Use a JS only tail implementation

lib/tail.js

@@ -3,64 +3,126 @@
 'use strict';
 
 const events = require('events');
-const childProcess = require('child_process');
-const tailStream = require('fs-tail-stream');
+const fs = require('fs');
+const TailLib = require('tail').Tail;
+const readLastLines = require('read-last-lines');
 const util = require('util');
 const CBuffer = require('CBuffer');
 const byline = require('byline');
-const commandExistsSync = require('command-exists').sync;
+
+const NB_OF_LINE_TO_PREFETCH = 50;
+const TAIL_RETRY_DELAY = 2000;
+const NEW_LINE_REGEX = /[\r]{0,1}\n/;
+const IS_WIN = process.platform === 'win32';
+
+function getLinePrefix(filePath, filePaths) {
+  const oneFileIsTailed = filePaths.length <= 1;
+  if (oneFileIsTailed) {
+    return '';
+  }
+  const paddingLength = Math.max(...filePaths.map((fileName) => fileName.length));
+  return `${filePath.padStart(paddingLength, ' ')} - `;
+}
+
+const fileLogger = (fileName) => ({
+  info: (...data) => {
+    console.info(fileName, ':', ...data);
+  },
+  error: (...data) => {
+    console.error(fileName, ':', ...data);
+  }
+});
+
+async function readLastLinesIfPossible(path, onLineCb) {
+  try {
+    await fs.promises.access(path, fs.constants.R_OK);
+  } catch (ex) {
+    // The file can not be read
+    return;
+  }
+
+  try {
+    const lines = await readLastLines.read(path, NB_OF_LINE_TO_PREFETCH)
+    const linesWithoutLastEmptyLine = lines.replace(/[\r]{0,1}\n$/gm, '');
+    if (linesWithoutLastEmptyLine === '') {
+      return;
+    }
+    linesWithoutLastEmptyLine.split(NEW_LINE_REGEX).forEach(onLineCb);
+  } catch (ex) {
+    fileLogger(path).error('Failed to prefetch the file content:', ex);
+  }
+}
+
+async function tailFile(path, onLineCb) {
+  const logger = fileLogger(path);
+  let tail;
+
+  function rescheduleTail() {
+    // Close the current tail
+    if (tail) {
+      try {
+        tail.unwatch();
+      } catch (ex) {
+        // Failed to shutdown the previous tail, ignore this since we try to stop it
+      }
+    }
+    setTimeout(() => tailFile(path, onLineCb), TAIL_RETRY_DELAY);
+  }
+
+  // Test that the file exists
+  try {
+    await fs.promises.access(path, fs.constants.R_OK);
+  } catch (ex) {
+    logger.error(`tail failure - ${ex}`);
+    rescheduleTail();
+    return;
+  }
+
+  try {
+    logger.info('starting to watch the file');
+    tail = new TailLib(path, {
+      useWatchFile: IS_WIN // Use watchfile on windows as a workaround to this issue: https://docs.microsoft.com/en-us/archive/blogs/asiasupp/file-date-modified-property-are-not-updating-while-modifying-a-file-without-closing-it
+    });
+  } catch (ex) {
+    logger.error(`tail failure - ${ex}`);
+    rescheduleTail();
+    return;
+  }
+
+  tail.on('line', onLineCb);
+  tail.on('error', (err) => {
+    logger.error('tail failure -', err);
+    rescheduleTail();
+  });
+}
 
 function Tail(path, opts) {
   events.EventEmitter.call(this);
+  const pathArray = Array.isArray(path) ? path : [path]; // Normalize the parameter
+
+  const pushLineToBuffer = (prefix) =>
+    (line) => {
+      const str = `${prefix}${line.toString()}`;
+      this._buffer.push(str);
+      this.emit('line', str);
+    };
 
   const options = opts || {
     buffer: 0
   };
   this._buffer = new CBuffer(options.buffer);
 
-  let stream;
-
-  if (path[0] === '-') {
-    stream = process.stdin;
-  } else {
-    /* Check if this os provides the `tail` command. */
-    const hasTailCommand = commandExistsSync('tail');
-    if (hasTailCommand) {
-      let followOpt = '-F';
-      if (process.platform === 'openbsd') {
-        followOpt = '-f';
-      }
-
-      const cp = childProcess.spawn(
-        'tail',
-        ['-n', options.buffer, followOpt].concat(path)
-      );
-      cp.stderr.on('data', (data) => {
-        // If there is any important error then display it in the console. Tail will keep running.
-        // File can be truncated over network.
-        if (data.toString().indexOf('file truncated') === -1) {
-          console.error(data.toString());
-        }
-      });
-      stream = cp.stdout;
-
-      process.on('exit', () => {
-        cp.kill();
-      });
+  // Start to tail on every parameter
+  pathArray.forEach((pathItem) => {
+    if (pathItem === '-') {
+      console.info('starting to watch stdin');
+      const linePrefix = getLinePrefix('stdin', pathArray);
+      byline(process.stdin, { keepEmptyLines: true }).on('data', pushLineToBuffer(linePrefix));
     } else {
-      /* This is used if the os does not support the `tail`command. */
-      stream = tailStream.createReadStream(path.join(), {
-        encoding: 'utf8',
-        start: options.buffer,
-        tail: true
-      });
+      const linePrefix = getLinePrefix(pathItem, pathArray);
+      readLastLinesIfPossible(pathItem, pushLineToBuffer(linePrefix));
+      tailFile(pathItem, pushLineToBuffer(linePrefix));
     }
-  }
-
-  byline(stream, { keepEmptyLines: true }).on('data', (line) => {
-    const str = line.toString();
-    this._buffer.push(str);
-    this.emit('line', str);
   });
 }
 util.inherits(Tail, events.EventEmitter);

package.json

@@ -18,10 +18,11 @@
     "cookie-parser": "~1.4.3",
     "daemon-fix41": "~1.1.2",
     "express-session": "~1.15.6",
-    "fs-tail-stream": "^1.1.0",
     "is-docker": "~1.1.0",
+    "read-last-lines": "^1.7.2",
     "serve-static": "~1.13.2",
     "socket.io": "^2.2.0",
+    "tail": "^2.0.3",
     "universal-analytics": "~0.4.20",
     "untildify": "~3.0.2",
     "uuid": "~3.3.2"

test/tail.js

@@ -5,7 +5,7 @@ const temp = require('temp');
 const tail = require('../lib/tail');
 
 const TEMP_FILE_PROFIX = '';
-const SPAWN_DELAY = 10;
+const SPAWN_DELAY = 40;
 
 function writeLines(fd, count) {
   for (let i = 0; i < count; i += 1) {
@@ -15,7 +15,10 @@ function writeLines(fd, count) {
 `
     );
   }
-  fs.closeSync(fd);
+}
+
+function endsWith(str, suffix) {
+  return str.indexOf(suffix, str.length - suffix.length) !== -1;
 }
 
 describe('tail', () => {
@@ -25,13 +28,42 @@ describe('tail', () => {
     temp.open(TEMP_FILE_PROFIX, (err, info) => {
       tail(info.path).on('line', (line) => {
         line.should.equal('line0');
+        fs.closeSync(info.fd);
         done();
       });
 
       setTimeout(writeLines, SPAWN_DELAY, info.fd, 1);
     });
   });
 
+  it('calls event line if new line appear in files', (done) => {
+    const buffer = [];
+
+    temp.open(TEMP_FILE_PROFIX, (err, info) => {
+      temp.open(TEMP_FILE_PROFIX, (err2, info2) => {
+        tail([info.path, info2.path]).on('line', (line) => {
+          buffer.push(line);
+        });
+
+        setTimeout(() => {
+          writeLines(info.fd, 2);
+          writeLines(info2.fd, 2);
+
+          setTimeout(() => {
+            buffer.length.should.equal(4);
+            endsWith(buffer[0], `${info.path} - line0`).should.be.true;
+            endsWith(buffer[1], `${info.path} - line1`).should.be.true;
+            endsWith(buffer[2], `${info2.path} - line0`).should.be.true;
+            endsWith(buffer[3], `${info2.path} - line1`).should.be.true;
+            fs.closeSync(info.fd);
+            fs.closeSync(info2.fd);
+            done();
+          }, SPAWN_DELAY);
+        }, SPAWN_DELAY);
+      });
+    });
+  });
+
   it('buffers lines on start', (done) => {
     temp.open(TEMP_FILE_PROFIX, (err, info) => {
       writeLines(info.fd, 20);
@@ -41,6 +73,7 @@ describe('tail', () => {
       });
       setTimeout(() => {
         tailer.getBuffer().should.be.eql(['line18', 'line19']);
+        fs.closeSync(info.fd);
         done();
       }, SPAWN_DELAY);
     });
@@ -53,6 +86,7 @@ describe('tail', () => {
       const tailer = tail(info.path);
       setTimeout(() => {
         tailer.getBuffer().should.be.empty;
+        fs.closeSync(info.fd);
         done();
       }, SPAWN_DELAY);
     });