readline: make tab size configurable

This adds the `tabSize` option to readline to allow different tab
sizes.

PR-URL: #31318
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/readline.md

@@ -456,6 +456,9 @@ the current position of the cursor down.
 <!-- YAML
 added: v0.1.98
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/31318
+    description: The `tabSize` option is supported now.
   - version: v8.3.0, 6.11.4
     pr-url: https://github.com/nodejs/node/pull/13497
     description: Remove max limit of `crlfDelay` option.
@@ -499,6 +502,8 @@ changes:
     can both form a complete key sequence using the input read so far and can
     take additional input to complete a longer key sequence).
     **Default:** `500`.
+  * `tabSize` {integer} The number of spaces a tab is equal to (minimum 1).
+    **Default:** `8`.
 
 The `readline.createInterface()` method creates a new `readline.Interface`
 instance.

lib/readline.js

@@ -45,7 +45,10 @@ const {
   ERR_INVALID_CURSOR_POS,
   ERR_INVALID_OPT_VALUE
 } = require('internal/errors').codes;
-const { validateString } = require('internal/validators');
+const {
+  validateString,
+  validateUint32,
+} = require('internal/validators');
 const {
   inspect,
   getStringWidth,
@@ -105,6 +108,7 @@ function Interface(input, output, completer, terminal) {
   this._sawKeyPress = false;
   this._previousKey = null;
   this.escapeCodeTimeout = ESCAPE_CODE_TIMEOUT;
+  this.tabSize = 8;
 
   EventEmitter.call(this);
   let historySize;
@@ -118,6 +122,11 @@ function Interface(input, output, completer, terminal) {
     completer = input.completer;
     terminal = input.terminal;
     historySize = input.historySize;
+    if (input.tabSize !== undefined) {
+      const positive = true;
+      validateUint32(input.tabSize, 'tabSize', positive);
+      this.tabSize = input.tabSize;
+    }
     removeHistoryDuplicates = input.removeHistoryDuplicates;
     if (input.prompt !== undefined) {
       prompt = input.prompt;
@@ -718,10 +727,9 @@ Interface.prototype._getDisplayPos = function(str) {
       offset = 0;
       continue;
     }
-    // Tabs must be aligned by an offset of 8.
-    // TODO(BridgeAR): Make the tab size configurable.
+    // Tabs must be aligned by an offset of the tab size.
     if (char === '\t') {
-      offset += 8 - (offset % 8);
+      offset += this.tabSize - (offset % this.tabSize);
       continue;
     }
     const width = getStringWidth(char);

test/parallel/test-readline-interface.js

@@ -138,6 +138,39 @@ function assertCursorRowsAndCols(rli, rows, cols) {
     name: 'RangeError',
     code: 'ERR_INVALID_OPT_VALUE'
   });
+
+  // Check for invalid tab sizes.
+  assert.throws(
+    () => new readline.Interface({
+      input,
+      tabSize: 0
+    }),
+    {
+      message: 'The value of "tabSize" is out of range. ' +
+                'It must be >= 1 && < 4294967296. Received 0',
+      code: 'ERR_OUT_OF_RANGE'
+    }
+  );
+
+  assert.throws(
+    () => new readline.Interface({
+      input,
+      tabSize: '4'
+    }),
+    { code: 'ERR_INVALID_ARG_TYPE' }
+  );
+
+  assert.throws(
+    () => new readline.Interface({
+      input,
+      tabSize: 4.5
+    }),
+    {
+      code: 'ERR_OUT_OF_RANGE',
+      message: 'The value of "tabSize" is out of range. ' +
+                'It must be an integer. Received 4.5'
+    }
+  );
 }
 
 // Sending a single character with no newline
@@ -632,6 +665,25 @@ function assertCursorRowsAndCols(rli, rows, cols) {
   rli.close();
 }
 
+// Multi-line input cursor position and long tabs
+{
+  const [rli, fi] = getInterface({ tabSize: 16, terminal: true, prompt: '' });
+  fi.columns = 10;
+  fi.emit('data', 'multi-line\ttext \t');
+  assert.strictEqual(rli.cursor, 17);
+  assertCursorRowsAndCols(rli, 3, 2);
+  rli.close();
+}
+
+// Check for the default tab size.
+{
+  const [rli, fi] = getInterface({ terminal: true, prompt: '' });
+  fi.emit('data', 'the quick\tbrown\tfox');
+  assert.strictEqual(rli.cursor, 19);
+  // The first tab is 7 spaces long, the second one 3 spaces.
+  assertCursorRowsAndCols(rli, 0, 27);
+}
+
 // Multi-line prompt cursor position
 {
   const [rli, fi] = getInterface({