[lldb-dev] Host vs. HostInfo

[Zachary Turner]

I've had some questions (both privately and in responses to other messages
on-list) about Host and HostInfo.  So I'll explain here in hopes of
answering for everyone.

First, the rationale: Host was getting too big and was starting to turn
into something like "well, I need to call a platform-specific API, I'll
make it a static method in Host.cpp".  As the platform matrix grows, so
does the complexity of managing this file.  Second, there was no attempt in
Host.cpp to group logically similar methods together in classes.  There
were filesystem methods, process spawning methods, thread manipulation
methods, methods to query the value of various os magic numbers, etc.  As I
started thinking about what will need to happen to support various things
on Windows, I imagined this file exploding in complexity.  Even with
HostWindows.cpp, you can see from looking at Host.cpp that it's not always
possible or easy to separate platform-specific logic into the platform
specific Host files.

So this refactor attempts to address all of these issues.

So far I've been focused on (and mostly completed) moving code from Host
into two different classes:  Filesystem and HostInfo.

HostInfo - answers queries about the operating system that LLDB is running
on.  Think of this class as being "const".  It doesn't modify your OS.  If
you want to know how much memory is available, or the page size, or the
path to lldb.exe, you ask HostInfo.  Instead of #include "Host.h" and
writing Host::method(), you #include "HostInfo.h" and write
HostInfo::method().  When adding new methods, put your method in the
least-derived class possible where it makes sense and will compile on all
corresponding platforms.

The advantage to this approach is that
a) No matter what host OS you're on, you always have all the functionality
of that host OS available to you through static binding (e.g. no casting to
a derived type)
b) Almost zero pre-processor complexity

FileSystem - Has methods like MakeDirectory, RemoveDirectory,
GetPermissions, etc.  Where before you would #include "Host.h" and write
Host::MakeDirectory(), now you #include "FileSystem.h" and write
FileSystem::MakeDirectory(...).

Remaining work to be done:
1) Nuke DynamicLibrary and use LLVM's
2) Make a HostProcess instantiatable, non-static class, which represents a
process which is running on the Host OS.  Move code from Host.cpp over
there.
3) Make a HostThread instantiatable, non-static class, which represents a
thread inside of a process on the Host OS.  Move code from Host.cpp over
there.
4) Make a HostProcessLauncher class, of which derived implementations would
be WindowsProcessLauncher, PosixSpawnProcessLauncher, XpcProcessLauncher,
etc.  Move code from Host.cpp over there.
5) Update Process plugins to use the appropriate HostProcessLauncher classes
6) Delete Host.cpp, as there will be no code left in it anymore.
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