR(data), io.ErrUnexpectedEOF
+	} else if len(data) == 0 {
+		// no data to process, request more data
 		return 0, nil, nil
 	}
 
 	// regex for shell prompt (e.g., `apc@apc>`, `apc>`, `some@dev>`, `other123>`, etc.)
 	re := regexp.MustCompile(`(\r\n|\r|\n)([A-Za-z0-9.]+@?)?[A-Za-z0-9.]+>`)
-
 	// find match for prompt
 	if index := re.FindStringIndex(string(data)); index != nil {
 		// advance starts after the prompt; token is everything before the prompt
 		return index[1], dropCR(data[0:index[0]]), nil
 	}
 
-	// If we're at EOF, we have a final, non-terminated line. Return it.
-	if atEOF {
-		return len(data), dropCR(data), nil
-	}
-
-	// Request more data.
+	// no match, request more data
 	return 0, nil, nil
 }