From: Derrick Stolee <dstolee@xxxxxxxxxxxxx> When working with very large repositories, an incremental 'git fetch' command can download a large amount of data. If there are many other users pushing to a common repo, then this data can rival the initial pack-file size of a 'git clone' of a medium-size repo. Users may want to keep the data on their local repos as close as possible to the data on the remote repos by fetching periodically in the background. This can break up a large daily fetch into several smaller hourly fetches. The task is called "prefetch" because it is work done in advance of a foreground fetch to make that 'git fetch' command much faster. However, if we simply ran 'git fetch <remote>' in the background, then the user running a foregroudn 'git fetch <remote>' would lose some important feedback when a new branch appears or an existing branch updates. This is especially true if a remote branch is force-updated and this isn't noticed by the user because it occurred in the background. Further, the functionality of 'git push --force-with-lease' becomes suspect. When running 'git fetch <remote> <options>' in the background, use the following options for careful updating: 1. --no-tags prevents getting a new tag when a user wants to see the new tags appear in their foreground fetches. 2. --refmap= removes the configured refspec which usually updates refs/remotes/<remote>/* with the refs advertised by the remote. 3. By adding a new refspec "+refs/heads/*:refs/prefetch/<remote>/*" we can ensure that we actually load the new values somewhere in our refspace while not updating refs/heads or refs/remotes. By storing these refs here, the commit-graph job will update the commit-graph with the commits from these hidden refs. 4. --prune will delete the refs/prefetch/<remote> refs that no longer appear on the remote. We've been using this step as a critical background job in Scalar [1] (and VFS for Git). This solved a pain point that was showing up in user reports: fetching was a pain! Users do not like waiting to download the data that was created while they were away from their machines. After implementing background fetch, the foreground fetch commands sped up significantly because they mostly just update refs and download a small amount of new data. The effect is especially dramatic when paried with --no-show-forced-udpates (through fetch.showForcedUpdates=false). [1] https://github.com/microsoft/scalar/blob/master/Scalar.Common/Maintenance/FetchStep.cs Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx> --- Documentation/git-maintenance.txt | 12 ++++++ builtin/gc.c | 64 ++++++++++++++++++++++++++++++- t/t7900-maintenance.sh | 24 ++++++++++++ 3 files changed, 99 insertions(+), 1 deletion(-) diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt index 9204762e21..0927643247 100644 --- a/Documentation/git-maintenance.txt +++ b/Documentation/git-maintenance.txt @@ -53,6 +53,18 @@ since it will not expire `.graph` files that were in the previous `commit-graph-chain` file. They will be deleted by a later run based on the expiration delay. +prefetch:: + The `fetch` task updates the object directory with the latest objects + from all registered remotes. For each remote, a `git fetch` command + is run. The refmap is custom to avoid updating local or remote + branches (those in `refs/heads` or `refs/remotes`). Instead, the + remote refs are stored in `refs/prefetch/<remote>/`. Also, tags are + not updated. ++ +This means that foreground fetches are still required to update the +remote refs, but the users is notified when the branches and tags are +updated on the remote. + gc:: Cleanup unnecessary files and optimize the local repository. "GC" stands for "garbage collection," but this task performs many diff --git a/builtin/gc.c b/builtin/gc.c index 5d99b4b805..969c127877 100644 --- a/builtin/gc.c +++ b/builtin/gc.c @@ -28,6 +28,7 @@ #include "blob.h" #include "tree.h" #include "promisor-remote.h" +#include "remote.h" #define FAILED_RUN "failed to run %s" @@ -700,7 +701,7 @@ int cmd_gc(int argc, const char **argv, const char *prefix) return 0; } -#define MAX_NUM_TASKS 2 +#define MAX_NUM_TASKS 3 static const char * const builtin_maintenance_usage[] = { N_("git maintenance run [<options>]"), @@ -781,6 +782,63 @@ static int maintenance_task_commit_graph(void) return 1; } +static int fetch_remote(const char *remote) +{ + int result; + struct argv_array cmd = ARGV_ARRAY_INIT; + struct strbuf refmap = STRBUF_INIT; + + argv_array_pushl(&cmd, "fetch", remote, "--prune", + "--no-tags", "--refmap=", NULL); + + strbuf_addf(&refmap, "+refs/heads/*:refs/prefetch/%s/*", remote); + argv_array_push(&cmd, refmap.buf); + + if (opts.quiet) + argv_array_push(&cmd, "--quiet"); + + result = run_command_v_opt(cmd.argv, RUN_GIT_CMD); + + strbuf_release(&refmap); + return result; +} + +static int fill_each_remote(struct remote *remote, void *cbdata) +{ + struct string_list *remotes = (struct string_list *)cbdata; + + string_list_append(remotes, remote->name); + return 0; +} + +static int maintenance_task_prefetch(void) +{ + int result = 0; + struct string_list_item *item; + struct string_list remotes = STRING_LIST_INIT_DUP; + + if (for_each_remote(fill_each_remote, &remotes)) { + error(_("failed to fill remotes")); + result = 1; + goto cleanup; + } + + /* + * Do not modify the result based on the success of the 'fetch' + * operation, as a loss of network could cause 'fetch' to fail + * quickly. We do not want that to stop the rest of our + * background operations. + */ + for (item = remotes.items; + item && item < remotes.items + remotes.nr; + item++) + fetch_remote(item->string); + +cleanup: + string_list_clear(&remotes, 0); + return result; +} + static int maintenance_task_gc(void) { int result; @@ -871,6 +929,10 @@ static void initialize_tasks(void) for (i = 0; i < MAX_NUM_TASKS; i++) tasks[i] = xcalloc(1, sizeof(struct maintenance_task)); + tasks[num_tasks]->name = "prefetch"; + tasks[num_tasks]->fn = maintenance_task_prefetch; + num_tasks++; + tasks[num_tasks]->name = "gc"; tasks[num_tasks]->fn = maintenance_task_gc; tasks[num_tasks]->enabled = 1; diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh index c09a9eb90b..8b04a04c79 100755 --- a/t/t7900-maintenance.sh +++ b/t/t7900-maintenance.sh @@ -44,4 +44,28 @@ test_expect_success 'run --task duplicate' ' test_i18ngrep "cannot be selected multiple times" err ' +test_expect_success 'run --task=prefetch with no remotes' ' + git maintenance run --task=prefetch 2>err && + test_must_be_empty err +' + +test_expect_success 'prefetch multiple remotes' ' + git clone . clone1 && + git clone . clone2 && + git remote add remote1 "file://$(pwd)/clone1" && + git remote add remote2 "file://$(pwd)/clone2" && + git -C clone1 switch -c one && + git -C clone2 switch -c two && + test_commit -C clone1 one && + test_commit -C clone2 two && + GIT_TRACE2_EVENT="$(pwd)/run-prefetch.txt" git maintenance run --task=prefetch && + grep ",\"fetch\",\"remote1\"" run-prefetch.txt && + grep ",\"fetch\",\"remote2\"" run-prefetch.txt && + test_path_is_missing .git/refs/remotes && + test_cmp clone1/.git/refs/heads/one .git/refs/prefetch/remote1/one && + test_cmp clone2/.git/refs/heads/two .git/refs/prefetch/remote2/two && + git log prefetch/remote1/one && + git log prefetch/remote2/two +' + test_done -- gitgitgadget
