Refactored server log scrubber into package

The server log scrubbing code from ticket #21304 is now refactored into
a safelog package, along with the appropriate tests

common/safelog/log.go

@@ -0,0 +1,59 @@
+//Package for a safer logging wrapper around the standard logging package
+
+//import "git.torproject.org/pluggable-transports/snowflake.git/common/safelog"
+package safelog
+
+import (
+	"bytes"
+	"io"
+	"regexp"
+)
+
+const ipv4Address = `\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}`
+const ipv6Address = `(([0-9a-fA-F]{0,4}:){2,7}([0-9a-fA-F]{0,4})?(` + ipv4Address + `))` +
+	`|(([0-9a-fA-F]{0,4}:){2,7}([0-9a-fA-F]{0,4})?)`
+const optionalPort = `(:\d{1,5})?`
+const addressPattern = `((` + ipv4Address + `)|(\[(` + ipv6Address + `)\])|(` + ipv6Address + `))` + optionalPort
+const fullAddrPattern = `(^|\s|[^\w:])` + addressPattern + `(\s|(:\s)|[^\w:]|$)`
+
+var scrubberPatterns = []*regexp.Regexp{
+	regexp.MustCompile(fullAddrPattern),
+}
+
+var addressRegexp = regexp.MustCompile(addressPattern)
+
+// An io.Writer that can be used as the output for a logger that first
+// sanitizes logs and then writes to the provided io.Writer
+type LogScrubber struct {
+	Output io.Writer
+	buffer []byte
+}
+
+func scrub(b []byte) []byte {
+	scrubbedBytes := b
+	for _, pattern := range scrubberPatterns {
+		// this is a workaround since go does not yet support look ahead or look
+		// behind for regular expressions.
+		scrubbedBytes = pattern.ReplaceAllFunc(scrubbedBytes, func(b []byte) []byte {
+			return addressRegexp.ReplaceAll(b, []byte("[scrubbed]"))
+		})
+	}
+	return 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:]
+	}
+}