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Return llvm::Error and llvm::Expected from DWARF parsing code.

Description

Return llvm::Error and llvm::Expected from DWARF parsing code.

The goal here is to improve our error handling and error recovery while
parsing DWARF, while at the same time getting us closer to being able to
merge LLDB's DWARF parser with LLVM's. To this end, I've udpated several
of the low-level parsing functions in LLDB to return llvm::Error and
llvm::Expected.

For now, this only updates LLDB parsing functions and not LLVM. In some
ways, this actually gets us *farther* from parity with the two
interfaces, because prior to this patch, at least the parsing interfaces
were the same (i.e. they all just returned bools, and now with this
patch they're diverging). But, I chose to do this for two primary
reasons.

LLDB has error logging code engrained deep within some of its parsing
functions. We don't want to lose this logging information, but obviously
LLVM has no logging mechanism at all. So if we're to merge the
interfaces, we have to find a way to still allow LLDB to properly report
parsing errors while not having the reporting code be inside of LLVM.
LLDB (and indeed, LLVM) overload the meaning of the false return value
from all of these extraction functions to mean both "We reached the null
entry at the end of a list of items, therefore everything was
successful" as well as "something bad and unrecoverable happened during
parsing". So you would have a lot code that would do something like:
while (foo.extract(...)) {

...

}

But when the loop stops, why did it stop? Did it stop because it
finished parsing, or because there was an error? Because of this, in
some cases we don't always know whether it is ok to proceed, or how to
proceed, but we were doing it anyway.

In this patch, I solve the second problem by introducing an
enumeration called DWARFEnumState which has two values MoreItems and
Complete. Both of these indicate success, but the latter indicates
that we reached the null entry. Then, I return this value instead of
bool, and convey parsing failure separately.

To solve the first problem (and convey parsing failure) these
functions now return either llvm::Error or llvm::Expected<DWARFEnumState>.
Having this extra bit of information allows us to properly convey all 3 of
"error, bail out", "success, call this function again", and "success,
don't call this function again".

In subsequent patches I plan to extend this pattern to the rest of the
parsing interfaces, which will ultimately get all of the log statements
and error reporting out of the low level parsing code and into the high
level parsing code (e.g. SymbolFileDWARF, DWARFASTParserClang, etc).

Eventually, these same changes will have to be backported to LLVM's
DWARF parser, but diverging in the short term is the easiest way to
converge in the long term.

Differential Revision: https://reviews.llvm.org/D59370

Details

Committed
zturnerMar 14 2019, 12:05 PM
Differential Revision
D59370: Return llvm::Error and llvm::Expected from some DWARF parsing functions
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