[lldb-dev] RFC: Moving debug info parsing out of process

Adrian Prantl via lldb-dev lldb-dev at lists.llvm.org
Sat Mar 2 14:56:31 PST 2019

> On Feb 25, 2019, at 10:21 AM, Zachary Turner via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> Hi all,
> We've got some internal efforts in progress, and one of those would benefit from debug info parsing being out of process (independently of whether or not the rest of LLDB is out of process).
> There's a couple of advantages to this, which I'll enumerate here:
> It improves one source of instability in LLDB which has been known to be problematic -- specifically, that debug info can be bad and handling this can often be difficult and bring down the entire debug session.  While other efforts have been made to address stability by moving things out of process, they have not been upstreamed, and even if they had I think we would still want this anyway, for reasons that follow.
Where do you draw the line between debug info and the in-process part of LLDB? I'm asking because I have never seen the mechanical parsing of DWARF to be a source of instability; most crashes in LLDB are when reconstructing Clang ASTs because we're breaking some subtle and badly enforced invariants in Clang's Sema. Perhaps parsing PDBs is less stable? If you do mean at the AST level then I agree with the sentiment that it is a common source of crashes, but I don't see a good way of moving that component out of process. Serializing ASTs or types in general is a hard problem, and I'd find the idea of inventing yet another serialization format for types that we would have to develop, test, and maintain quite scary.
> It becomes theoretically possible to move debug info parsing not just to another process, but to another machine entirely.  In a broader sense, this decouples the physical debug info location (and for that matter, representation) from the debugger host.
I can see how that can be useful in some settings. You'd need a really low latency network connection to make interactive debugging work but I expect you've got that covered :-)
> It becomes testable as an independent component, because you can just send requests to it and dump the results and see if they make sense.  Currently there is almost zero test coverage of this aspect of LLDB apart from what you can get after going through many levels of indirection via spinning up a full debug session and doing things that indirectly result in symbol queries.
You are right that the type system debug info ingestion and AST reconstruction is primarily tested end-to-end.

> The big win here, at least from my point of view, is the second one.  Traditional symbol servers operate by copying entire symbol files (DSYM, DWP, PDB) from some machine to the debugger host.  These can be very large -- we've seen 12+ GB in some cases -- which ranges from "slow bandwidth hog" to "complete non-starter" depending on the debugger host and network. 

12 GB sounds suspiciously large. Do you know how this breaks down between line table, types, and debug locations? If it's types, are you deduplicating them? For comparison, the debug info of LLDB (which contains two compilers and a debugger) compresses to under 500MB, but perhaps the binaries you are working with are really just that much larger.

> In this kind of scenario, one could theoretically run the debug info process on the same NAS, cloud, or whatever as the symbol server.  Then, rather than copying over an entire symbol file, it responds only to the query you issued -- if you asked for a type, it just returns a packet describing the type you requested.
> The API itself would be stateless (so that you could make queries for multiple targets in any order) as well as asynchronous (so that responses might arrive out of order).  Blocking could be implemented in LLDB, but having the server be asynchronous means multiple clients could connect to the same server instance.  This raises interesting possibilities.  For example, one can imagine thousands of developers connecting to an internal symbol server on the network and being able to debug remote processes or core dumps over slow network connections or on machines with very little storage (e.g. chromebooks).

You *could* just run LLDB remotely ;-)

That sounds all cool, but in my opinion you are leaving out the really important part: what is the abstraction level of the API going to be?

To be blunt, I'm against inventing yet another serialization format for *types* not just because of the considerable engineering effort it will take to get this right, but also because of the maintenance burden it would impose. We already have to support loading types from DWARF, PDB, Clang modules, the Objective-C runtime, Swift modules, and probably more sources, all of these operate to some degree at different levels of abstraction. Adding another source or abstraction layer into the mix needs to be really well thought out and justified.

> On the LLDB side, all of this is hidden behind the SymbolFile interface, so most of LLDB doesn't have to change at all.   While this is in development, we could have SymbolFileRemote and keep the existing local codepath the default, until such time that it's robust and complete enough that we can switch the default.

The SymbolFile interface ultimately vends compiler types so now I'm really curious what kind of data you are planning to send over the wire.

thanks for sharing,

> Thoughts?
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