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optimise-store: Fix race condition in temporary link creation

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Replace rand() with atomic counter for unique temp link names and add
retry loop to handle EEXIST errors when multiple processes optimize
the store simultaneously.
This commit is contained in:
Jörg Thalheim 2025-06-12 17:40:11 +02:00
parent 371fcf91c3
commit ef05a2fa0f
1 changed files with 43 additions and 28 deletions
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71
src/libstore/optimise-store.cc
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@ -223,38 +223,53 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
its timestamp back to 0. */
MakeReadOnly makeReadOnly(mustToggle ? dirOfPath : "");
std::filesystem::path tempLink = fmt("%1%/.tmp-link-%2%-%3%", config->realStoreDir, getpid(), rand());
/* Retry loop for temporary link creation to handle race conditions */
while (true) {
Path tempLink = makeTempPath(config->realStoreDir.get(), ".tmp-link");
try {
std::filesystem::create_hard_link(linkPath, tempLink);
inodeHash.insert(st.st_ino);
} catch (std::filesystem::filesystem_error & e) {
if (e.code() == std::errc::too_many_links) {
/* Too many links to the same file (>= 32000 on most file
systems). This is likely to happen with empty files.
Just shrug and ignore. */
if (st.st_size)
printInfo("'%1%' has maximum number of links", linkPath);
return;
try {
std::filesystem::create_hard_link(linkPath, tempLink);
inodeHash.insert(st.st_ino);
} catch (std::filesystem::filesystem_error & e) {
if (e.code() == std::errc::too_many_links) {
/* Too many links to the same file (>= 32000 on most file
systems). This is likely to happen with empty files.
Just shrug and ignore. */
if (st.st_size)
printInfo("'%1%' has maximum number of links", linkPath);
return;
}
throw SysError("creating temporary link '%1%'", tempLink);
}
throw;
}
/* Atomically replace the old file with the new hard link. */
try {
std::filesystem::rename(tempLink, path);
} catch (std::filesystem::filesystem_error & e) {
std::filesystem::remove(tempLink);
printError("unable to unlink '%1%'", tempLink);
if (e.code() == std::errc::too_many_links) {
/* Some filesystems generate too many links on the rename,
rather than on the original link. (Probably it
temporarily increases the st_nlink field before
decreasing it again.) */
debug("'%s' has reached maximum number of links", linkPath);
return;
/* Atomically replace the old file with the new hard link. */
try {
std::filesystem::rename(tempLink, path);
break; /* Success! */
} catch (std::filesystem::filesystem_error & e) {
try {
std::filesystem::remove(tempLink);
} catch (...) {
/* Ignore errors removing the temp link */
}
if (e.code() == std::errc::too_many_links) {
/* Some filesystems generate too many links on the rename,
rather than on the original link. (Probably it
temporarily increases the st_nlink field before
decreasing it again.) */
debug("%s has reached maximum number of links", linkPath);
return;
}
if (e.code() == std::errc::file_exists) {
/* Race condition: another process created the same temp link.
Try again with a different name. */
continue;
}
throw SysError("renaming temporary link '%1%' to '%2%'", tempLink, path);
}
throw;
}
stats.filesLinked++;