Hi,
I found a file descriptor leak in the pg_dump compression backends
(gzip, lz4, zstd, and the uncompressed "none" backend), introduced
in commit e9960732a9 ("Introduce a generic pg_dump compression API",
PostgreSQL 16).
== The Bug ==
All four compression open functions use this pattern when an existing
file descriptor is passed in:
if (fd >= 0)
fp = fdopen(dup(fd), mode); /* or gzdopen() */
if (fp == NULL)
return false; /* dup'd fd is leaked here */
The problem is that dup(fd) and fdopen()/gzdopen() are two separate
steps, and their failure modes must be handled independently:
1. If dup() fails (returns -1), no fd is created -- this case was
not checked at all in the original code.
2. If dup() succeeds but fdopen()/gzdopen() fails (e.g., due to a
failed malloc(3) for the FILE structure), POSIX explicitly states:
"If the fdopen() function fails, the file descriptor is
not closed."
-- POSIX.1-2017, fdopen() specification
The duplicated fd therefore remains open with no owner, leaking
until the process exits.
== When Can dup() Fail? ==
The most realistic trigger for dup() returning EMFILE is parallel
pg_dump (pg_dump -j N) against a large database. Each worker opens
multiple file descriptors for tables, indexes, TOC files, and
compression streams simultaneously. On systems with a low per-process
fd limit (e.g., ulimit -n 1024), or when dumping a schema with a very
large number of objects, the process fd count can approach the limit.
At that point, dup() fails with EMFILE and the subsequent NULL-check
on fp returns false without any cleanup.
While fdopen() failure (EMFILE/OOM) is less common, it is equally
incorrect to ignore -- and it is precisely the case that POSIX calls
out as the caller's responsibility to close.
== The Fix ==
Save the result of dup() in a local variable. Check it immediately.
If fdopen()/gzdopen() subsequently fails, explicitly close the
duplicated fd before returning false.
if (fd >= 0)
{
int dup_fd = dup(fd);
if (dup_fd < 0)
return false;
fp = fdopen(dup_fd, mode);
if (fp == NULL)
{
close(dup_fd); /* POSIX: fdopen does not close on
failure */
return false;
}
}
else
{
fp = fopen(path, mode);
if (fp == NULL)
return false;
}
The zstd fix additionally ensures that the previously allocated
zstdcs structure is freed on all new failure paths.
== Affected Versions ==
PostgreSQL 16 and 17, and current master.
The compress_* files were introduced in commit e9960732a9 (Feb 2023).
== Patch ==
Patch attached. It applies cleanly to current master (as of
commit 18bcdb75d15).
Regards,
Jianghua Yang
