Async-signal safety in signal handlers

Hi,

we have recently observed a Git process which has been hanging around for more than a month on one of our servers in production. A backtrace showed that the git-fetch(1) process was deadlocked in its signal handler while trying to free memory. Functions like malloc, free and most I/O functions aren't reentrant though, which means they must not be executed in async signal handlers as specified in signal-safety(7).

The fix for git-fetch(1) is rather simple: we can just unlink(2) the lockfiles, which is indeed allowed, but skip free'ing memory. But in fact, this is a wider issue we have: we mostly didn't pay attention to those restrictions, and thus we freely call non-async-signal-safe functions. It's less clear what to do about this in most of the cases though:

- git-clone(1) tries to clean up the ".git" directory and its worktree on being killed, but needs to allocate memory to compute corresponding paths. We can try to preallocate the buffer, but it's not clear whether there is a proper upper boundary.

- git-gc(1) will try to commit "gc.log" and write to stderr, both of which aren't allowed. I think we'll have to just bail and leave it behind in a partially-written state.

- git-repack(1) tries to remove "pack/.tmp-*" files, calling opendir(3P), readdir(3P), closedir(3P) and allocates memory. We probably have to keep track of all temporary files we create in a global list, which we can then access in our signal handler.

- git-worktree(1) is doing the same as git-clone(1), trying to prune the new worktree if it's killed. Again, we'd probably have to preallocate a buffer to compute paths.

- HTTP pushes do all sorts of HTTP requests in their signal handler to unlock the remote server. I don't really see what to do about this except drop the code -- setting a global "please clean up and exit now" flags is probably not going to fly well.

The tempfiles and tmp-objdir code already handles signals correctly.

Patrick

Patrick Steinhardt (1):
fetch: fix deadlock when cleaning up lockfiles in async signals

 builtin/clone.c |  2 +-
 builtin/fetch.c | 17 +++++++++++------
 transport.c     | 11 ++++++++---
 transport.h     | 14 +++++++++++++-
 4 files changed, 33 insertions(+), 11 deletions(-)

2.34.1