[lldb-dev] Host vs. HostInfo

[Greg Clayton]

As a final cleanup to these changes, can we get get rid of the preprocessor macro in the HostInfo::GetLLDBPath() and change over to using std::once, or switch to using a static HostInfo::Initialize()?

> On Aug 25, 2014, at 11:42 AM, Zachary Turner <zturner at google.com> wrote:
> 
> 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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