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openConnection(): Don't throw exceptions in forked child

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On hydra.nixos.org the queue runner had child processes that were
stuck handling an exception:

  Thread 1 (Thread 0x7f501f7fe640 (LWP 1413473) "bld~v54h5zkhmb3"):
  #0  futex_wait (private=0, expected=2, futex_word=0x7f50c27969b0 <_rtld_local+2480>) at ../sysdeps/nptl/futex-internal.h:146
  #1  __lll_lock_wait (futex=0x7f50c27969b0 <_rtld_local+2480>, private=0) at lowlevellock.c:52
  #2  0x00007f50c21eaee4 in __GI___pthread_mutex_lock (mutex=0x7f50c27969b0 <_rtld_local+2480>) at ../nptl/pthread_mutex_lock.c:115
  #3  0x00007f50c1854bef in __GI___dl_iterate_phdr (callback=0x7f50c190c020 <_Unwind_IteratePhdrCallback>, data=0x7f501f7fb040) at dl-iteratephdr.c:40
  #4  0x00007f50c190d2d1 in _Unwind_Find_FDE () from /nix/store/65hafbsx91127farbmyyv4r5ifgjdg43-glibc-2.33-117/lib/libgcc_s.so.1
  #5  0x00007f50c19099b3 in uw_frame_state_for () from /nix/store/65hafbsx91127farbmyyv4r5ifgjdg43-glibc-2.33-117/lib/libgcc_s.so.1
  #6  0x00007f50c190ab90 in uw_init_context_1 () from /nix/store/65hafbsx91127farbmyyv4r5ifgjdg43-glibc-2.33-117/lib/libgcc_s.so.1
  #7  0x00007f50c190b08e in _Unwind_RaiseException () from /nix/store/65hafbsx91127farbmyyv4r5ifgjdg43-glibc-2.33-117/lib/libgcc_s.so.1
  #8  0x00007f50c1b02ab7 in __cxa_throw () from /nix/store/dd8swlwhpdhn6bv219562vyxhi8278hs-gcc-10.3.0-lib/lib/libstdc++.so.6
  #9  0x00007f50c1d01abe in nix::parseURL (url="root@cb893012.packethost.net") at src/libutil/url.cc:53
  #10 0x0000000000484f55 in extraStoreArgs (machine="root@cb893012.packethost.net") at build-remote.cc:35
  #11 operator() (__closure=0x7f4fe9fe0420) at build-remote.cc:79
  ...

Maybe the fork happened while another thread was holding some global
stack unwinding lock
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71744). Anyway, since
the hanging child inherits all file descriptors to SSH clients,
shutting down remote builds (via 'child.to = -1' in
State::buildRemote()) doesn't work and 'child.pid.wait()' hangs
forever.

So let's not do any significant work between fork and exec.
This commit is contained in:
Eelco Dolstra 2022-03-30 22:39:48 +02:00
parent bbb0998699
commit bcaad1c934
1 changed files with 23 additions and 25 deletions
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28
src/hydra-queue-runner/build-remote.cc
View file

@ -55,25 +55,11 @@ static void openConnection(Machine::ptr machine, Path tmpDir, int stderrFD, Chil
to.create();
from.create();
child.pid = startProcess([&]() {
restoreProcessContext();
if (dup2(to.readSide.get(), STDIN_FILENO) == -1)
throw SysError("cannot dup input pipe to stdin");
if (dup2(from.writeSide.get(), STDOUT_FILENO) == -1)
throw SysError("cannot dup output pipe to stdout");
if (dup2(stderrFD, STDERR_FILENO) == -1)
throw SysError("cannot dup stderr");
Strings argv;
if (machine->isLocalhost()) {
pgmName = "nix-store";
argv = {"nix-store", "--builders", "", "--serve", "--write"};
}
else {
} else {
pgmName = "ssh";
auto sshName = machine->sshName;
Strings extraArgs = extraStoreArgs(sshName);
@ -92,6 +78,18 @@ static void openConnection(Machine::ptr machine, Path tmpDir, int stderrFD, Chil
append(argv, extraArgs);
}
child.pid = startProcess([&]() {
restoreProcessContext();
if (dup2(to.readSide.get(), STDIN_FILENO) == -1)
throw SysError("cannot dup input pipe to stdin");
if (dup2(from.writeSide.get(), STDOUT_FILENO) == -1)
throw SysError("cannot dup output pipe to stdout");
if (dup2(stderrFD, STDERR_FILENO) == -1)
throw SysError("cannot dup stderr");
execvp(argv.front().c_str(), (char * *) stringsToCharPtrs(argv).data()); // FIXME: remove cast
throw SysError("cannot start %s", pgmName);