allow escaping the dot character in paths using a backslash

Occasionally expvar output contains keys that are hostnames,
IP addresses, or filenames that contain the "." character.  For
example:

{
  "bleve": {
    "bootDuration": 16559,
    "indexes": {
      "bench.bleve": {
        "index": {
          "analysis_time": 10889841135,
          "batches": 145,
          "deletes": 0,
          "errors": 0,
          "index_time": 21277401883,
          "lookup_queue_len": 0,
          "updates": 14500
        },
        "search_time": 0,
        "searches": 0
      }
    }
  }
...
}

I can now chart the lookup_queue_len value using the var:

bleve.indexes.bench\.bleve.index.lookup_queue_len

This partially addresses divan#11 (still does not escape colon character).

var.go

@@ -38,7 +38,7 @@ const (
 // Example: "mem:memstats.Alloc" => []string{"memstats", "Alloc"}
 func (v VarName) ToSlice() []string {
 	start := strings.IndexRune(string(v), ':') + 1
-	slice := strings.FieldsFunc(string(v)[start:], func(r rune) bool { return r == '.' })
+	slice := DottedFieldsToSliceEscaped(string(v)[start:])
 	return slice
 }
 
@@ -128,3 +128,47 @@ func roundDuration(d time.Duration) time.Duration {
 	}
 	return d
 }
+
+func DottedFieldsToSliceEscaped(s string) []string {
+	rv := make([]string, 0)
+	lastSlash := false
+	curr := ""
+	for _, r := range s {
+		// base case, dot not after slash
+		if !lastSlash && r == '.' {
+			if len(curr) > 0 {
+				rv = append(rv, curr)
+				curr = ""
+			}
+			continue
+		} else if !lastSlash {
+			// any character not after slash
+			curr += string(r)
+			if r == '\\' {
+				lastSlash = true
+			} else {
+				lastSlash = false
+			}
+			continue
+		} else if r == '\\' {
+			// last was slash, and so is this
+			lastSlash = false // 2 slashes = 0
+			// we already appended a single slash on first
+			continue
+		} else if r == '.' {
+			// we see \. but already appended \ last time
+			// replace it with .
+			curr = curr[:len(curr)-1] + "."
+			lastSlash = false
+		} else {
+			// \ and any other character, ignore
+			curr += string(r)
+			lastSlash = false
+			continue
+		}
+	}
+	if len(curr) > 0 {
+		rv = append(rv, curr)
+	}
+	return rv
+}

var_test.go

@@ -44,4 +44,49 @@ func TestVarName(t *testing.T) {
 	if kind != KindDuration {
 		t.Fatalf("Expecting kind to be %v, but got: %v", KindDuration, kind)
 	}
+
+	// single \. escapes the dot
+	v = VarName(`bleve.indexes.bench\.bleve.index.lookup_queue_len`)
+
+	slice = v.ToSlice()
+	if len(slice) != 5 || slice[0] != "bleve" || slice[1] != "indexes" || slice[2] != "bench.bleve" ||
+		slice[3] != "index" || slice[4] != "lookup_queue_len" {
+		t.Fatalf("ToSlice failed: %v", slice)
+	}
+
+	// double \\. escapes backslash, not dot
+	v = VarName(`bleve.indexes.bench\\.bleve.index.lookup_queue_len`)
+
+	slice = v.ToSlice()
+	if len(slice) != 6 || slice[0] != "bleve" || slice[1] != "indexes" || slice[2] != "bench\\" ||
+		slice[3] != "bleve" || slice[4] != "index" || slice[5] != "lookup_queue_len" {
+		t.Fatalf("ToSlice failed: %v", slice)
+	}
+
+	// triple \\\. escapes backslash then dot
+	v = VarName(`bleve.indexes.bench\\\.bleve.index.lookup_queue_len`)
+
+	slice = v.ToSlice()
+	if len(slice) != 5 || slice[0] != "bleve" || slice[1] != "indexes" || slice[2] != "bench\\.bleve" ||
+		slice[3] != "index" || slice[4] != "lookup_queue_len" {
+		t.Fatalf("ToSlice failed: %v", slice)
+	}
+
+	// quadruple \\\\. escapes two backslashes, not dot
+	v = VarName(`bleve.indexes.bench\\\\.bleve.index.lookup_queue_len`)
+
+	slice = v.ToSlice()
+	if len(slice) != 6 || slice[0] != "bleve" || slice[1] != "indexes" || slice[2] != "bench\\\\" ||
+		slice[3] != "bleve" || slice[4] != "index" || slice[5] != "lookup_queue_len" {
+		t.Fatalf("ToSlice failed: %v", slice)
+	}
+
+	// unsupported \x passes through unaltered
+	v = VarName(`bleve.indexes.bench\xbleve.index.lookup_queue_len`)
+
+	slice = v.ToSlice()
+	if len(slice) != 5 || slice[0] != "bleve" || slice[1] != "indexes" || slice[2] != "bench\\xbleve" ||
+		slice[3] != "index" || slice[4] != "lookup_queue_len" {
+		t.Fatalf("ToSlice failed: %v", slice)
+	}
 }