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avformat/shared: don't read directly into cache file when racing writes

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Instead, read to the output/temporary buffer (write_back path). This is to
lessen the impact of racing the write against other clients trying to race
the same pending block.

Sponsored-by: nxtedition AB
Signed-off-by: Niklas Haas <git@haasn.dev>
This commit is contained in:
Niklas Haas
2026-06-10 14:00:20 +02:00
parent 58e5ec8a5a
commit 9140cbe583
1 changed files with 6 additions and 3 deletions
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9
libavformat/shared.c
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@@ -600,7 +600,7 @@ static int shared_read(URLContext *h, unsigned char *buf, int size)
Block *const block = &s->spacemap->blocks[block_id];
unsigned short state = atomic_load_explicit(&block->state, memory_order_acquire);
int64_t pending_since = 0;
int verify_read = 0;
int verify_read = 0, is_race = 0;
retry:
switch (state) {
@@ -661,11 +661,14 @@ retry:
case BLOCK_PENDING:
/* Another thread is busy fetching this block, wait for it to finish */
if (!s->timeout) {
is_race = 1;
break; /* no timeout requested, immediately race to fetch block */
} else if (pending_since) {
int64_t new = av_gettime_relative();
if (new - pending_since >= s->timeout)
if (new - pending_since >= s->timeout) {
is_race = 1;
break; /* timeout expired, try to fetch the block ourselves */
}
} else {
pending_since = av_gettime_relative();
}
@@ -720,7 +723,7 @@ retry:
}
int write_back = 1;
if (s->cache_data) {
if (s->cache_data && !is_race) {
/* Read directly into memory mapped cache file */
tmp = s->cache_data + block_pos;
write_back = 0;