Handle "simple" (missing origin) source mappings in minifier output (#…

…928)

Summary:
Pull Request resolved: #928

Fixes #927

Terser, unlike the previous default minifier UglifyES, produces source maps whose entries may contain "simple mappings" - that is entries with a generated line+col but no original location.

When Metro processes minified output ahead of serialization, it doesn't correctly handle these entries. Instead it incorrectly detects them as complete mappings and produces a merged source map with `-1` origin line numbers in place of simple mappings.

Some tools will reject the whole source map (which is otherwise correct) based on the appearance of a negative line number.

The issue comes down to the conversion of raw mappings to `BabelSourceMapSegments` in `toBabelSegments`, which currently generates entries for simple mappings with `null` `origin` *properties*, such as:

```
{
  generated: {
    column: 2,
    line: 1
  },
  origin: {
    column: null,
    line: null
  },
  // ...
}
```

According to the Flow type for `BabelSourceMapSegments`, that's incorrect. `origin` itself is optional and should be missing - the properties of `origin` are not nullable.

Based on the presence of the `origin` property in that object, `toSegmentTuple` generates a 4-tuple and we go on to add a "source mapping" rather than a "simple mapping".

This fixes `toBabelSegments` to omit `origin` unless `line` and `column` are both populated. The final source map output is then correct.

Changelog: [Fix] Source maps may have invalid entries when using Terser minification.

Reviewed By: motiz88

Differential Revision: D43351987

fbshipit-source-id: 5cd46fdd001f3c3a3599ada44f525bec467854e1

packages/metro-source-map/src/__tests__/__snapshots__/source-map-test.js.snap

@@ -8,10 +8,6 @@ Array [
       "line": 1,
     },
     "name": null,
-    "original": Object {
-      "column": null,
-      "line": null,
-    },
     "source": null,
   },
   Object {
@@ -56,10 +52,6 @@ Array [
       "line": 12,
     },
     "name": null,
-    "original": Object {
-      "column": null,
-      "line": null,
-    },
     "source": null,
   },
   Object {
@@ -80,10 +72,6 @@ Array [
       "line": 25,
     },
     "name": null,
-    "original": Object {
-      "column": null,
-      "line": null,
-    },
     "source": null,
   },
   Object {

packages/metro-source-map/src/source-map.js

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @flow
+ * @flow strict-local
  * @format
  * @oncall react_native
  */
@@ -21,6 +21,7 @@ const Consumer = require('./Consumer');
 const normalizeSourcePath = require('./Consumer/normalizeSourcePath');
 const {generateFunctionMap} = require('./generateFunctionMap');
 const Generator = require('./Generator');
+// $FlowFixMe[untyped-import] - source-map
 const SourceMap = require('source-map');
 
 export type {IConsumer};
@@ -85,6 +86,15 @@ export type IndexMap = {
 
 export type MixedSourceMap = IndexMap | BasicSourceMap;
 
+type SourceMapConsumerMapping = {
+  generatedLine: number,
+  generatedColumn: number,
+  originalLine: ?number,
+  originalColumn: ?number,
+  source: ?string,
+  name: ?string,
+};
+
 function fromRawMappingsImpl(
   isBlocking: boolean,
   onDone: Generator => void,
@@ -205,20 +215,33 @@ function toBabelSegments(
 ): Array<BabelSourceMapSegment> {
   const rawMappings: Array<BabelSourceMapSegment> = [];
 
-  new SourceMap.SourceMapConsumer(sourceMap).eachMapping(map => {
-    rawMappings.push({
-      generated: {
-        line: map.generatedLine,
-        column: map.generatedColumn,
-      },
-      original: {
-        line: map.originalLine,
-        column: map.originalColumn,
-      },
-      source: map.source,
-      name: map.name,
-    });
-  });
+  new SourceMap.SourceMapConsumer(sourceMap).eachMapping(
+    (map: SourceMapConsumerMapping) => {
+      rawMappings.push(
+        map.originalLine == null || map.originalColumn == null
+          ? {
+              generated: {
+                line: map.generatedLine,
+                column: map.generatedColumn,
+              },
+              source: map.source,
+              name: map.name,
+            }
+          : {
+              generated: {
+                line: map.generatedLine,
+                column: map.generatedColumn,
+              },
+              original: {
+                line: map.originalLine,
+                column: map.originalColumn,
+              },
+              source: map.source,
+              name: map.name,
+            },
+      );
+    },
+  );
 
   return rawMappings;
 }
@@ -274,11 +297,13 @@ function addMapping(
   if (n === 2) {
     generator.addSimpleMapping(line, column);
   } else if (n === 4) {
-    const sourceMap: SourceMapping = (mapping: any);
+    // $FlowIssue[invalid-tuple-arity] Arity is ensured by conidition on length
+    const sourceMap: SourceMapping = mapping;
 
     generator.addSourceMapping(line, column, sourceMap[2], sourceMap[3]);
   } else if (n === 5) {
-    const sourceMap: SourceMappingWithName = (mapping: any);
+    // $FlowIssue[invalid-tuple-arity] Arity is ensured by conidition on length
+    const sourceMap: SourceMappingWithName = mapping;
 
     generator.addNamedSourceMapping(
       line,