Add 'clone' method to snapshot an instance

Cloned instances share a common input
Allows multiple output Streams to use a single input Stream

test/unit/clone.js

@@ -0,0 +1,58 @@
+'use strict';
+
+var fs = require('fs');
+var assert = require('assert');
+
+var sharp = require('../../index');
+var fixtures = require('../fixtures');
+
+sharp.cache(0);
+
+describe('Clone', function() {
+  it('Read from Stream and write to multiple Streams', function(done) {
+    var finishEventsExpected = 2;
+    // Output stream 1
+    var output1 = fixtures.path('output.multi-stream.1.jpg');
+    var writable1 = fs.createWriteStream(output1);
+    writable1.on('finish', function() {
+      sharp(output1).toBuffer(function(err, data, info) {
+        if (err) throw err;
+        assert.strictEqual(true, data.length > 0);
+        assert.strictEqual(data.length, info.size);
+        assert.strictEqual('jpeg', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fs.unlinkSync(output1);
+        finishEventsExpected--;
+        if (finishEventsExpected === 0) {
+          done();
+        }
+      });
+    });
+    // Output stream 2
+    var output2 = fixtures.path('output.multi-stream.2.jpg');
+    var writable2 = fs.createWriteStream(output2);
+    writable2.on('finish', function() {
+      sharp(output2).toBuffer(function(err, data, info) {
+        if (err) throw err;
+        assert.strictEqual(true, data.length > 0);
+        assert.strictEqual(data.length, info.size);
+        assert.strictEqual('jpeg', info.format);
+        assert.strictEqual(100, info.width);
+        assert.strictEqual(122, info.height);
+        fs.unlinkSync(output2);
+        finishEventsExpected--;
+        if (finishEventsExpected === 0) {
+          done();
+        }
+      });
+    });
+    // Create parent instance
+    var rotator = sharp().rotate(90);
+    // Cloned instances with differing dimensions
+    rotator.clone().resize(320, 240).pipe(writable1);
+    rotator.clone().resize(100).pipe(writable2);
+    // Go
+    fs.createReadStream(fixtures.inputJpg).pipe(rotator);
+  });
+});