Support for reading byte ranges from files using fs.createReadStream.

doc/api.markdown

@@ -1583,6 +1583,14 @@ Returns a new ReadStream object (See `Readable Stream`).
     , 'bufferSize': 4 * 1024
     }
 
+`options` can include `start` and `end` values to read a range of bytes from
+the file instead of the entire file.  Both `start` and `end` are inclusive and
+start at 0.  When used, both the limits must be specified always.
+
+An example to read the last 10 bytes of a file which is 100 bytes long:
+
+    fs.createReadStream('sample.txt', {start: 90, end: 99});
+
 
 ## fs.WriteStream
 

lib/fs.js

@@ -615,6 +615,16 @@ var ReadStream = fs.ReadStream = function(path, options) {
     this[key] = options[key];
   }
 
+  if(this.start || this.end) {
+    if(this.start === undefined || this.end === undefined) {
+      self.emit('error', new Error('Both start and end are needed for range streaming.'));
+    } else if(this.start > this.end) {
+      self.emit('error', new Error('start must be <= end'));
+    } else {
+      this.firstRead = true;
+    }
+  }
+
   if (this.fd !== null) {
     return;
   }
@@ -653,6 +663,11 @@ ReadStream.prototype._read = function () {
     allocNewPool();
   }
 
+  if(this.start && this.firstRead) {
+    this.pos = this.start;
+    this.firstRead = false;
+  }
+
   // Grab another reference to the pool in the case that while we're in the
   // thread pool another read() finishes up the pool, and allocates a new
   // one.
@@ -660,6 +675,10 @@ ReadStream.prototype._read = function () {
   var toRead = Math.min(pool.length - pool.used, this.bufferSize);
   var start = pool.used;
 
+  if(this.pos) {
+    toRead = Math.min(this.end - this.pos + 1, toRead);
+  }
+
   function afterRead (err, bytesRead) {
     if (err) {
       self.emit('error', err);
@@ -692,7 +711,11 @@ ReadStream.prototype._read = function () {
     self._read();
   }
 
-  fs.read(self.fd, pool, pool.used, toRead, undefined, afterRead);
+  fs.read(self.fd, pool, pool.used, toRead, this.pos, afterRead);
+
+  if(self.pos) {
+    self.pos += toRead;
+  }
   pool.used += toRead;
 };
 

test/simple/test-fs-read-stream.js

@@ -9,6 +9,7 @@ Buffer = require('buffer').Buffer;
 path = require('path');
 fs = require('fs');
 fn = path.join(common.fixturesDir, 'elipses.txt');
+rangeFile = path.join(common.fixturesDir, 'x.txt');
 
 callbacks = { open: 0, end: 0, close: 0, destroy: 0 };
 
@@ -86,3 +87,12 @@ process.addListener('exit', function() {
   assert.equal(30000, file.length);
   assert.equal(10000, file3.length);
 });
+
+var file4 = fs.createReadStream(rangeFile, {start: 1, end: 2});
+var contentRead = '';
+file4.addListener('data', function(data) {
+	contentRead += data.toString('utf-8');
+});
+file4.addListener('end', function(data) {
+	assert.equal(contentRead, 'yz');
+});