2 files changed, 42 insertions, 4 deletions

diff --git a/std/log/handlers.ts b/std/log/handlers.ts
index ffcd846cc..3c8dd6896 100644
--- a/std/log/handlers.ts
+++ b/std/log/handlers.ts
@@ -1,5 +1,5 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
-const { open, openSync, close, renameSync, statSync } = Deno;
+const { open, openSync, close, renameSync, stat } = Deno;
 type File = Deno.File;
 type Writer = Deno.Writer;
 type OpenOptions = Deno.OpenOptions;
@@ -142,6 +142,8 @@ interface RotatingFileHandlerOptions extends FileHandlerOptions {
 export class RotatingFileHandler extends FileHandler {
   #maxBytes: number;
   #maxBackupCount: number;
+  #currentFileSize = 0;
+  #encoder = new TextEncoder();
 
   constructor(levelName: LevelName, options: RotatingFileHandlerOptions) {
     super(levelName, options);
@@ -176,6 +178,8 @@ export class RotatingFileHandler extends FileHandler {
           );
         }
       }
+    } else {
+      this.#currentFileSize = (await stat(this._filename)).size;
     }
   }
 
@@ -183,9 +187,12 @@ export class RotatingFileHandler extends FileHandler {
     if (this.level > logRecord.level) return;
 
     const msg = this.format(logRecord);
-    const currentFileSize = statSync(this._filename).size;
-    if (currentFileSize + msg.length > this.#maxBytes) {
+    const msgByteLength = this.#encoder.encode(msg).byteLength + 1;
+    if (this.#currentFileSize + msgByteLength > this.#maxBytes) {
       this.rotateLogFiles();
+      this.#currentFileSize = msgByteLength;
+    } else {
+      this.#currentFileSize += msgByteLength;
     }
 
     return this.log(msg);
diff --git a/std/log/handlers_test.ts b/std/log/handlers_test.ts
index c8b76347f..5275a5ca0 100644
--- a/std/log/handlers_test.ts
+++ b/std/log/handlers_test.ts
@@ -1,6 +1,11 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 const { test } = Deno;
-import { assert, assertEquals, assertThrowsAsync } from "../testing/asserts.ts";
+import {
+  assert,
+  assertEquals,
+  assertThrowsAsync,
+  assertNotEquals,
+} from "../testing/asserts.ts";
 import {
   LogLevels,
   LogLevelNames,
@@ -303,3 +308,29 @@ test({
     );
   },
 });
+
+test({
+  name: "RotatingFileHandler fileSize equal to bytelength of message + 1",
+  async fn() {
+    const fileHandler = new RotatingFileHandler("WARNING", {
+      filename: LOG_FILE,
+      maxBytes: 100,
+      maxBackupCount: 1,
+      mode: "w",
+    });
+    await fileHandler.setup();
+
+    const msg = "。";
+    const msgLength = msg.length;
+    const msgByteLength = new TextEncoder().encode(msg).byteLength;
+    await fileHandler.log(msg);
+    const fileSzie = (await Deno.stat(LOG_FILE)).size;
+
+    assertEquals(fileSzie, msgByteLength + 1);
+    assertNotEquals(fileSzie, msgLength);
+    assertNotEquals(fileSzie, msgLength + 1);
+
+    await fileHandler.destroy();
+    Deno.removeSync(LOG_FILE);
+  },
+});