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Refactoring in order to reduce Git's global state ================================================= PERSONAL INFORMATION -------------------- Name: Tian Yuchen E-mail: a3205153416@gmail.com Phone number: +65 98740318 Time-zone: UTC + 08:00 Github: https://github.com/malon7782 Education: NTU, Singapore Year: Year 1 semester 2 Degree: Electrical and Electronic Engineering (EEE) PRE GSOC -------- I have always held a deep passion for the open-source community. Although I wasn't a computer science major, I tinkered with open-source projects long before college. I have solid hands-on experience in C programming and system-level debugging. I use Ubuntu 24.04 on a daily basis, so I am proficient in using the Linux command line and CLI tools. I have contributed to the Git community by sending patches. Since my first commit (17/1/2026), I have maintained a nearly daily contribution. Here is the list of contributions I have made: * [PATCH v1] t1005: modernize "! test -f" to "test_path_is_missing" https://lore.kernel.org/git/20260117062515.319664-1-a3205153416@gmail.com/ This patch is my microproject, the first contribution I made to the codebase. [Graduated to 'master'] * [PATCH v2] t2203: avoid masking exit codes in git status https://lore.kernel.org/git/20260118043537.338769-1-a3205153416@gmail.com/#t * [PATCH v2] symlinks: use unsigned int for flags https://lore.kernel.org/git/20260120152219.398999-1-a3205153416@gmail.com/ [Will merge to 'next'] * [PATCH v4] t/perf/p3400: speed up setup using fast-import https://lore.kernel.org/git/20260130170123.642344-1-a3205153416@gmail.com/ [Will merge to 'master'] * Re: [PATCH] [RFC] attr: use local repository state in read_attr https://lore.kernel.org/git/cc2f400e-49c2-4de0-9c51-9a5c0294735e@gmail.com/ Code review. To verify the performance loss, I wrote a test script to measure the time difference before and after the modification. * Re: Bug: git add :!x . exits with error when x is in .gitignore https://lore.kernel.org/git/1d560aa1-d452-47f5-aaf2-4cb1ccdab100@gmail.com/ Code review. Pointed out logical error. * [PATCH v10] setup: allow cwd/.git to be a symlink to a directory https://lore.kernel.org/git/20260220164512.216901-1-a3205153416@gmail.com/ [Under review] After over half a month of discussions, repeated refactoring, and code reviews, I delved deep into setup.c. I gained insights into Git's design philosophy, and learned the art of striking a balance in developer communication. It took me a large amount of time and effort to thoroughly understand every line of the code. I often found myself poring over the call chain of a single function well into the night.... But I persevered until the end, and I believe my patience will see me through even larger projects. ABOUT THE PROJECT ----------------- -- Synopsis As far as I know, the Git community is actively working towards 'libification' - making Git's internal machinery reusable as a C library. The extensive reliance on global state is a major roadblock to this goal. Many core functions implicitly read environment variables and store them in global static variables. This can cause several issues: 1. Global variables prevent Git's core functions from being executed safely in multi-threaded contexts. For example, When unexpected states (e.g., a permission denied error when probing a directory), they often rely on the global state to decide whether to call die(), which internally calls exit(). It’s fine for a standalone CLI tool, but for a linked C library used by a long-running multi-threaded server, a single die() call will kill the entire host process. Structured status, instead of fatal exits, should be returned. 2. When Git is called multiple times within the same process, global states can lead to memory leaks or incorrect behaviors. 3. Unit testing becomes difficult because the environment must be artificially manipulated before calling functions. Take a look at this example from environment.c: 206 const char *get_commit_output_encoding(void) 207 { 208 return git_commit_encoding ? git_commit_encoding : "UTF-8"; 209 } If Git is invoked as a C library by a multi-threaded server: - Thread A formats a commit for Repo A (using GBK); - Thread B concurrently formats a commit for Repo B (using UTF-8); Then they will race to read and overwrite the exact same global `git_commit_encoding` pointer, which is not what we expect. Therefore, we have to refactor these environment variables by moving them from global scope into a well-defined and encapsulated context. -- Approach The task at hand goes beyond simply repackaging the global variables into the struct repository structure. Based on my recent experience refactoring setup.c, I realized that libification requires careful management of variable lifecycles and api boundaries: [ Current ] Core functions --------reads-------> Global variables (via getenv) [Thread unsafe] [ Target ] Core functions ----passes context--> struct repository | owns v struct git_env Although the principle is simple, the scope of changes is extensive. The following three-step approach can serve as a guiding principle for it: 1. Identify isolated environment variables currently residing in the global scope. Introduce a dedicated structure to hold these states, e.g. `struct git_env` within the `struct repository`. 2. Instead of blindly passing struct repository *repo down into every single low-level library function, bubbling the dependency up is the true goal. External callers of the functions must be carefully audited to prevent regressions. 3. Safely remove the old global variables and macro definitions. Tools such as AddressSanitizer can be helpful to ensure that the new struct-based lifecycle introduces zero memory leaks. 4. Many globals like are parsed once and remain available globally. New data flow might need to be designed to maintain the lazy- loading efficiency. Additionally, given the anticipated high volume of commits, we must ensure each patch is independent and atomic, preventing any user-untraceable or unexplainable bugs from occurring in the codebase at any state. AVAILABILITY ------------ Fortunately, my summer vacation coincides with the GSoC work period. I will treat this project as my primary focus, dedicating a minimum of 35 hours per week. If needed, I can work a 9-to-5 schedule. I will have a significant head start to draft RFC patches before the official coding period even begins. Having this buffer period allows me to go through the rigorous code review process within the Git community with greater ease. TIMELINE & MILESTONES --------------------- Considering the differences between this project and other projects on the idea list, rather than hoarding massive changes, I will submit 3-to-5-patch series frequently to respect reviewers' time and maintain a steady velocity. Below is the tentative schedule I have prepared for myself: * Community Bonding (May 1 - May 25): Planning & RFC - May 1 - May 7: Wrap up university finals. Discuss and finalize the prioritized list of subsystems with my mentor. - May 8 - May 25: Define the core context container. Draft and submit the initial RFC patch series for this new data structure. * Phase 1 (May 26 - July 10): Foundation - Weeks 1-2: Plumb the context pointer (`struct repository *repo`) through call chains for simple variables (e.g., boolean flags or integer configs). - Weeks 3-4: Audit and update external callers to use the new API. - Weeks 5-6: Submit the first major refactoring patch series. Address mailing list feedback and resolve merge conflicts. (Midterm Evaluation) * Phase 2 (July 11 - August 18): Complex Migration & Cleanup - Weeks 7-8: Refactor higher-complexity variables (e.g., path-related globals). - Weeks 9-10: Compile the codebase with AddressSanitizer and run the full test suite to execute strict memory leak checks. - Weeks 11-12: Remove unused global macro definitions and static variables. Update internal documentation and write the final GSoC report. (The above is for reference only. Personally, I always finish tasks faster than planned 😉) ~$ git checkout HEAD@{postGSoC} ------------------------------- This past month since joining the Git community has been the most enjoyable month of my programming journey. To quote a close friend of mine (who is applying for the Neovim GSoC project): "Only fools chase trends; open source is the game for the brave." The words may be blunt, but the logic holds true. This statement surely resonates with me (and maybe many other GSoC contributors): our passion for code and open-source drives us forward. Even if I didn't make the cut, so what? ~$ git reset --hard... Just kidding. The Git codebase is far too interesting to abandon now. ------------------------------------------------------------------------- Changes since V3: - Based on reviewing last year's contributors' changes and recent experience modifying setup.c, additional descriptions have been added to the synopsis & approach section.