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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. 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