Hi Philip
On 18/02/2024 07:33, Philip Peterson via GitGitGadget wrote:
Hello all, this is my first patchset so thank you for being patient with me
as I learn the process and conventions of your very fine project. These
patches are intended as part of the Libification effort, to show how we
could use a Promise structure to help return values from functions.
I agree that we could do with a better way of propagating errors ups the
call-chain that (a) allows us to pass more detailed information about
the error to the caller and (b) add useful context to the error message
as the stack in unwound. I'm afraid I do not think that the promise
implementation in this patch series is a good way forward for several
reasons.
1) It is hard to see how we can wrap the return value of the function in
a promise and preserve type safety. Even if we used some kind of union
the compiler will not warn us if the caller reads a different member to
the one that the callee set.
2) It obscures the return type of the function and forces callers to
unpack the value from the promise adding complexity to the calling code.
3) It imposes a cost in terms of dynamic memory allocation on code that
is called synchronously and therefore does not need to allocate the
promise on the heap. This adds complexity and is sure to result in
memory leaks.
4) If the function fails we need to propagate the error using
PROMISE_BUBBLE_UP() which forces the caller to add more context to the
error message even if it does covey anything useful to the user. For
example in patch 5 we see
error:
could not find header
caused by:
could not find file diff header
caused by:
git diff header lacks filename information (line 4)" >expected
The error message starts by saying it couldn't find the header and ends
by saying it did actually find the header but it could not parse it.
5) The cover letter talks about adding asynchronous examples in the
future but it is not clear what part of the git code base it is
referring to.
I think we'd be better served by some kind of structured error type like
the failure_result in this patch series that is allocated on the stack
by the caller at the entry point to the library and passed down the call
chain. That avoids the need for lots of dynamic allocations and allows
us to continue allocating "out" parameters on the stack. For example
int f(struct repository *r) {
struct object_id oid;
if (repo_get_oid(r, "HEAD", &oid))
return error(_("could not parse HEAD"))
/* use oid here */
}
would become
int f(struct repository *r, struct error *err) {
struct object_id oid;
if (repo_get_oid(r, "HEAD", &oid))
return error(&err, _("could not parse HEAD"))
/* use oid here */
}
I'm sure this has been discussed in the past but I didn't manage to turn
anything up with a quick search of the archive on lore.kernel.org.
Best Wishes
Phillip
Problems
========
We seek to make libification easier by establishing a pattern for tracking
whether a function errored in a rich way. Currently, any given function
could immediately die(), or use error() to print directly to the console,
bypassing any relevant verbosity checks. The use of die() currently makes
use of Git as a library inconvenient since it is not graceful.
Additionally, returning using return error(...) (as is commonly done) always
just returns a generic error value, -1, which provides little information.
Approach
========
I solve this problem by splitting the single return value into two return
values: error, and message. However, managing two output variables can
require some coordination, and this coordination can be abstracted away by
use of an existing pattern named Promise.
Promise Concept
===============
A promise is a contract representing "some task" that will eventually
complete. Initially a promise is considered in a pending state. When it
completes, one of two codepaths will eventually be entered: reject, or
resolve. Once resolved or rejected, the promise enters a different state
representing the result. Reject or resolve may only be called once on a
given promise.
Until now, everything I described up to this point is consistent with other
implementations, such as the ECMAScript standard for promises. However, this
implementation departs from the complexity of those promises. In this
implementation, promises are simple and canNOT be chained using .then(...)
and do NOT have any notion of automatic bubbling (via re-entering the
pending state).
Sample output and reproduction
==============================
During an error, we can have richer feedback as to what caused the problem.
% git apply garbage.patch
error:
could not find header
caused by:
patch fragment without header at line 1: @@ -2 +2 @@
To reproduce this output, you can use the following patch (garbage.patch):
@@ -2 +2 @@
Goals
=====
I would love to get feedback on this approach. This patchset is kept small,
so as to serve as a minimal proof of concept. It is intended to abstract to
asynchronous use-cases even though this is only a synchronous one.
Eventually, any top-level function, such as apply_all_patches(...) would
return its output via a promise to make the library interface as clean as
possible, but this patchset does not accomplish this goal. Hopefully it can
provide a direction to go in to achieve that.
Diversion
=========
While building this patchset, I noted a bug that may not have a concrete
repro case in the master branch. The bug is that when invoking git am, it
can call out to git apply, passing many flags but interestingly not the
--quiet flag. I included a fix for this issue in the patchset.
Potential Issue
===============
There is one difficulty with this approach, which is the high level of
repetition in the code required. Tracking which promise is which is its own
source of complexity and may make mistakes more prone to happen. If anyone
has suggestions for how to make the code cleaner, I would love to hear.
Thank you, Philip
Philip Peterson (5):
promise: add promise pattern to track success/error from operations
apply: use new promise structures in git-apply logic as a proving
ground
apply: update t4012 test suite
apply: pass through quiet flag to fix t4150
am: update test t4254 by adding the new error text
Makefile | 1 +
apply.c | 133 +++++++++++++++++++++++++++--------------
apply.h | 9 ++-
builtin/am.c | 5 ++
promise.c | 89 +++++++++++++++++++++++++++
promise.h | 71 ++++++++++++++++++++++
range-diff.c | 14 +++--
t/t4012-diff-binary.sh | 4 +-
t/t4254-am-corrupt.sh | 9 ++-
9 files changed, 279 insertions(+), 56 deletions(-)
create mode 100644 promise.c
create mode 100644 promise.h
base-commit: 2996f11c1d11ab68823f0939b6469dedc2b9ab90
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1666%2Fphilip-peterson%2Fpeterson%2Femail-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1666/philip-peterson/peterson/email-v1
Pull-Request: https://github.com/git/git/pull/1666
