core, log: track field length and pad to align

log/format.go

@@ -10,6 +10,7 @@ import (
 	"sync"
 	"sync/atomic"
 	"time"
+	"unicode/utf8"
 )
 
 const (
@@ -43,6 +44,13 @@ var locationEnabled uint32
 // padded to to aid in alignment.
 var locationLength uint32
 
+// fieldPadding is a global map with maximum field value lengths seen until now
+// to allow padding log contexts in a bit smarter way.
+var fieldPadding = make(map[string]int)
+
+// fieldPaddingLock is a global mutex protecting the field padding map.
+var fieldPaddingLock sync.RWMutex
+
 type Format interface {
 	Format(r *Record) []byte
 }
@@ -163,15 +171,29 @@ func logfmt(buf *bytes.Buffer, ctx []interface{}, color int, term bool) {
 		}
 
 		// XXX: we should probably check that all of your key bytes aren't invalid
+		fieldPaddingLock.RLock()
+		padding := fieldPadding[k]
+		fieldPaddingLock.RUnlock()
+
+		length := utf8.RuneCountInString(v)
+		if padding < length {
+			padding = length
+
+			fieldPaddingLock.Lock()
+			fieldPadding[k] = padding
+			fieldPaddingLock.Unlock()
+		}
 		if color > 0 {
-			fmt.Fprintf(buf, "\x1b[%dm%s\x1b[0m=%s", color, k, v)
+			fmt.Fprintf(buf, "\x1b[%dm%s\x1b[0m=", color, k)
 		} else {
 			buf.WriteString(k)
 			buf.WriteByte('=')
-			buf.WriteString(v)
+		}
+		buf.WriteString(v)
+		if i < len(ctx)-2 {
+			buf.Write(bytes.Repeat([]byte{' '}, padding-length))
 		}
 	}
-
 	buf.WriteByte('\n')
 }