Extract getFlag fmap detection to a tested module

lib/get-write-flag.js

@@ -0,0 +1,16 @@
+// Get the appropriate flag to use for creating files
+// We use fmap on Windows platforms for files less than
+// 512kb.  This is a fairly low limit, but avoids making
+// things slower in some cases.  Since most of what this
+// library is used for is extracting tarballs of many
+// relatively small files in npm packages and the like,
+// it can be a big boost on Windows platforms.
+// Only supported in Node v12.9.0 and above.
+const platform = process.env.__FAKE_PLATFORM__ || process.platform
+const isWindows = platform === 'win32'
+const fs = require('fs')
+const { O_CREAT, O_TRUNC, O_WRONLY, UV_FS_O_FILEMAP = 0 } = fs.constants
+const fMapEnabled = isWindows && !!UV_FS_O_FILEMAP
+const fMapLimit = 512 * 1024
+const fMapFlag = UV_FS_O_FILEMAP | O_TRUNC | O_CREAT | O_WRONLY
+module.exports = size => (fMapEnabled && size < fMapLimit) ? fMapFlag : 'w'

lib/unpack.js

@@ -10,7 +10,6 @@ const assert = require('assert')
 const EE = require('events').EventEmitter
 const Parser = require('./parse.js')
 const fs = require('fs')
-const { O_CREAT, O_TRUNC, O_WRONLY, UV_FS_O_FILEMAP = 0 } = fs.constants
 const fsm = require('fs-minipass')
 const path = require('path')
 const mkdir = require('./mkdir.js')
@@ -43,6 +42,7 @@ const DOCHOWN = Symbol('doChown')
 const UID = Symbol('uid')
 const GID = Symbol('gid')
 const crypto = require('crypto')
+const getFlag = require('./get-write-flag.js')
 
 /* istanbul ignore next */
 const neverCalled = () => {
@@ -93,13 +93,6 @@ const uint32 = (a, b, c) =>
   : b === b >>> 0 ? b
   : c
 
-/* istanbul ignore next */
-const fMapEnabled = process.platform === 'win32' && !!UV_FS_O_FILEMAP
-const fMapLimit = 512 * 1024
-const fMapFlag = UV_FS_O_FILEMAP | O_TRUNC | O_CREAT | O_WRONLY
-/* istanbul ignore next */
-const getFlag = size => (fMapEnabled && size < fMapLimit) ? fMapFlag : 'w'
-
 class Unpack extends Parser {
   constructor (opt) {
     if (!opt)

test/get-write-flag.js

@@ -0,0 +1,51 @@
+const t = require('tap')
+
+// run three scenarios
+// unix (no fmap)
+// win32 (without fmap support)
+// win32 (with fmap support)
+
+const fs = require('fs')
+const hasFmap = !!fs.constants.UV_FS_O_FILEMAP
+const platform = process.platform
+
+switch (process.argv[2]) {
+  case 'win32-fmap': {
+    if (!hasFmap)
+      fs.constants.UV_FS_O_FILEMAP = 0x20000000
+    const { O_CREAT, O_TRUNC, O_WRONLY, UV_FS_O_FILEMAP } = fs.constants
+    if (platform !== 'win32')
+      process.env.__FAKE_PLATFORM__ = 'win32'
+    const getFlag = require('../lib/get-write-flag.js')
+    t.equal(getFlag(1), UV_FS_O_FILEMAP | O_TRUNC | O_CREAT | O_WRONLY)
+    t.equal(getFlag(512 * 1024 + 1), 'w')
+    break
+  }
+
+  case 'win32-nofmap': {
+    if (hasFmap)
+      fs.constants.UV_FS_O_FILEMAP = 0
+    if (platform !== 'win32')
+      process.env.__FAKE_PLATFORM__ = 'win32'
+    const getFlag = require('../lib/get-write-flag.js')
+    t.equal(getFlag(1), 'w')
+    t.equal(getFlag(512 * 1024 + 1), 'w')
+    break
+  }
+
+  case 'unix': {
+    if (platform === 'win32')
+      process.env.__FAKE_PLATFORM__ = 'darwin'
+    const getFlag = require('../lib/get-write-flag.js')
+    t.equal(getFlag(1), 'w')
+    t.equal(getFlag(512 * 1024 + 1), 'w')
+    break
+  }
+
+  default: {
+    const node = process.execPath
+    t.spawn(node, [__filename, 'win32-fmap'])
+    t.spawn(node, [__filename, 'win32-nofmap'])
+    t.spawn(node, [__filename, 'unix'])
+  }
+}