Some internal docs on buildmessage

tools/tools-docs/buildmessage.md

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+# Build Message
+
+We have a build process that does a lot of nested things. The build system
+should report errors in the process to the user: compilation problems, incorrect
+packages, etc.
+
+We could have used exceptions but not only it would be hard to pass exceptions
+around, it also wouldn't be correct because users making mistakes is part of the
+expected use.
+
+Also BuildMessage knows how to report errors better, recover on errors to
+collect as many errors in one pass as possible. For example, if several files
+fail to be parsed, we want to report all of them.
+
+try/catch usually marks a code call and says "everything called from this try
+block, dynamically, is handled by this catch block". Build Message also has such
+property.
+
+`buildmessage.capture` - takes a function and runs it in a buildmessage
+"context" and returns a message set (a set of errors). When the code is running
+inside that "capture", the errors from it go to this message set.
+
+You can check if there were any errors by checking `messages.hasMessages()`.
+
+There are different applications of `capture`, a lot of parts of the code wrap
+it. There is `captureAndExit` that exits immediately after printing. In catalog
+we have a wrapper that tries to run a function, if it has messages, refreshes
+the catalog and tries again.
+
+In the lower-level code you usually never print the messages, you pass them up
+along the stack to be handled by something else (printing by cli or displaying
+by gui, etc).
+
+`capture` is not useful without jobs. You use `buildmessage.enterJob` to create
+a new job. It has a title.
+
+`buildmessage.error` is the function you use to say "something bad happened".
+The innermost job will provide more information to the error.
+Ex.: "While building package X: an error happened"
+
+It should always be called inside a job. It will throw an exception if it is not
+inside, but you should not do this.
+
+There is also a `buildmessage.exception`, the idea is, you use it to report the
+actual exceptions. Usually you use it to rethrow exceptions from a user package
+or a build plugin. It has some fancy stack-trace parsing and formatting.
+Ex.: "An exception while running your package.js file: TypeError ...".
+
+Another function that you would run a lot is `job.hasMessages()`. Unlike
+exceptions, when you notify the buildmessage of errors, it doesn't stop the
+execution, it keeps going. This gives you an opportunity to recover and go on to
+get more errors. But if you don't want to keep going, you can check if there are
+any errors in the current job by calling `job.hasMessages()`. This call also
+includes messages from nested jobs.
+
+Initially buildmessage was used only for error reporting, but now it is also
+used for the progress bars. It doesn't hurt the error reporting, it gives the
+error messages better locality (assuming it is useful).
+
+## Quick recap:
+
+- `buildmessage.capture` - the same as a `try-catch` block
+- `buildmessage.error` - is like throwing an exception (except it doesn't stop there)
+- `buildmessage.enterJob` - is a concept similar to a stack-frame
+- `capture.messages()` - get all errors
+- `job.hasMessages()` - check if there are any errors so far
+
+
+## Misc
+
+Helpers for asserting the code structure:
+
+- `assertInJob`
+- `assertInCapture`
+
+A helper to run the user code:
+
+- `files.runJavascript` - a helper to run the user code
+- `buildmessage.markBoundary` - a helper that marks the entry point of the user
+  code (like a build plugin), later when an `buildmessage.exception` is called,
+  the buildmessage will remove unnecessary stack-frames from the stack-trace so
+  the error displayed to the user doesn't contain any frames from tool.
+
+Ex.:
+
+```
+var result = null;
+f = function () { user provided code };
+try {
+  var markedF = buildmessage.markBoundary(f);
+  result = markedF();
+} catch (e) {
+  buildmessage.exception(e);
+  // clean up
+  ...
+  // pretend nothing happened so we can recover and move on to find other errors
+  result = { name: "dummy" };
+}
+```
+