builtin/commit-graph.c: introduce split strategy 'replace'

When using split commit-graphs, it is sometimes useful to completely
replace the commit-graph chain with a new base.

For example, consider a scenario in which a repository builds a new
commit-graph incremental for each push. Occasionally (say, after some
fixed number of pushes), they may wish to rebuild the commit-graph chain
with all reachable commits.

They can do so with

  $ git commit-graph write --reachable

but this removes the chain entirely and replaces it with a single
commit-graph in 'objects/info/commit-graph'. Unfortunately, this means
that the next push will have to move this commit-graph into the first
layer of a new chain, and then write its new commits on top.

Avoid such copying entirely by allowing the caller to specify that they
wish to replace the entirety of their commit-graph chain, while also
specifying that the new commit-graph should become the basis of a fresh,
length-one chain.

This addresses the above situation by making it possible for the caller
to instead write:

  $ git commit-graph write --reachable --split=replace

which writes a new length-one chain to 'objects/info/commit-graphs',
making the commit-graph incremental generated by the subsequent push
relatively cheap by avoiding the aforementioned copy.

In order to do this, remove an assumption in 'write_commit_graph_file'
that chains are always at least two incrementals long.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
index a4c4a641e509ed9de4ec3bf26c9badafe38243c8..46f7f7c573d50a118f987e9609525849f8ce741a 100644 (file)
--- a/Documentation/git-commit-graph.txt
+++ b/Documentation/git-commit-graph.txt
@@ -64,9 +64,10 @@ strategy and other splitting options. The new commits not already in the
 commit-graph are added in a new "tip" file. This file is merged with the
 existing file if the following merge conditions are met:
 * If `--split=no-merge` is specified, a merge is never performed, and
-the remaining options are ignored. A bare `--split` defers to the
-remaining options. (Note that merging a chain of commit graphs replaces
-the existing chain with a length-1 chain where the first and only
+the remaining options are ignored. `--split=replace` overwrites the
+existing chain with a new one. A bare `--split` defers to the remaining
+options. (Note that merging a chain of commit graphs replaces the
+existing chain with a length-1 chain where the first and only
 incremental holds the entire graph).
 +
 * If `--size-multiple=<X>` is not specified, let `X` equal 2. If the new

diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
index 9234b95ecfb6ff0b301110ca4a033947ce50de66..e0dd2876a15366371f636fca3aacafcdedb20279 100644 (file)
--- a/builtin/commit-graph.c
+++ b/builtin/commit-graph.c
@@ -126,6 +126,8 @@ static int write_option_parse_split(const struct option *opt, const char *arg,
 
        if (!strcmp(arg, "no-merge"))
                *flags = COMMIT_GRAPH_SPLIT_MERGE_PROHIBITED;
+       else if (!strcmp(arg, "replace"))
+               *flags = COMMIT_GRAPH_SPLIT_REPLACE;
        else
                die(_("unrecognized --split argument, %s"), arg);
 

diff --git a/commit-graph.c b/commit-graph.c
index 71f488839ca56a049926dc69ea5c9ece1f09a47d..36e1d6f9b36cd5566d7830b51330033a61ff1c28 100644 (file)
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -866,7 +866,7 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 
                        if (edge_value >= 0)
                                edge_value += ctx->new_num_commits_in_base;
-                       else {
+                       else if (ctx->new_base_graph) {
                                uint32_t pos;
                                if (find_commit_in_graph(parent->item,
                                                         ctx->new_base_graph,
@@ -897,7 +897,7 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 
                        if (edge_value >= 0)
                                edge_value += ctx->new_num_commits_in_base;
-                       else {
+                       else if (ctx->new_base_graph) {
                                uint32_t pos;
                                if (find_commit_in_graph(parent->item,
                                                         ctx->new_base_graph,
@@ -964,7 +964,7 @@ static void write_graph_chunk_extra_edges(struct hashfile *f,
 
                        if (edge_value >= 0)
                                edge_value += ctx->new_num_commits_in_base;
-                       else {
+                       else if (ctx->new_base_graph) {
                                uint32_t pos;
                                if (find_commit_in_graph(parent->item,
                                                         ctx->new_base_graph,
@@ -1037,6 +1037,8 @@ static void close_reachable(struct write_commit_graph_context *ctx)
 {
        int i;
        struct commit *commit;
+       enum commit_graph_split_flags flags = ctx->split_opts ?
+               ctx->split_opts->flags : COMMIT_GRAPH_SPLIT_UNSPECIFIED;
 
        if (ctx->report_progress)
                ctx->progress = start_delayed_progress(
@@ -1066,8 +1068,9 @@ static void close_reachable(struct write_commit_graph_context *ctx)
                if (!commit)
                        continue;
                if (ctx->split) {
-                       if (!parse_commit(commit) &&
-                           commit->graph_pos == COMMIT_NOT_FROM_GRAPH)
+                       if ((!parse_commit(commit) &&
+                            commit->graph_pos == COMMIT_NOT_FROM_GRAPH) ||
+                           flags == COMMIT_GRAPH_SPLIT_REPLACE)
                                add_missing_parents(ctx, commit);
                } else if (!parse_commit_no_graph(commit))
                        add_missing_parents(ctx, commit);
@@ -1287,6 +1290,8 @@ static uint32_t count_distinct_commits(struct write_commit_graph_context *ctx)
 static void copy_oids_to_commits(struct write_commit_graph_context *ctx)
 {
        uint32_t i;
+       enum commit_graph_split_flags flags = ctx->split_opts ?
+               ctx->split_opts->flags : COMMIT_GRAPH_SPLIT_UNSPECIFIED;
 
        ctx->num_extra_edges = 0;
        if (ctx->report_progress)
@@ -1303,11 +1308,14 @@ static void copy_oids_to_commits(struct write_commit_graph_context *ctx)
                ALLOC_GROW(ctx->commits.list, ctx->commits.nr + 1, ctx->commits.alloc);
                ctx->commits.list[ctx->commits.nr] = lookup_commit(ctx->r, &ctx->oids.list[i]);
 
-               if (ctx->split &&
+               if (ctx->split && flags != COMMIT_GRAPH_SPLIT_REPLACE &&
                    ctx->commits.list[ctx->commits.nr]->graph_pos != COMMIT_NOT_FROM_GRAPH)
                        continue;
 
-               parse_commit_no_graph(ctx->commits.list[ctx->commits.nr]);
+               if (ctx->split && flags == COMMIT_GRAPH_SPLIT_REPLACE)
+                       parse_commit(ctx->commits.list[ctx->commits.nr]);
+               else
+                       parse_commit_no_graph(ctx->commits.list[ctx->commits.nr]);
 
                num_parents = commit_list_count(ctx->commits.list[ctx->commits.nr]->parents);
                if (num_parents > 2)
@@ -1488,8 +1496,12 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
                }
 
                if (ctx->base_graph_name) {
-                       const char *dest = ctx->commit_graph_filenames_after[
-                                               ctx->num_commit_graphs_after - 2];
+                       const char *dest;
+                       int idx = ctx->num_commit_graphs_after - 1;
+                       if (ctx->num_commit_graphs_after > 1)
+                               idx--;
+
+                       dest = ctx->commit_graph_filenames_after[idx];
 
                        if (strcmp(ctx->base_graph_name, dest)) {
                                result = rename(ctx->base_graph_name, dest);
@@ -1546,9 +1558,13 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
 
        g = ctx->r->objects->commit_graph;
        num_commits = ctx->commits.nr;
-       ctx->num_commit_graphs_after = ctx->num_commit_graphs_before + 1;
+       if (flags == COMMIT_GRAPH_SPLIT_REPLACE)
+               ctx->num_commit_graphs_after = 1;
+       else
+               ctx->num_commit_graphs_after = ctx->num_commit_graphs_before + 1;
 
-       if (flags != COMMIT_GRAPH_SPLIT_MERGE_PROHIBITED) {
+       if (flags != COMMIT_GRAPH_SPLIT_MERGE_PROHIBITED &&
+           flags != COMMIT_GRAPH_SPLIT_REPLACE) {
                while (g && (g->num_commits <= size_mult * num_commits ||
                            (max_commits && num_commits > max_commits))) {
                        if (g->odb != ctx->odb)
@@ -1561,7 +1577,11 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
                }
        }
 
-       ctx->new_base_graph = g;
+       if (flags != COMMIT_GRAPH_SPLIT_REPLACE)
+               ctx->new_base_graph = g;
+       else if (ctx->num_commit_graphs_after != 1)
+               BUG("split_graph_merge_strategy: num_commit_graphs_after "
+                   "should be 1 with --split=replace");
 
        if (ctx->num_commit_graphs_after == 2) {
                char *old_graph_name = get_commit_graph_filename(g->odb);
@@ -1772,6 +1792,7 @@ int write_commit_graph(struct object_directory *odb,
        struct write_commit_graph_context *ctx;
        uint32_t i, count_distinct = 0;
        int res = 0;
+       int replace = 0;
 
        if (!commit_graph_compatible(the_repository))
                return 0;
@@ -1806,6 +1827,9 @@ int write_commit_graph(struct object_directory *odb,
                                g = g->base_graph;
                        }
                }
+
+               if (ctx->split_opts)
+                       replace = ctx->split_opts->flags & COMMIT_GRAPH_SPLIT_REPLACE;
        }
 
        ctx->approx_nr_objects = approximate_object_count();
@@ -1866,13 +1890,14 @@ int write_commit_graph(struct object_directory *odb,
                goto cleanup;
        }
 
-       if (!ctx->commits.nr)
+       if (!ctx->commits.nr && !replace)
                goto cleanup;
 
        if (ctx->split) {
                split_graph_merge_strategy(ctx);
 
-               merge_commit_graphs(ctx);
+               if (!replace)
+                       merge_commit_graphs(ctx);
        } else
                ctx->num_commit_graphs_after = 1;
 

diff --git a/commit-graph.h b/commit-graph.h
index 8752afb88d0f6056cbb870cbe9b38251cdd9b773..718433d79b772a437885141e44fc760e3720f7db 100644 (file)
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -84,7 +84,8 @@ enum commit_graph_write_flags {
 
 enum commit_graph_split_flags {
        COMMIT_GRAPH_SPLIT_UNSPECIFIED      = 0,
-       COMMIT_GRAPH_SPLIT_MERGE_PROHIBITED = 1
+       COMMIT_GRAPH_SPLIT_MERGE_PROHIBITED = 1,
+       COMMIT_GRAPH_SPLIT_REPLACE          = 2
 };
 
 struct split_commit_graph_opts {

diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 29de47e834bb385bcb582cfc257c024e7237986b..87a9148fbeb390a8313ec7aea27c74170b896060 100755 (executable)
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -355,4 +355,23 @@ test_expect_success '--split=no-merge always writes an incremental' '
        test_line_count = 2 $graphdir/commit-graph-chain
 '
 
+test_expect_success '--split=replace replaces the chain' '
+       rm -rf $graphdir $infodir/commit-graph &&
+       git reset --hard commits/3 &&
+       git rev-list -1 HEAD~2 >a &&
+       git rev-list -1 HEAD~1 >b &&
+       git rev-list -1 HEAD >c &&
+       git commit-graph write --split=no-merge --stdin-commits <a &&
+       git commit-graph write --split=no-merge --stdin-commits <b &&
+       git commit-graph write --split=no-merge --stdin-commits <c &&
+       test_line_count = 3 $graphdir/commit-graph-chain &&
+       git commit-graph write --stdin-commits --split=replace <b &&
+       test_path_is_missing $infodir/commit-graph &&
+       test_path_is_file $graphdir/commit-graph-chain &&
+       ls $graphdir/graph-*.graph >graph-files &&
+       test_line_count = 1 graph-files &&
+       verify_chain_files_exist $graphdir &&
+       graph_read_expect 2
+'
+
 test_done