initial commit (tested working)

README.md

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+safe-replace
+============
+
+A micro-module for safely replacing a file.
+
+Commandline Reference
+---------------------
+
+If I want to safely replace a file with a new version, I would do so like this:
+
+```bash
+# create the new version
+touch keep.txt.new
+
+# remove the previous backup
+rm -f keep.txt.bak
+
+# move the current version to the backup
+mv keep.txt keep.txt.bak
+
+# move the new version to the current
+mv keep.txt.new keep.txt
+```
+
+If `keep.txt` became corrupt and I wanted to use the backup,
+I would do this:
+
+```bash
+# copy the backup to the new version
+rsync keep.txt.bak keep.txt
+```
+
+In Node
+-------
+
+I ported that proccess to node.
+
+```js
+// default behavior is to concat (filename + '.' + 'new')
+var safeReplace = require('safe-replace').create({ new: 'new', bak: 'bak' });
+
+var data = new Buffer('A priceless document');
+safeReplace.writeFile('keep.txt', data, 'ascii').then(function () {
+  fs.readdir('.', function (nodes) {
+    console.log('file system nodes', nodes);
+    // keep.txt
+    // keep.txt.bak
+  });
+});
+
+// let's say I wrote keep.txt.new with my own mechanism
+safeReplace.commit('keep.txt').then(function () {
+  fs.readdir('.', function (nodes) {
+    console.log('file system nodes', nodes);
+    // keep.txt
+    // keep.txt.bak
+  });
+});
+
+// let's say I want to revert the file from the '.bak'
+safeReplace.revert('keep.txt').then(function () {
+  fs.readdir('.', function (nodes) {
+    console.log('file system nodes', nodes);
+    // keep.txt
+    // keep.txt.bak
+  });
+});
+```

index.js

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+'use strict';
+
+var PromiseA = require('bluebird').Promise;
+var fs = PromiseA.promisifyAll(require('fs'));
+
+function noop() {
+}
+
+function create(options) {
+  if (!options) {
+    options = {};
+  }
+  if (!options.new) {
+    options.new = 'new';
+  }
+  if (!options.bak) {
+    options.bak = 'bak';
+  }
+  if (options.new === options.bak) {
+    throw new Error("'new' and 'bak' suffixes cannot be the same... duh");
+  }
+
+  var newnamefn = options.newnamefn || function (pathname) {
+    return pathname + '.' + options.new;
+  };
+  var baknamefn = options.baknamefn || function (pathname) {
+    return pathname + '.' + options.bak;
+  };
+  var namefn = options.namefn || function (pathname) {
+    return pathname;
+  };
+
+  var sfs = {
+    writeFile: function (filename, data, options) {
+      //console.log(newnamefn(filename));
+      return fs.writeFileAsync(newnamefn(filename), data, options).then(function () {
+        //console.log(namefn(filename));
+        return sfs.commit(namefn(filename));
+      });
+    }
+  , commit: function (filename) {
+      // this may not exist
+      return fs.unlinkAsync(baknamefn(filename)).then(noop, noop).then(function () {
+        // this may not exist
+        //console.log(namefn(filename), '->', baknamefn(filename));
+        return fs.renameAsync(namefn(filename), baknamefn(filename)).then(function () {
+          //console.log('created bak');
+        }, noop);
+      }).then(function () {
+        // this must be successful
+        //console.log(newnamefn(filename), '->', namefn(filename));
+        return fs.renameAsync(newnamefn(filename), namefn(filename)).then(noop, function (err) {
+          //console.error(err);
+          return sfs.revert(filename).then(function () {
+            return PromiseA.reject(err);
+          });
+        });
+      });
+    }
+  , revert: function (filename) {
+      return new PromiseA(function (resolve, reject) {
+        var reader = fs.createReadStream(baknamefn(filename));
+        var writer = fs.createWriteStream(namefn(filename));
+
+        reader.on('error', reject);
+        writer.on('error', reject);
+
+        reader.pipe(writer);
+        writer.on('close', resolve);
+      });
+    }
+  };
+
+  return sfs;
+}
+
+module.exports.create = create;

test.js

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+'use strict';
+
+var safeReplace = require('./').create();
+//var safeReplace = require('safe-replace').create();
+var fs = require('fs');
+
+safeReplace.writeFile('keep.txt', 'my precious').then(function () {
+  fs.readdir('.', function (nodes) {
+    console.log('file system nodes', nodes);
+  });
+});