Uri is not thread/domain safe

[polytypic]

We encountered the following:

Fatal error: exception CamlinternalLazy.Undefined
Raised at CamlinternalLazy.force_gen_lazy_block in file "camlinternalLazy.ml", line 75, characters 9-24
Called from CamlinternalLazy.force_lazy_block in file "camlinternalLazy.ml" (inlined), line 78, characters 27-67
Called from Angstrom.fix_direct.(fun) in file "lib/angstrom.ml", line 460, characters 4-18
Called from Angstrom__Parser.parse_bigstring in file "lib/parser.ml", line 43, characters 52-93
Called from Uri.Parser.uri_reference.(fun) in file "lib/uri.ml", line 1066, characters 12-63
Called from Angstrom__Parser.Monad.(>>|).(fun).succ' in file "lib/parser.ml", line 64, characters 61-66
Called from Angstrom__Parser.parse_bigstring in file "lib/parser.ml", line 43, characters 52-93
Called from Uri.of_string in file "lib/uri.ml", line 1148, characters 8-68

Our program runs multiple domains in parallel.

I spent a brief moment to look through the Uri and Angstrom code bases. I strongly suspect the problem is that the Uri.Parser module uses the Angstrom fix combinator, which uses lazy internally. The problem with that is that Lazy is not safe to use concurrently / in parallel — an attempt to do so may raise the Undefined exception. As a workaround we will try to force the parser before spawning domains, but this should really be fixed properly.

[polytypic]

I was able to reproduce the Undefined exception with the following:

let () =
  let barrier = Atomic.make 2 in
  let parse () =
    Atomic.decr barrier;
    while Atomic.get barrier <> 0 do
      Domain.cpu_relax ()
    done;
    Uri.of_string "http://localhost:8080/" |> ignore
  in
  let d1 = Domain.spawn parse and d2 = Domain.spawn parse in
  Domain.join d1 |> ignore;
  Domain.join d2 |> ignore;
  ()

NOTE: It may take a few retries!

EDIT: I updated the above to use a different URI.

[edwintorok]

Interestingly Uri-re isn't safe either:

module Uri = Uri_legacy
let () =
  Printexc.record_backtrace true;
  let barrier = Atomic.make 2 in
  let parse () =
    Printexc.record_backtrace true;
    Atomic.decr barrier;
    while Atomic.get barrier <> 0 do
      Domain.cpu_relax ()
    done;
    try
      Uri.of_string "http://[::1]:8080" |> ignore
    with e ->
     (* Domain backtraces are broken on 5.2 *)
      let bt = Printexc.get_raw_backtrace () in
      Printexc.print_raw_backtrace stderr bt;
      Printexc.to_string e |> prerr_endline;
      Printexc.raise_with_backtrace e bt
  in
  let d1 = Domain.spawn parse and d2 = Domain.spawn parse in
  Domain.join d1 |> ignore;
  Domain.join d2 |> ignore;
  ()

fails after a couple of tries with:

Raised at Stdlib.invalid_arg in file "stdlib.ml", line 30, characters 20-45
Called from Stdlib__String.sub in file "string.ml" (inlined), line 43, characters 2-23
Called from Re__Group.get in file "lib/group.ml", line 20, characters 2-29
Called from Uri_legacy.of_string.get_opt in file "lib_re/uri_legacy.ml", line 708, characters 33-51
Called from Uri_legacy.of_string in file "lib_re/uri_legacy.ml", line 713, characters 15-29
Called from Dune__exe__Lazybug.parse in file "lazybug.ml", line 12, characters 6-39
Invalid_argument("String.sub / Bytes.sub")
Fatal error: exception Invalid_argument("String.sub / Bytes.sub")
Raised at Stdlib__Domain.join in file "domain.ml", line 296, characters 16-24
Called from Dune__exe__Lazybug in file "lazybug.ml", line 21, characters 2-16

[edwintorok]

Uri_legacy uses Lazy too internally :(

[edwintorok]

Here is a slightly tweaked testcase that crashes on OCaml 4.14.2 too (with a segfault too sometimes :( ):

let check _ =
  Thread.yield ();
  None
let () =
  let tracker = Gc.Memprof.{null_tracker with alloc_minor = check; alloc_major = check } in
  Gc.Memprof.start ~sampling_rate:1e-1 ~callstack_size:0 tracker;
  let n = 100 in
  let barrier = Atomic.make n in
  let parse _ =
    Atomic.decr barrier;
    while Atomic.get barrier <> 0 do
      Thread.yield ()
    done;
    for _ = 1 to 100 do
      Sys.opaque_identity (Uri.of_string "http://[::1]:8080") |> ignore
    done
  in

  let threads = Array.init n (Thread.create parse) in
  Array.iter Thread.join threads

dune exec ./lazybug.exe
Thread 91 killed on uncaught exception CamlinternalLazy.Undefined
Thread 88 killed on uncaught exception CamlinternalLazy.Undefined
Thread 86 killed on uncaught exception CamlinternalLazy.Undefined
Thread 83 killed on uncaught exception CamlinternalLazy.Undefined
Thread 78 killed on uncaught exception CamlinternalLazy.Undefined
Thread 57 killed on uncaught exception CamlinternalLazy.Undefined
[3]    233673 segmentation fault (core dumped)  dune exec ./lazybug.exe

FWIW I couldn't get the equivalent uri-re code to crash/fail on OCaml 4 yet, although that doesn't mean it is thread safe there.

[edwintorok]

Potential workaround is to do this in Angstrom:

diff --git a/lib/angstrom.ml b/lib/angstrom.ml
index e546fbc63856..286992f4e870 100644
--- a/lib/angstrom.ml
+++ b/lib/angstrom.ml
@@ -459,6 +459,7 @@ let fix_direct f =
   and r = { run = fun buf pos more fail succ ->
     (Lazy.force p).run buf pos more fail succ }
   in
+  (Sys.opaque_identity (Lazy.force p) |> ignore);
   r

Or this:

diff --git a/lib/angstrom.ml b/lib/angstrom.ml
index e546fbc63856..304d3f3014de 100644
--- a/lib/angstrom.ml
+++ b/lib/angstrom.ml
@@ -455,10 +455,11 @@ let choice ?(failure_msg="no more choices") ps =
   List.fold_right (<|>) ps (fail failure_msg)

 let fix_direct f =
-  let rec p = lazy (f r)
-  and r = { run = fun buf pos more fail succ ->
-    (Lazy.force p).run buf pos more fail succ }
+  let p = ref None in
+  let r = { run = fun buf pos more fail succ ->
+    (Option.get !p).run buf pos more fail succ }
   in
+  p := Some (f r);
   r

There might be other ways of fixing it too

[avsm]

It's somewhat worrying to see that test case segfaulting. Do you have a backtrace for that, @edwintorok?

[edwintorok]

It's somewhat worrying to see that test case segfaulting. Do you have a backtrace for that, @edwintorok?

OCaml 4.14.2, uri 4.4.0 and angstrom 0.16.0

$ ocamlfind ocamlopt lazybug.ml -g -o lazybug.exe -package uri,threads.posix -thread -linkpkg
$ ulimit -c unlimited
$ ./lazybug.exe
$ gdb -c core
(gdb) bt
#0  0x0000000000447ed9 in caml_apply5 ()
#1  0x0000000000458155 in camlAngstrom__fun_2531 () at lib/angstrom.ml:615
#2  0x0000000000000001 in ?? ()
#3  0x00007f38fbdfff70 in ?? ()
#4  0x0000000000458155 in camlAngstrom__fun_2531 () at lib/angstrom.ml:615
#5  0x0000000000448434 in camlUri__compare_opt_377 () at lib/uri.ml:65
#6  0x000000000000001d in ?? ()
#7  0x00007f38a8000b70 in ?? ()
#8  0x00007f38fbdffd50 in ?? ()
#9  0x00000000000038c0 in ?? ()
#10 0x00007f38a8000030 in ?? ()
#11 0x00000000004c8e0d in caml_start_program ()
#12 0x9e612da82678d500 in ?? ()
#13 0x00007f38fbe00cdc in ?? ()
#14 0x00007fff786e2200 in ?? ()
#15 0x000000000000001d in ?? ()
#16 0xffffffffffffff88 in ?? ()
#17 0x00007f38fbe006c0 in ?? ()
#18 0x00007f38fbdfff70 in ?? ()
#19 0x00007f399a7f5180 in ?? ()
#20 0x00000000004bf3bd in default_fatal_uncaught_exception (exn=1) at printexc.c:130
#21 caml_fatal_uncaught_exception (exn=1) at printexc.c:155
#22 0x00000000018e5140 in ?? ()
#23 0x0000000000000000 in ?? ()

(gdb) disass 0x0000000000447ed9
Dump of assembler code for function caml_apply5:
   0x0000000000447ea0 <+0>:	sub    $0x28,%rsp
   0x0000000000447ea4 <+4>:	mov    0x8(%rcx),%r8
   0x0000000000447ea8 <+8>:	sar    $0x38,%r8
   0x0000000000447eac <+12>:	cmp    $0x5,%r8
   0x0000000000447eb0 <+16>:	jne    0x447ec0 <caml_apply5+32>
   0x0000000000447eb2 <+18>:	mov    0x10(%rcx),%r8
   0x0000000000447eb6 <+22>:	add    $0x28,%rsp
   0x0000000000447eba <+26>:	jmp    *%r8
   0x0000000000447ebd <+29>:	nopl   (%rax)
   0x0000000000447ec0 <+32>:	mov    %rdx,0x18(%rsp)
   0x0000000000447ec5 <+37>:	mov    %rsi,0x10(%rsp)
   0x0000000000447eca <+42>:	mov    %rdi,0x8(%rsp)
   0x0000000000447ecf <+47>:	mov    %rbx,(%rsp)
   0x0000000000447ed3 <+51>:	mov    (%rcx),%rdi
   0x0000000000447ed6 <+54>:	mov    %rcx,%rbx
=> 0x0000000000447ed9 <+57>:	call   *%rdi
   0x0000000000447edb <+59>:	mov    %rax,%rbx
   0x0000000000447ede <+62>:	mov    (%rbx),%rdi
   0x0000000000447ee1 <+65>:	mov    (%rsp),%rax
   0x0000000000447ee5 <+69>:	call   *%rdi
   0x0000000000447ee7 <+71>:	mov    %rax,%rbx
   0x0000000000447eea <+74>:	mov    (%rbx),%rdi
   0x0000000000447eed <+77>:	mov    0x8(%rsp),%rax
   0x0000000000447ef2 <+82>:	call   *%rdi
   0x0000000000447ef4 <+84>:	mov    %rax,%rbx
   0x0000000000447ef7 <+87>:	mov    (%rbx),%rdi
   0x0000000000447efa <+90>:	mov    0x10(%rsp),%rax
   0x0000000000447eff <+95>:	call   *%rdi
   0x0000000000447f01 <+97>:	mov    %rax,%rbx
   0x0000000000447f04 <+100>:	mov    (%rbx),%rdi
   0x0000000000447f07 <+103>:	mov    0x18(%rsp),%rax
   0x0000000000447f0c <+108>:	add    $0x28,%rsp
   0x0000000000447f10 <+112>:	jmp    *%rdi
End of assembler dump.
(gdb) print rdi
No symbol "rdi" in current context.
(gdb) info registers
rax            0x7ffff7be6fe8      140737349840872
rbx            0x466da0            4615584
rcx            0x466da0            4615584
rdx            0x7ffff7b49470      140737349194864
rsi            0x7ffff7b5e830      140737349281840
rdi            0x10438b4808ec8348  1171933469550084936
rbp            0x7ffec59fff70      0x7ffec59fff70
rsp            0x7ffec59ffc50      0x7ffec59ffc50
r8             0x0                 0
r9             0x502580            5252480
r10            0x4                 4
r11            0x7ffec59ffc10      140732214017040
r12            0x7ffff7b4c9c0      140737349208512
r13            0x7ffff7b4ca48      140737349208648
r14            0x55c2a0            5620384
r15            0x7ffff7b49468      140737349194856
rip            0x447ed9            0x447ed9 <caml_apply5+57>
eflags         0x10293             [ CF AF SF IF RF ]
cs             0x33                51
ss             0x2b                43
ds             0x0                 0
es             0x0                 0
fs             0x0                 0
gs             0x0                 0
fs_base        0x7ffec5a006c0      140732214019776
gs_base        0x0                 0
(gdb) print 0x10438b4808ec8348
$1 = 1171933469550084936
(gdb) print *0x10438b4808ec8348
Cannot access memory at address 0x10438b4808ec8348

Disabling compaction doesn't help and the debug runtime doesn't give any particularly useful clues either.

Perhaps we should move debugging this part to an ocaml/ocaml issue on 4.14 if it is not meant to SIGSEGV (which I asume is not, it is meant to raise the exception).