Fix compressed source-maps have non-terminated segments (#342)

Currently when `terser` compresses a given input file that comes
with a source-map, but the source-map does not have mappings for
all code parts in the input file. e.g. the input file has been generated
and some specific code-parts have been generated which are not
originating from any source-file.

In that case if mappings before the "generated code" are collected, and
the original file location for the "generated code" comes after previous
segments for the same line, Terser just ignores the fact that there is no
original location and the previous segment is not terminated. This causes
the "generated code" incorrectly to be mapped to the previous segment/
original source location.

lib/sourcemap.js

@@ -74,12 +74,21 @@ function SourceMap(options) {
     }
 
     function add(source, gen_line, gen_col, orig_line, orig_col, name) {
+        var generatedPos = { line: gen_line + options.dest_line_diff, column: gen_col };
         if (orig_map) {
             var info = orig_map.originalPositionFor({
                 line: orig_line,
                 column: orig_col
             });
             if (info.source === null) {
+                if (generatedPos.column !== 0) {
+                    generator.addMapping({
+                        generated: generatedPos,
+                        original: null,
+                        source: null,
+                        name: null
+                    });
+                }
                 return;
             }
             source = info.source;
@@ -88,7 +97,7 @@ function SourceMap(options) {
             name = info.name || name;
         }
         generator.addMapping({
-            generated : { line: gen_line + options.dest_line_diff, column: gen_col },
+            generated : generatedPos,
             original  : { line: orig_line + options.orig_line_diff, column: orig_col },
             source    : source,
             name      : name

test/mocha/input-sourcemaps.js

@@ -1,6 +1,7 @@
 var Uglify = require('../../');
 var assert = require("assert");
 var SourceMapConsumer = require("source-map").SourceMapConsumer;
+var SourceMapGenerator = require("source-map").SourceMapGenerator;
 
 describe("input sourcemaps", function() {
     var transpilemap, map;
@@ -63,4 +64,47 @@ describe("input sourcemaps", function() {
             assert.ok(pos.line <= 2, msg);
         })
     });
+
+    it("Should preserve unmapped segments in output source map", function() {
+        var generator = new SourceMapGenerator();
+
+        generator.addMapping({
+            source: "source.ts",
+            generated: {line: 1, column: 0},
+            original: {line: 1, column: 0},
+        });
+
+        generator.addMapping({
+            source: null,
+            original: null,
+            generated: {line: 1, column: 38}
+        });
+
+        generator.addMapping({
+            source: "source.ts",
+            generated: {line: 1, column: 51},
+            original: {line: 2, column: 0},
+        });
+
+        // Everything except the "say('hello');" part is mapped to "source.ts". The "say"
+        // function call is not mapped to any original source location. e.g. this can
+        // happen when a code transformer inserts generated code in between existing code.
+        var inputFile = "function say(msg) {console.log(msg)};say('hello');process.exit(1);";
+        var result = Uglify.minify(inputFile, {
+            sourceMap: {
+                content: JSON.parse(generator.toString())
+            }
+        });
+
+        var transformedMap = new SourceMapConsumer(result.map);
+        var hasMappedSource = true;
+
+        for (let i = 0; i < result.code.length; i++) {
+            var info = transformedMap.originalPositionFor({line: 1, column: i});
+            hasMappedSource = hasMappedSource && !!info.source;
+        }
+
+        assert.equal(hasMappedSource, false, "Expected transformed source map to preserve the " +
+            "mapping without original source location");
+    });
 });