Improve error reporting when handling SystemTap SDT probes

This patch improves the error reporting when handling SystemTap SDT
probes.  "Handling", in this case, mostly means "parsing".

On gdb/probe.h, only trivial changes on functions' comments in order
to explicitly mention that some of them can throw exceptions.  This is
just to make the API a bit more clear.

On gdb/stap-probe.c, I have s/internal_error/error/ on two functions
that are responsible for parsing specific bits of the probes'
arguments: stap_get_opcode and stap_get_expected_argument_type.  It is
not correct to call internal_error on such situations because it is
not really GDB's fault if the probes have malformed arguments.  I also
improved the error reported on stap_get_expected_argument_type by also
including the probe name on it.

Aside from that, and perhaps most importantly, I added a check on
stap_get_arg to make sure that we don't try to extract an argument
from a probe that has no arguments.  This check issues an
internal_error, because it really means that GDB is doing something it
shouldn't.

Although it can be considered almost trivial, and despite the fact
that I am the maintainer for this part of the code, I am posting this
patch for review.  I will wait a few days, and if nobody has anything
to say, I will go ahead and push it.

gdb/ChangeLog:
2015-09-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	* probe.h (struct probe_ops) <get_probe_argument_count,
	evaluate_probe_argument, enable_probe, disable_probe>: Mention in
	the comment that the function can throw an exception.
	(get_probe_argument_count): Likewise.
	(evaluate_probe_argument): Likewise.
	* stap-probe.c (stap_get_opcode): Call error instead of
	internal_error.
	(stap_get_expected_argument_type): Likewise.  Add argument
	'probe'.  Improve error message by mentioning the probe's name.
	(stap_parse_probe_arguments): Adjust call to
	stap_get_expected_argument_type.
	(stap_get_arg): Add comment.  Assert that 'probe->args_parsed' is
	not zero.  Call internal_error if GDB requests an argument but the
	probe has no arguments.

gdb/ChangeLog

@@ -1,3 +1,20 @@
+2015-09-01  Sergio Durigan Junior  <sergiodj@redhat.com>
+
+	* probe.h (struct probe_ops) <get_probe_argument_count,
+	evaluate_probe_argument, enable_probe, disable_probe>: Mention in
+	the comment that the function can throw an exception.
+	(get_probe_argument_count): Likewise.
+	(evaluate_probe_argument): Likewise.
+	* stap-probe.c (stap_get_opcode): Call error instead of
+	internal_error.
+	(stap_get_expected_argument_type): Likewise.  Add argument
+	'probe'.  Improve error message by mentioning the probe's name.
+	(stap_parse_probe_arguments): Adjust call to
+	stap_get_expected_argument_type.
+	(stap_get_arg): Add comment.  Assert that 'probe->args_parsed' is
+	not zero.  Call internal_error if GDB requests an argument but the
+	probe has no arguments.
+
 2015-09-01  Pierre-Marie de Rodat  <derodat@adacore.com>
 
 	* ada-lang.c (ada_resolve_function): Do not ask the user what

gdb/probe.h

@@ -72,7 +72,8 @@ struct probe_ops
     CORE_ADDR (*get_probe_address) (struct probe *probe,
 				    struct objfile *objfile);
 
-    /* Return the number of arguments of PROBE.  */
+    /* Return the number of arguments of PROBE.  This function can
+       throw an exception.  */
 
     unsigned (*get_probe_argument_count) (struct probe *probe,
 					  struct frame_info *frame);
@@ -84,7 +85,8 @@ struct probe_ops
     int (*can_evaluate_probe_arguments) (struct probe *probe);
 
     /* Evaluate the Nth argument from the PROBE, returning a value
-       corresponding to it.  The argument number is represented N.  */
+       corresponding to it.  The argument number is represented N.
+       This function can throw an exception.  */
 
     struct value *(*evaluate_probe_argument) (struct probe *probe,
 					      unsigned n,
@@ -143,13 +145,15 @@ struct probe_ops
 
     /* Enable a probe.  The semantics of "enabling" a probe depend on
        the specific backend and the field can be NULL in case enabling
-       probes is not supported.  */
+       probes is not supported.  This function can throw an
+       exception.  */
 
     void (*enable_probe) (struct probe *probe);
 
     /* Disable a probe.  The semantics of "disabling" a probe depend
        on the specific backend and the field can be NULL in case
-       disabling probes is not supported.  */
+       disabling probes is not supported.  This function can throw an
+       exception.  */
 
     void (*disable_probe) (struct probe *probe);
   };
@@ -266,7 +270,9 @@ extern struct cmd_list_element **info_probes_cmdlist_get (void);
 extern CORE_ADDR get_probe_address (struct probe *probe,
 				    struct objfile *objfile);
 
-/* Return the argument count of the specified probe.  */
+/* Return the argument count of the specified probe.
+
+   This function can throw an exception.  */
 
 extern unsigned get_probe_argument_count (struct probe *probe,
 					  struct frame_info *frame);
@@ -278,7 +284,9 @@ extern unsigned get_probe_argument_count (struct probe *probe,
 extern int can_evaluate_probe_arguments (struct probe *probe);
 
 /* Evaluate argument N of the specified probe.  N must be between 0
-   inclusive and get_probe_argument_count exclusive.  */
+   inclusive and get_probe_argument_count exclusive.
+
+   This function can throw an exception.  */
 
 extern struct value *evaluate_probe_argument (struct probe *probe,
 					      unsigned n,

gdb/stap-probe.c

@@ -313,8 +313,7 @@ stap_get_opcode (const char **s)
       break;
 
     default:
-      internal_error (__FILE__, __LINE__,
-		      _("Invalid opcode in expression `%s' for SystemTap"
+      error (_("Invalid opcode in expression `%s' for SystemTap"
 	       "probe"), *s);
     }
 
@@ -326,7 +325,8 @@ stap_get_opcode (const char **s)
 
 static struct type *
 stap_get_expected_argument_type (struct gdbarch *gdbarch,
-				 enum stap_arg_bitness b)
+				 enum stap_arg_bitness b,
+				 const struct stap_probe *probe)
 {
   switch (b)
     {
@@ -361,8 +361,8 @@ stap_get_expected_argument_type (struct gdbarch *gdbarch,
       return builtin_type (gdbarch)->builtin_uint64;
 
     default:
-      internal_error (__FILE__, __LINE__,
-		      _("Undefined bitness for probe."));
+      error (_("Undefined bitness for probe '%s'."),
+	     probe->p.name);
       break;
     }
 }
@@ -1172,7 +1172,8 @@ stap_parse_probe_arguments (struct stap_probe *probe, struct gdbarch *gdbarch)
       else
 	arg.bitness = STAP_ARG_BITNESS_UNDEFINED;
 
-      arg.atype = stap_get_expected_argument_type (gdbarch, arg.bitness);
+      arg.atype = stap_get_expected_argument_type (gdbarch, arg.bitness,
+						   probe);
 
       expr = stap_parse_argument (&cur, arg.atype, gdbarch);
 
@@ -1278,12 +1279,26 @@ stap_is_operator (const char *op)
   return ret;
 }
 
+/* Return argument N of probe PROBE.
+
+   If the probe's arguments have not been parsed yet, parse them.  If
+   there are no arguments, throw an exception (error).  Otherwise,
+   return the requested argument.  */
+
 static struct stap_probe_arg *
 stap_get_arg (struct stap_probe *probe, unsigned n, struct gdbarch *gdbarch)
 {
   if (!probe->args_parsed)
     stap_parse_probe_arguments (probe, gdbarch);
 
+  gdb_assert (probe->args_parsed);
+  if (probe->args_u.vec == NULL)
+    internal_error (__FILE__, __LINE__,
+		    _("Probe '%s' apparently does not have arguments, but \n"
+		      "GDB is requesting its argument number %u anyway.  "
+		      "This should not happen.  Please report this bug."),
+		    probe->p.name, n);
+
   return VEC_index (stap_probe_arg_s, probe->args_u.vec, n);
 }