Wiring up existing source maps of transpiled codes

When `generateSourceMaps` option is enabled, r.js now detects if the
module to be concatenated already declares a source map. It tries to
load the existing source map, and translates line numbers of all
mappings in that source map to those of the final output.

This fixes most of requirejs#470, and works well as long as `optimize` option is
set to `none`.  Some work still remains to pass a correct
`--in-source-map` option to UglifyJS when also uglifying.  Currently,
UglifyJS ignores r.js's carefully generated source map producing a bogus
one if `optimize` option is set to `uglify2`.

Used https://github.com/lydell/source-map-url (v0.2.0) for detecting
sourceMappingURL= comments from the code.

build/jslib/build.js

@@ -24,6 +24,8 @@ define(function (require) {
         env = require('env'),
         commonJs = require('commonJs'),
         SourceMapGenerator = require('source-map/source-map-generator'),
+        SourceMapConsumer = require('source-map/source-map-consumer'),
+        sourceMappingURL = require('source-map-url'),
         hasProp = lang.hasProp,
         getOwn = lang.getOwn,
         falseProp = lang.falseProp,
@@ -1910,27 +1912,67 @@ define(function (require) {
                                 }
                             }
 
+                            //See if the file already has a source map
+                            var sourceMapConsumer = null;
+                            try {
+                                var existingSourceMapURL = sourceMappingURL.get(singleContents);
+                                if (existingSourceMapURL) {
+                                    //Load referenced source map
+                                    var sourceMapContents = file.readFile(build.makeAbsPath(existingSourceMapURL, file.parent(path)));
+                                    var existingSourceMap = JSON.parse(String(sourceMapContents));
+                                    sourceMapConsumer = existingSourceMap ? new SourceMapConsumer.SourceMapConsumer(existingSourceMap) : null;
+                                }
+                            } catch (e) {
+                                sourceMapConsumer = null;
+                            }
+                            //Remove the sourceMappingURL comment, so it doesn't
+                            //interfere with the new one to be generated.
+                            singleContents = sourceMappingURL.remove(singleContents);
+
                             sourceMapLineNumber = fileContents.split('\n').length - 1;
                             lineCount = singleContents.split('\n').length;
-                            for (var i = 1; i <= lineCount; i += 1) {
-                                sourceMapGenerator.addMapping({
-                                    generated: {
-                                        line: sourceMapLineNumber + i,
-                                        column: 0
-                                    },
-                                    original: {
-                                        line: i,
-                                        column: 0
-                                    },
-                                    source: sourceMapPath
+                            if (sourceMapConsumer) {
+                                sourceMapConsumer.eachMapping(function(m) {
+                                    if (m.generatedLine > lineCount)
+                                        return;
+                                    //Translate the line numbers in the existing source map
+                                    sourceMapGenerator.addMapping({
+                                        generated: {
+                                            line: m.generatedLine + sourceMapLineNumber,
+                                            column: m.generatedColumn
+                                        },
+                                        original: {
+                                            line: m.originalLine,
+                                            column: m.originalColumn
+                                        },
+                                        source: m.source,
+                                        name: m.name
+                                    });
                                 });
-                            }
 
-                            //Store the content of the original in the source
-                            //map since other transforms later like minification
-                            //can mess up translating back to the original
-                            //source.
-                            sourceMapGenerator.setSourceContent(sourceMapPath, singleContents);
+                                // TODO Preserve sourcesContent of existing source map
+                                // TODO by calling sourceMapGenerator.setSourceContent() for each non-null item in sourceMapConsumer.sourcesContent
+                            } else {
+                                for (var i = 1; i <= lineCount; i += 1) {
+                                    sourceMapGenerator.addMapping({
+                                        generated: {
+                                            line: sourceMapLineNumber + i,
+                                            column: 0
+                                        },
+                                        original: {
+                                            line: i,
+                                            column: 0
+                                        },
+                                        source: sourceMapPath
+                                    });
+                                }
+
+                                //Store the content of the original in the source
+                                //map since other transforms later like minification
+                                //can mess up translating back to the original
+                                //source.
+                                sourceMapGenerator.setSourceContent(sourceMapPath, singleContents);
+                            }
                         }
 
                         //Add the file to the final contents

build/jslib/source-map-url.js

@@ -0,0 +1,78 @@
+// Copyright 2014 Simon Lydell
+
+void (function(root, factory) {
+  if (typeof define === "function" && define.amd) {
+    define(factory)
+  } else if (typeof exports === "object") {
+    module.exports = factory()
+  } else {
+    root.sourceMappingURL = factory()
+  }
+}(this, function(undefined) {
+
+  var innerRegex = /[#@] sourceMappingURL=([^\s'"]*)/
+  var newlineRegex = /\r\n?|\n/
+
+  var regex = RegExp(
+    "(^|(?:" + newlineRegex.source + "))" +
+    "(?:" +
+      "/\\*" +
+      "(?:\\s*(?:" + newlineRegex.source + ")(?://)?)?" +
+      "(?:" + innerRegex.source + ")" +
+      "\\s*" +
+      "\\*/" +
+      "|" +
+      "//(?:" + innerRegex.source + ")" +
+    ")" +
+    "\\s*(?:$|(?:" + newlineRegex.source + "))"
+  )
+
+  function SourceMappingURL(commentSyntax) {
+    this._commentSyntax = commentSyntax
+  }
+
+  SourceMappingURL.prototype.regex = regex
+  SourceMappingURL.prototype._innerRegex = innerRegex
+  SourceMappingURL.prototype._newlineRegex = newlineRegex
+
+  SourceMappingURL.prototype.get = function(code) {
+    var match = code.match(this.regex)
+    if (!match) {
+      return null
+    }
+    return match[2] || match[3] || ""
+  }
+
+  SourceMappingURL.prototype.set = function(code, url, commentSyntax) {
+    if (!commentSyntax) {
+      commentSyntax = this._commentSyntax
+    }
+    // Use a newline present in the code, or fall back to '\n'.
+    var newline = String(code.match(this._newlineRegex) || "\n")
+    var open = commentSyntax[0], close = commentSyntax[1] || ""
+    code = this.remove(code)
+    return code + newline + open + "# sourceMappingURL=" + url + close
+  }
+
+  SourceMappingURL.prototype.remove = function(code) {
+    return code.replace(this.regex, "")
+  }
+
+  SourceMappingURL.prototype.insertBefore = function(code, string) {
+    var match = code.match(this.regex)
+    if (match) {
+      var hasNewline = Boolean(match[1])
+      return code.slice(0, match.index) +
+        string +
+        (hasNewline ? "" : "\n") +
+        code.slice(match.index)
+    } else {
+      return code + string
+    }
+  }
+
+  SourceMappingURL.prototype.SourceMappingURL = SourceMappingURL
+
+  return new SourceMappingURL(["/*", " */"])
+
+}));

dist.js

@@ -57,6 +57,7 @@ var fs = require('fs'),
         'build/jslib/source-map/source-node.js',
         'build/jslib/source-map/util.js',
         'build/jslib/source-map.js',
+        'build/jslib/source-map-url.js',
         'build/jslib/uglifyjs2.js',
         'build/jslib/parse.js',
         'build/jslib/transform.js',