From 49eb02676983a2b480cf69195b86d9a70cf473c2 Mon Sep 17 00:00:00 2001
From: Lorenzo Blasa <lblasa@fb.com>
Date: Thu, 26 May 2022 09:44:30 -0700
Subject: [PATCH] Tail

Summary:
Added a new type, Tail. It observes changes made to a file and notifies the subscriber.

It is useful and used to tail flipper-server logs, for example.

Reviewed By: passy

Differential Revision: D36473529

fbshipit-source-id: b0f905f4f8419d9e13386440d2d2cdabd528295b
---
 desktop/flipper-frontend-core/package.json    |   1 +
 desktop/flipper-frontend-core/src/index.tsx   |   1 +
 .../flipper-frontend-core/src/utils/tail.tsx  | 144 ++++++++++++++++++
 3 files changed, 146 insertions(+)
 create mode 100644 desktop/flipper-frontend-core/src/utils/tail.tsx

diff --git a/desktop/flipper-frontend-core/package.json b/desktop/flipper-frontend-core/package.json
index 5ce9753a5..09cb6ae72 100644
--- a/desktop/flipper-frontend-core/package.json
+++ b/desktop/flipper-frontend-core/package.json
@@ -13,6 +13,7 @@
     "eventemitter3": "^4.0.7",
     "flipper-common": "0.0.0",
     "flipper-plugin": "0.0.0",
+    "fs-extra": "^10.1.0",
     "immer": "^9.0.12",
     "js-base64": "^3.7.2",
     "p-map": "^5.3.0",
diff --git a/desktop/flipper-frontend-core/src/index.tsx b/desktop/flipper-frontend-core/src/index.tsx
index 2beca1acb..641e8ce9b 100644
--- a/desktop/flipper-frontend-core/src/index.tsx
+++ b/desktop/flipper-frontend-core/src/index.tsx
@@ -15,3 +15,4 @@ export * from './globalObject';
 export * from './plugins';
 export * from './flipperLibImplementation';
 export * from './client/FlipperServerClient';
+export * from './utils/tail';
diff --git a/desktop/flipper-frontend-core/src/utils/tail.tsx b/desktop/flipper-frontend-core/src/utils/tail.tsx
new file mode 100644
index 000000000..a861ad56a
--- /dev/null
+++ b/desktop/flipper-frontend-core/src/utils/tail.tsx
@@ -0,0 +1,144 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import EventEmitter from 'eventemitter3';
+import fs, {FSWatcher} from 'fs-extra';
+import path from 'path';
+
+export type TailOptions = {
+  separator: RegExp;
+  encoding: BufferEncoding;
+  fromBeginning: boolean;
+};
+
+type TailBlock = {
+  start: number;
+  end: number;
+};
+
+export class Tail extends EventEmitter {
+  absPath: string;
+  dispatcher = new EventEmitter();
+  buffer = '';
+  queue: TailBlock[] = [];
+  isWatching = false;
+  currentCursorPosition = 0;
+  watcher: FSWatcher | undefined = undefined;
+
+  constructor(
+    private readonly filename: string,
+    private readonly options: TailOptions = {
+      separator: /[\r]{0,1}\n/,
+      fromBeginning: true,
+      encoding: 'utf8',
+    },
+  ) {
+    super();
+
+    this.absPath = path.dirname(this.filename);
+    this.dispatcher.on('next', () => {
+      this.read_();
+    });
+  }
+
+  async watch() {
+    if (this.isWatching) {
+      return;
+    }
+    this.isWatching = true;
+
+    let startingCursor = 0;
+    if (!this.options.fromBeginning) {
+      startingCursor = await this.latestPosition_();
+    }
+
+    this.watch_(startingCursor, this.options.fromBeginning);
+  }
+
+  unwatch() {
+    if (this.watcher) {
+      this.watcher.close();
+    }
+    this.isWatching = false;
+  }
+
+  private async latestPosition_(): Promise<number> {
+    return (await fs.stat(this.filename)).size;
+  }
+
+  private read_() {
+    if (this.queue.length >= 1) {
+      const block = this.queue[0];
+      if (block.end > block.start) {
+        const stream = fs.createReadStream(this.filename, {
+          start: block.start,
+          end: block.end - 1,
+          encoding: this.options.encoding,
+        });
+        stream.on('error', (error) => {
+          this.emit('error', error);
+        });
+        stream.on('end', () => {
+          this.queue.shift();
+          if (this.queue.length > 0) {
+            this.dispatcher.emit('next');
+          }
+          if (this.buffer.length > 0) {
+            this.emit('line', this.buffer);
+            this.buffer = '';
+          }
+        });
+        stream.on('data', (d) => {
+          this.buffer += d;
+          const parts = this.buffer.split(this.options.separator);
+          // The last part may be an incomplete chunk, so
+          // push that back into the buffer. Otherwise, reset buffer.
+          this.buffer = parts.pop() || '';
+          for (const chunk of parts) {
+            this.emit('line', chunk);
+          }
+        });
+      }
+    }
+  }
+
+  private async fileUpdated_() {
+    const latestPosition = await this.latestPosition_();
+    // Case where text is not appended but it's actually a w+
+    if (latestPosition < this.currentCursorPosition) {
+      this.currentCursorPosition = latestPosition;
+    } else if (latestPosition > this.currentCursorPosition) {
+      this.queue.push({start: this.currentCursorPosition, end: latestPosition});
+      this.currentCursorPosition = latestPosition;
+      // Only emit if the queue was empty and now is not.
+      if (this.queue.length == 1) {
+        this.dispatcher.emit('next');
+      }
+    }
+  }
+
+  private watch_(cursor: number, readPrevious: boolean) {
+    this.currentCursorPosition = cursor;
+    if (readPrevious) {
+      this.fileUpdated_();
+    }
+
+    this.watcher = fs.watch(this.filename, {}, (eventType, filename) => {
+      this.watchEvent_(eventType, filename);
+    });
+  }
+
+  private watchEvent_(eventType: string, _filename: string) {
+    try {
+      if (eventType === 'change') {
+        this.fileUpdated_();
+      }
+    } catch (err) {}
+  }
+}