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midx: support custom --base for incremental MIDX writes

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Both `compact` and `write --incremental` fix the base of the resulting
MIDX layer: `compact` always places the compacted result on top of
"from's" immediate parent in the chain, and `write --incremental` always
appends a new layer to the existing tip. In both cases the base is not
configurable.

Future callers need additional flexibility. For instance, the incremental
MIDX-based repacking code may wish to write a layer based on some
intermediate ancestor rather than the current tip, or produce a root
layer when replacing the bottommost entries in the chain.

Introduce a new `--base` option for both subcommands to specify the
checksum of the MIDX layer to use as the base. The given checksum must
refer to a valid layer in the MIDX chain that is an ancestor of the
topmost layer being written or compacted.

The special value "none" is accepted to produce a root layer with no
parent. This will be needed when the incremental repacking machinery
determines that the bottommost layers of the chain should be replaced.

If no `--base` is given, behavior is unchanged: `compact` uses "from's"
immediate parent in the chain, and `write` appends to the existing tip.

For the `write` subcommand, `--base` requires `--no-write-chain-file`. A plain
`write --incremental` appends a new layer to the live chain tip with no
mechanism to atomically replace it; overriding the base would produce a
layer that does not extend the tip, breaking chain invariants. With
`--no-write-chain-file` the chain is left unmodified and the caller is
responsible for assembling a valid chain.

For `compact`, no such restriction applies. The compaction operation
atomically replaces the compacted range in the chain file, so writing
the result on top of any valid ancestor preserves chain invariants.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Taylor Blau
2026-05-19 11:57:54 -04:00
committed by Junio C Hamano
parent 8d342ed4b5
commit 0cd2255e64
6 changed files with 178 additions and 9 deletions
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34
midx-write.c
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@@ -1247,6 +1247,7 @@ struct write_midx_opts {
const char *preferred_pack_name;
const char *refs_snapshot;
const char *incremental_base;
unsigned flags;
};
@@ -1330,11 +1331,32 @@ static int write_midx_internal(struct write_midx_opts *opts)
/*
* If compacting MIDX layer(s) in the range [from, to], then the
* compacted MIDX will share the same base MIDX as 'from'.
* compacted MIDX will share the same base MIDX as 'from',
* unless a custom --base is specified (see below).
*/
if (ctx.compact)
ctx.base_midx = ctx.compact_from->base_midx;
if (opts->incremental_base) {
if (!strcmp(opts->incremental_base, "none")) {
ctx.base_midx = NULL;
} else {
while (ctx.base_midx) {
const char *cmp = midx_get_checksum_hex(ctx.base_midx);
if (!strcmp(opts->incremental_base, cmp))
break;
ctx.base_midx = ctx.base_midx->base_midx;
}
if (!ctx.base_midx) {
error(_("could not find base MIDX '%s'"),
opts->incremental_base);
goto cleanup;
}
}
}
ctx.nr = 0;
ctx.alloc = ctx.m ? ctx.m->num_packs + ctx.m->num_packs_in_base : 16;
ctx.info = NULL;
@@ -1827,7 +1849,8 @@ cleanup:
int write_midx_file(struct odb_source *source,
const char *preferred_pack_name,
const char *refs_snapshot, unsigned flags)
const char *refs_snapshot,
unsigned flags)
{
struct write_midx_opts opts = {
.source = source,
@@ -1842,13 +1865,16 @@ int write_midx_file(struct odb_source *source,
int write_midx_file_only(struct odb_source *source,
struct string_list *packs_to_include,
const char *preferred_pack_name,
const char *refs_snapshot, unsigned flags)
const char *refs_snapshot,
const char *incremental_base,
unsigned flags)
{
struct write_midx_opts opts = {
.source = source,
.packs_to_include = packs_to_include,
.preferred_pack_name = preferred_pack_name,
.refs_snapshot = refs_snapshot,
.incremental_base = incremental_base,
.flags = flags,
};
@@ -1858,12 +1884,14 @@ int write_midx_file_only(struct odb_source *source,
int write_midx_file_compact(struct odb_source *source,
struct multi_pack_index *from,
struct multi_pack_index *to,
const char *incremental_base,
unsigned flags)
{
struct write_midx_opts opts = {
.source = source,
.compact_from = from,
.compact_to = to,
.incremental_base = incremental_base,
.flags = flags | MIDX_WRITE_COMPACT,
};