Change all null checking expressions to use 'is null' and 'is not null'
instead of '== null' and '!= null'.
This was mostly done with sed, so code can probably be improved in many
other ways with manual inspection, but that will come over time.
Also add code to the autoformat script to automatically fix these issues in the future.
Also limit the output from the native compiler, so that we don't overload the
IDEs with output if the native compiler produces tens of thousands of errors.
Fixes https://github.com/xamarin/xamarin-macios/issues/6526.
Also change output to use the full path to files as nodes, instead of just the
filename, and instead use a label to set what's shown to just the filename.
This makes the graph correct when we have multiple files with the same name,
but different paths.
The previous build system kept a forward-pointing single linked list of tasks
to execute: task X had a list of subsequent tasks to execute. If task X was
up-to-date, it was not created (and the next tasks were directly added to the
list of tasks to execute).
In this world it became complicated to merge output from tasks (for instance
if the output of task X and task Y should be a consumed by a single task
producing a single output, since the corresponding task would end up in both
X's and Y's list of subsequent tasks).
Example: creating a single framework from the aot-compiled output of multiple
assemblies.
So I've reversed the logic: now we keep track of the final output, and then
each task has a list of dependencies that must be built.
This makes it trivial to create merging tasks (for the previous example, there
could for instance be a CreateFrameworkTask, where its dependencies would be
all the corresponding AotTasks).
We also always create every task, and then each task decides when its executed
whether it should do anything or not. This makes it unnecessary to 'forward-
delete' files when creating tasks (say you have three tasks, A, B, C; B
depends on A, and C depends on B; if A's output isn't up-to-date, it has to
delete its own output if it exists, otherwise B would not detect that it would
have to re-execute, because at task *creation* time, B's input hadn't
changed).
Additionally make it based on async/await, since much of the work happens in
externel processes (and we don't need to spin up additional threads just to
run external processes). This makes us have less code run on background
threads, which makes any issues with thread-safety less likely.
Rolf Bjarne Kvinge
Waleed Chaudhry
Chris Hamons