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

Zachary Turner via lldb-dev lldb-dev at lists.llvm.org
Mon Mar 4 10:32:31 PST 2019

On Sat, Mar 2, 2019 at 2:56 PM Adrian Prantl <aprantl at apple.com> wrote:

> 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.
If anything I think parsing PDBs is more stable.  There is close to zero
flexibility in how types and symbols can be represented in PDB / CodeView,
and on top of that, there are very few producers.  Combined, this means we
can assume almost everything about the structure of the records.

Yes the crashes *happen* at the AST level (most of them anyway, not all -
there are definitely examples of crashing in the actual parsing code), but
the fact that there is so much flexibility in how records can be specified
in DWARF exacerbates the problem by complicating the parsing code, which is
then not well tested because of all the different code paths.

>    - 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.
Do you consider this something worth addressing by testing the debug info
ingestion in isolation?

> 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.
They really are that large.

> 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.
Let's ignore whether the format can be serialized and instead focus on the
abstraction level of the API.  Personally, I think the format should be
higher level than DWARF DIEs but lower level than an AST.  By making it
higher level than DWARF DIEs, we could use the same abstraction to
represent PDB types and symbols as well, and by making it lower level than
ASTs, we could support non-clang TypeSystems.  This way, you have one API
which gives you "something" that you can trust and works with any
underlying debug info format, and one codepath that builds the AST from it,
regardless of which Debug info format and programming language it describes.

In a way, this is like separating the DWARFASTParserClang / SymbolFileDWARF
and PDBASTBuilder / SymbolFileNativePDB, and instead have some library
called DebugInfoParser, and a single ASTParser class which says
DIParser->ParseTypes() and then builds an AST from it without knowing what
format it orignated from.

> 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.

So again, let's ignore "the wire" for the sake of this discussion.
SymbolFile does vend compiler types, but that doesn't mean we can't have a
single "master" SymbolFile implementation which a) calls into
DebugInfoParser (which need not be out of process), and then b) uses the
result of these library calls to construct an AST.
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