Modified log scrubber to handle split lines

Log scrubber now scrubs addresses even in the case where they are split
across calls to the scrubber's output io.Writer.

Added test cases to test that the writer behaves correctly across split
lines.

server/server.go

@@ -3,6 +3,7 @@
 package main
 
 import (
+	"bytes"
 	"crypto/tls"
 	"errors"
 	"flag"
@@ -66,15 +67,32 @@ var scrubberPatterns = []*regexp.Regexp{
 // sanitizes logs and then writes to the provided io.Writer
 type logScrubber struct {
 	output io.Writer
+	buffer []byte
 }
 
-func (ls *logScrubber) Write(b []byte) (n int, err error) {
+func scrub(b []byte) []byte {
 	scrubbedBytes := b
 	for _, pattern := range scrubberPatterns {
 		scrubbedBytes = pattern.ReplaceAll(scrubbedBytes, []byte("[scrubbed]"))
 	}
+	return scrubbedBytes
+}
 
-	return ls.output.Write(scrubbedBytes)
+func (ls *logScrubber) Write(b []byte) (n int, err error) {
+	n = len(b)
+	ls.buffer = append(ls.buffer, b...)
+	for {
+		i := bytes.LastIndexByte(ls.buffer, '\n')
+		if i == -1 {
+			return
+		}
+		fullLines := ls.buffer[:i+1]
+		_, err = ls.output.Write(scrub(fullLines))
+		if err != nil {
+			return
+		}
+		ls.buffer = ls.buffer[i+1:]
+	}
 }
 
 // An abstraction that makes an underlying WebSocket connection look like an
@@ -308,7 +326,7 @@ func main() {
 		logOutput = f
 	}
 	//We want to send the log output through our scrubber first
-	log.SetOutput(&logScrubber{logOutput})
+	log.SetOutput(&logScrubber{output: logOutput})
 
 	if !disableTLS && acmeHostnamesCommas == "" {
 		log.Fatal("the --acme-hostnames option is required")