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Document the hairy gfi_unpack_entry part of fast-import

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Junio pointed out this part of fast-import wasn't very clear on
initial read, and it took some time for someone who was new to
fast-import's "dirty little tricks" to understand how this was
even working.  So a little bit of commentary in the proper place
may help future readers.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Shawn O. Pearce 2008-01-20 23:37:01 -05:00 committed by Junio C Hamano
parent bb23fdfa6c
commit 7422bac441
1 changed files with 33 additions and 0 deletions
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33
fast-import.c
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@ -1125,6 +1125,24 @@ static int store_object(
return 0;
}
/* All calls must be guarded by find_object() or find_mark() to
* ensure the 'struct object_entry' passed was written by this
* process instance. We unpack the entry by the offset, avoiding
* the need for the corresponding .idx file. This unpacking rule
* works because we only use OBJ_REF_DELTA within the packfiles
* created by fast-import.
*
* oe must not be NULL. Such an oe usually comes from giving
* an unknown SHA-1 to find_object() or an undefined mark to
* find_mark(). Callers must test for this condition and use
* the standard read_sha1_file() when it happens.
*
* oe->pack_id must not be MAX_PACK_ID. Such an oe is usually from
* find_mark(), where the mark was reloaded from an existing marks
* file and is referencing an object that this fast-import process
* instance did not write out to a packfile. Callers must test for
* this condition and use read_sha1_file() instead.
*/
static void *gfi_unpack_entry(
struct object_entry *oe,
unsigned long *sizep)
@ -1132,7 +1150,22 @@ static void *gfi_unpack_entry(
enum object_type type;
struct packed_git *p = all_packs[oe->pack_id];
if (p == pack_data && p->pack_size < (pack_size + 20)) {
/* The object is stored in the packfile we are writing to
* and we have modified it since the last time we scanned
* back to read a previously written object. If an old
* window covered [p->pack_size, p->pack_size + 20) its
* data is stale and is not valid. Closing all windows
* and updating the packfile length ensures we can read
* the newly written data.
*/
close_pack_windows(p);
/* We have to offer 20 bytes additional on the end of
* the packfile as the core unpacker code assumes the
* footer is present at the file end and must promise
* at least 20 bytes within any window it maps. But
* we don't actually create the footer here.
*/
p->pack_size = pack_size + 20;
}
return unpack_entry(p, oe->offset, &type, sizep);