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.

1 files changed, 22 insertions, 7 deletions

diff --git a/gdb/stap-probe.c b/gdb/stap-probe.c
index 266402e..912505c 100644
--- a/gdb/stap-probe.c
+++ b/gdb/stap-probe.c
@@ -313,9 +313,8 @@ stap_get_opcode (const char **s)
       break;
 
     default:
-      internal_error (__FILE__, __LINE__,
-		      _("Invalid opcode in expression `%s' for SystemTap"
-			"probe"), *s);
+      error (_("Invalid opcode in expression `%s' for SystemTap"
+	       "probe"), *s);
     }
 
   return op;
@@ -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);
 }