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The `meteor debug` command behaves like Node's `--inspect-brk` flag, in
that it attempts to pause the server before executing any server code.
However, simply passing the `--inspect-brk` flag to Node causes execution
to pause on the very first line of code, which is not good for setting any
breakpoints, because no server code has actually loaded yet.
Instead, the `meteor debug` command uses Node's `--inspect` flag to enable
debugging without an initial pause, then manually pauses at an appropriate
moment during server startup. Ideally, the pause should last until an
inspector client has been attached to the process, at which point the
developer has a chance to set any desired breakpoints, then clicks the
continue button to proceed with server startup.
The most difficult part of this process is detecting when the inspector
client has attached. Previously, the parent process listened for the child
process to print a "Debugger attached" message to STDERR, which happens as
a result of this `fprintf` call in Native C++:
7cff6e80bf/src/inspector_io.cc (L396)
However, this message was not printed in some cases, especially on Windows
(#9165), and required inter-process communication even in the ideal case.
All of that logic is gone now, thanks to this commit.
This commit takes advantage of a difference in behavior of the `debugger`
keyword depending on whether or not an inspector client is attached. When
no client is attached, the `debugger` keyword is a no-op that takes no
time (or very little time) to execute. Once a client has attached, the
`debugger` keyword triggers a breakpoint that lasts until the developer
explicitly continues execution through the client UI. Needless to say,
this makes the `debugger` keyword take longer than a no-op.
Because the `debugger` keyword does nothing until a client connects, we
can safely poll a `pause` function containing a `debugger` keyword at a
frequent interval (say, every 500ms). Once a client connects, the
`debugger` keyword will become active, pausing the server at exactly the
point we hoped. The difference is easy to detect by timing the `pause()`
function call. Once the `debugger` keyword becomes active, we stop polling
and allow server startup to continue.
Elegant!
Fixes #9165.
Runners
The Meteor tool process would run multiple big "parts" that should be managed: started, stopped, restarted, monitored for crashes, etc.
The interface that Meteor tool uses is called a "runner".
Some of the runners run sub-processes. The main runner is called run-all, it
has sub-runners such as run-app, run-mongo and run-proxy. Other runners
are used for different CLI commands.
Run Log
The output of the runners (compilation errors, state change, etc), should be reported through the Run Log. Run Log is also something that users see in their browsers, when their app breaks.