OCAMLRUNPARAM=b=2 => load debuginfo

Load the debug information during runtime startup if OCAMLRUNPARAM=b=2.
This guards against the specific case of running out of fds, since the
debug information can't then be loaded.

Changes

@@ -65,7 +65,8 @@ OCaml 4.12.0
 
 ### Runtime system:
 
-- #2195: Improve error message in bytecode stack trace printing
+- #2195: Improve error message in bytecode stack trace printing and load
+  debug information during bytecode startup if OCAMLRUNPARAM=b=2.
   (David Allsopp, review by Gabriel Scherer and Xavier Leroy)
 
 - #9756: garbage collector colors change

manual/manual/cmds/runtime.etex

@@ -112,8 +112,13 @@ The following environment variables are also consulted:
 \fi
   \begin{options}
   \item[b] (backtrace) Trigger the printing of a stack backtrace
-        when an uncaught exception aborts the program.
-        This option takes no argument.
+        when an uncaught exception aborts the program. An optional argument can
+        be provided: "b=0" turns backtrace printing off; "b=1" is equivalent to
+        "b" and turns backtrace printing on; "b=2" turns backtrace printing on
+        and forces the runtime system to load debugging information at program
+        startup time instead of at backtrace printing time. "b=2" can be used if
+        the runtime is unable to load debugging information at backtrace
+        printing time, for example if there are no file descriptors available.
   \item[p] (parser trace) Turn on debugging support for
         "ocamlyacc"-generated parsers.  When this option is on,
         the pushdown automaton that executes the parsers prints a

runtime/backtrace.c

@@ -123,6 +123,7 @@ CAMLexport void caml_print_exception_backtrace(void)
     }
   }
 
+  /* See also printexc.ml */
   switch (caml_debug_info_status()) {
   case FILE_NOT_FOUND:
     fprintf(stderr,
@@ -143,7 +144,7 @@ CAMLexport void caml_print_exception_backtrace(void)
     fprintf(stderr,
             "(Cannot print locations:\n "
              "bytecode executable program file cannot be opened;\n "
-             "-- too many open files)\n");
+             "-- too many open files. Try running with OCAMLRUNPARAM=b=2)\n");
     break;
   }
 }

runtime/backtrace_byt.c

@@ -421,6 +421,13 @@ CAMLexport void caml_init_debug_info(void)
   caml_add_debug_info(caml_start_code, Val_long(caml_code_size), Val_unit);
 }
 
+CAMLexport void caml_load_main_debug_info(void)
+{
+  if (Caml_state->backtrace_active > 1) {
+    read_main_debug_info(caml_debug_info.contents[0]);
+  }
+}
+
 int caml_debug_info_available(void)
 {
   return (caml_debug_info.size != 0);

runtime/caml/backtrace.h

@@ -109,6 +109,7 @@ CAMLextern char_os * caml_cds_file;
  * different prototype. */
 extern void caml_stash_backtrace(value exn, value * sp, int reraise);
 
+CAMLextern void caml_load_main_debug_info(void);
 #endif
 
 

runtime/startup_aux.c

@@ -111,7 +111,7 @@ void caml_parse_ocamlrunparam(void)
       switch (*opt++){
       case 'a': scanmult (opt, &p); caml_set_allocation_policy ((intnat) p);
         break;
-      case 'b': scanmult (opt, &p); caml_record_backtrace(Val_bool (p));
+      case 'b': scanmult (opt, &p); caml_record_backtrace(Val_int (p));
         break;
       case 'c': scanmult (opt, &p); caml_cleanup_on_exit = (p != 0); break;
       case 'h': scanmult (opt, &caml_init_heap_wsz); break;

runtime/startup_byt.c

@@ -454,6 +454,8 @@ CAMLexport void caml_main(char_os **argv)
   caml_oldify_mopup ();
   /* Initialize system libraries */
   caml_sys_init(exe_name, argv + pos);
+  /* Load debugging info, if b>=2 */
+  caml_load_main_debug_info();
 #ifdef _WIN32
   /* Start a thread to handle signals */
   if (caml_secure_getenv(T("CAMLSIGPIPE")))
@@ -545,6 +547,8 @@ CAMLexport value caml_startup_code_exn(
   caml_section_table_size = section_table_size;
   /* Initialize system libraries */
   caml_sys_init(exe_name, argv);
+  /* Load debugging info, if b>=2 */
+  caml_load_main_debug_info();
   /* Execute the program */
   caml_debugger(PROGRAM_START, Val_unit);
   return caml_interprete(caml_start_code, caml_code_size);

stdlib/printexc.ml

@@ -301,7 +301,7 @@ let errors = [| "";
   (* NO_FDS *)
   "(Cannot print locations:\n \
       bytecode executable program file cannot be opened;\n \
-      -- too many open files)"
+      -- too many open files. Try running with OCAMLRUNPARAM=b=2)"
 |]
 
 let default_uncaught_exception_handler exn raw_backtrace =

testsuite/tests/backtrace/pr2195-locs.byte.reference

@@ -0,0 +1,4 @@
+Fatal error: exception Stdlib.Exit
+Raised by primitive operation at Stdlib.open_in_gen in file "stdlib.ml", line 399, characters 28-54
+Called from Pr2195 in file "pr2195.ml", line 24, characters 6-19
+Re-raised at Pr2195 in file "pr2195.ml", line 29, characters 4-41

testsuite/tests/backtrace/pr2195-nolocs.byte.reference

@@ -3,4 +3,4 @@ Raised by primitive operation at unknown location
 Called from unknown location
 (Cannot print locations:
  bytecode executable program file cannot be opened;
- -- too many open files)
+ -- too many open files. Try running with OCAMLRUNPARAM=b=2)

testsuite/tests/backtrace/pr2195.ml

@@ -2,7 +2,14 @@
    flags += "-g"
    exit_status = "2"
    * bytecode
-     reference = "${test_source_directory}/pr2195.byte.reference"
+     ocamlrunparam += ",b=0"
+     reference = "${test_source_directory}/pr2195-nolocs.byte.reference"
+   * bytecode
+     ocamlrunparam += ",b=1"
+     reference = "${test_source_directory}/pr2195-nolocs.byte.reference"
+   * bytecode
+     ocamlrunparam += ",b=2"
+     reference = "${test_source_directory}/pr2195-locs.byte.reference"
    * native
      reference = "${test_source_directory}/pr2195.opt.reference"
      compare_programs = "false"

testsuite/tests/backtrace/pr2195.opt.reference

@@ -1,5 +1,5 @@
 Fatal error: exception Stdlib.Exit
 Raised by primitive operation at Stdlib.open_in_gen in file "stdlib.ml", line 399, characters 28-54
 Called from Stdlib.open_in in file "stdlib.ml" (inlined), line 404, characters 2-45
-Called from Pr2195 in file "pr2195.ml", line 17, characters 6-19
-Re-raised at Pr2195 in file "pr2195.ml", line 22, characters 4-41
+Called from Pr2195 in file "pr2195.ml", line 24, characters 6-19
+Re-raised at Pr2195 in file "pr2195.ml", line 29, characters 4-41