dart-sdk/pkg/sourcemap_testing/README.md

# Testing of source maps

Currently this package consists of two "frameworks":
1) stacktrace_helper.dart
2) stepping_helper.dart

It is intended to be a shared resource between DDC and dart2js.

## Stacktraces ("stacktrace_helper.dart")

TODO

## Debugging tests (step tests, "stepping_helper.dart")

This is supposed to work in a few steps:
1) Create the JS
2) Run the JS with D8, setting instructed breakpoints etc.
3) Translating JS positions to Dart positions
4) Validating the stepped positions.

In the above, the helper assumes
1) The dart file is called "test.dart" and is placed in the output directory before compiling to
"js.js" in the output dir.

And then performs via
2) `runD8AndStep`
3) `checkD8Steps`
4) (done in above step)

The test files themselves contain information about where to stop, which breakpoints to expect etc.
They contain this information in comments inlined in the code as in `/*key*/` where `key` can be one
of the below.

_Not context sensitive_

These comments can be anywhere in the file and their position does not matter.

* **fail**: Will fail the test. Useful for debugging in conjunction with debug being set to true
(see below).
* **Debugger:stepOver**: Will step over breakpoints. Default (i.e. without this) is to step into.

_Context sensitive_

These comments should be placed at the wanted position: The line and possibly column position of the
comment matters. They refer to the next non-whitespace position in the source.

* **bl** (break line): insert a breakpoint on this line. This does not add any new expected breaks.
* **s:{i}** (stop): adds an expected stop as the `i`th stop (1-indexed).
* **sl:{i}** (stop at line): adds an expected stop as the `i`th stop (1-indexed). Only check the
line number.
* **nb** (no break): The debugger should never break on this line.
* **nbc** (no break column): The debugger should never break on this line and column.
* **nbb:{i}:{j}** (no break between): The debugger should not break on this line between expectation
`i` and `j` (1-indexed). Note that `from` can also be the special value `0` meaning from the very
first stop. For example `nbb:0:1` means not before first expected stop.
* **nm** (no mapping): There's not allowed to be any mapping to this line.
* **bc:{i}** (break column): inserts a breakpoint at this line and column and adds an expected stop
as the `i`th stop (1-indexed).

Note that in an ideal world `bc:{i}` would not be unnecessary: Stopping at a line and stepping
should generally be enough. Because of the current behavior of d8 though, for instance
```
baz(foo(), bar())
```
will stop at `baz`, go into `foo`, stop at `bar`, go into `bar` and stop at `baz`.
From a Dart perspective we would instead expect it to stop at `foo`, go into `foo`, stop at `bar`,
go into `bar` and stop a `baz`.
Having **bc:{i}** allows us to force this behavior as d8 can actually stop at `foo` too.

All of these annotations are removed before compiling to js and the expected output thus refers to
the unannotated code.

When the test confirms that the debugger stopped at the expected locations it allows for additional
breakpoints before, between and after the expected breakpoints.

### Debugging a test

By calling `checkD8Steps` with the `debug` parameter set to `true` one can get information like this
dumped to standard out:

```
Stop #1

test.main = function() {                            |  main() {
  try {                                             |    try {
    let value = /*STOP*/"world";                    |      var value = /*STOP*/"world";
    dart.throw(dart.str`Hello, ${value}`);          |      // Comment
  } catch (e) {                                     |      throw "Hello, $value";

Stop #2

  try {                                             |      var value = "world";
    let value = "world";                            |      // Comment
    /*STOP*/dart.throw(dart.str`Hello, ${value}`);  |      /*STOP*/throw "Hello, $value";
  } catch (e) {                                     |    }
    let st = dart.stackTrace(e);                    |    // Comment

Stop #3

    dart.throw(dart.str`Hello, ${value}`);          |    }
  } catch (e) {                                     |    // Comment
    let st = /*STOP*/dart.stackTrace(e);            |    catch (e, /*STOP*/st) {
    {                                               |      print(e);
      core.print(e);                                |      print(st);

[...]
```

This can for instance be useful in combination with `/*fail*/` when adding new tests to see all the
places where the debugger stopped.

### Technical details

Some of the logic comes from https://github.com/ChromeDevTools/devtools-frontend/, for instance see
https://github.com/ChromeDevTools/devtools-frontend/blob/fa18d70a995f06cb73365b2e5b8ae974cf60bd3a/front_end/sources/JavaScriptSourceFrame.js#L1520-L1523
for how a line breakpoint is resolved:
Basically the line asked to break on in user code (e.g. in dart code) is asked for first and last
javascript positions; these are then used to get possible breakpoints in that part. If there are
none it tries the next line (etc for a number of lines). Once it finds something (in javascript
positions) it converts that to user code position (e.g. in dart code), normalizes it by converting
to javascript position and back to user code position again, then converts to javascript position
and sets the breakpoint.
This is to some extend mimicked here when setting a line break (though not a "column break").