[gdb/breakpoint] Handle setting breakpoint on label without address

Consider test-case test.c:
...
$ cat test.c
int main (void) {
  return 0;
 L1:
  (void)0;
}
...

Compiled with debug info:
...
$ gcc test.c -g
...

When attempting to set a breakpoint at L1, which is a label without address:
...
 <1><f4>: Abbrev Number: 2 (DW_TAG_subprogram)
    <f5>   DW_AT_name        : main
 <2><115>: Abbrev Number: 3 (DW_TAG_label)
    <116>   DW_AT_name        : L1
    <119>   DW_AT_decl_file   : 1
    <11a>   DW_AT_decl_line   : 5
 <2><11b>: Abbrev Number: 0
...
we run into an internal-error:
...
$ gdb -batch a.out -ex "b main:L1"
linespec.c:3233: internal-error: void \
  decode_line_full(const event_location*, int, program_space*, symtab*, \
  int, linespec_result*, const char*, const char*): \
  Assertion `result.size () == 1 || canonical->pre_expanded' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
...

Fix this by detecting the error condition in decode_line_full instead, and
throwing an error, such that we have instead:
...
(gdb) b main:L1
Location main:L1 not available
(gdb)
...

Unfortunately, to call event_location_to_string, which is used to get the
location name in the error message, we need to pass a non-const struct
event_location, because the call may cache the string in the struct (See
EL_STRING).  So, we change the prototype of decode_line_full accordingly, and
everywhere this propages to.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-08-28  Tom de Vries  <tdevries@suse.de>

	PR breakpoint/26544
	* breakpoint.c (parse_breakpoint_sals): Remove const from struct
	event_location.
	(create_breakpoint): Same.
	(base_breakpoint_decode_location): Same.
	(bkpt_create_sals_from_location): Same.
	(bkpt_decode_location): Same.
	(bkpt_probe_create_sals_from_location): Same.
	(bkpt_probe_decode_location): Same.
	(tracepoint_create_sals_from_location): Same.
	(tracepoint_decode_location): Same.
	(tracepoint_probe_decode_location): Same.
	(strace_marker_create_sals_from_location): Same.
	(strace_marker_decode_location): Same.
	(create_sals_from_location_default): Same.
	(decode_location_default): Same.
	* breakpoint.h (struct breakpoint_ops): Same.
	(create_breakpoint): Same.
	* linespec.h (decode_line_full): Same.
	* linespec.c (decode_line_full): Same.  Throw error if
	result.size () == 0.

gdb/testsuite/ChangeLog:

2020-08-28  Tom de Vries  <tdevries@suse.de>

	* gdb.base/label-without-address.c: New test.
	* gdb.base/label-without-address.exp: New file.

1 files changed, 18 insertions, 18 deletions

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 8f75618..670cba0 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -89,7 +89,7 @@ static void map_breakpoint_numbers (const char *,
 static void breakpoint_re_set_default (struct breakpoint *);
 
 static void
-  create_sals_from_location_default (const struct event_location *location,
+  create_sals_from_location_default (struct event_location *location,
 				     struct linespec_result *canonical,
 				     enum bptype type_wanted);
 
@@ -104,7 +104,7 @@ static void create_breakpoints_sal_default (struct gdbarch *,
 					    int, int, int, unsigned);
 
 static std::vector<symtab_and_line> decode_location_default
-  (struct breakpoint *b, const struct event_location *location,
+  (struct breakpoint *b, struct event_location *location,
    struct program_space *search_pspace);
 
 static int can_use_hardware_watchpoint
@@ -8948,7 +8948,7 @@ create_breakpoints_sal (struct gdbarch *gdbarch,
    the caller's responsibility to free them.  */
 
 static void
-parse_breakpoint_sals (const struct event_location *location,
+parse_breakpoint_sals (struct event_location *location,
 		       struct linespec_result *canonical)
 {
   struct symtab_and_line cursal;
@@ -9213,7 +9213,7 @@ breakpoint_ops_for_event_location (const struct event_location *location,
 
 int
 create_breakpoint (struct gdbarch *gdbarch,
-		   const struct event_location *location,
+		   struct event_location *location,
 		   const char *cond_string,
 		   int thread, const char *extra_string,
 		   int parse_extra,
@@ -12266,7 +12266,7 @@ base_breakpoint_print_recreate (struct breakpoint *b, struct ui_file *fp)
 
 static void
 base_breakpoint_create_sals_from_location
-  (const struct event_location *location,
+  (struct event_location *location,
    struct linespec_result *canonical,
    enum bptype type_wanted)
 {
@@ -12291,7 +12291,7 @@ base_breakpoint_create_breakpoints_sal (struct gdbarch *gdbarch,
 
 static std::vector<symtab_and_line>
 base_breakpoint_decode_location (struct breakpoint *b,
-				 const struct event_location *location,
+				 struct event_location *location,
 				 struct program_space *search_pspace)
 {
   internal_error_pure_virtual_called ();
@@ -12514,7 +12514,7 @@ bkpt_print_recreate (struct breakpoint *tp, struct ui_file *fp)
 }
 
 static void
-bkpt_create_sals_from_location (const struct event_location *location,
+bkpt_create_sals_from_location (struct event_location *location,
 				struct linespec_result *canonical,
 				enum bptype type_wanted)
 {
@@ -12545,7 +12545,7 @@ bkpt_create_breakpoints_sal (struct gdbarch *gdbarch,
 
 static std::vector<symtab_and_line>
 bkpt_decode_location (struct breakpoint *b,
-		      const struct event_location *location,
+		      struct event_location *location,
 		      struct program_space *search_pspace)
 {
   return decode_location_default (b, location, search_pspace);
@@ -12718,7 +12718,7 @@ bkpt_probe_remove_location (struct bp_location *bl,
 }
 
 static void
-bkpt_probe_create_sals_from_location (const struct event_location *location,
+bkpt_probe_create_sals_from_location (struct event_location *location,
 				      struct linespec_result *canonical,
 				      enum bptype type_wanted)
 {
@@ -12732,7 +12732,7 @@ bkpt_probe_create_sals_from_location (const struct event_location *location,
 
 static std::vector<symtab_and_line>
 bkpt_probe_decode_location (struct breakpoint *b,
-			    const struct event_location *location,
+			    struct event_location *location,
 			    struct program_space *search_pspace)
 {
   std::vector<symtab_and_line> sals = parse_probes (location, search_pspace, NULL);
@@ -12826,7 +12826,7 @@ tracepoint_print_recreate (struct breakpoint *self, struct ui_file *fp)
 }
 
 static void
-tracepoint_create_sals_from_location (const struct event_location *location,
+tracepoint_create_sals_from_location (struct event_location *location,
 				      struct linespec_result *canonical,
 				      enum bptype type_wanted)
 {
@@ -12857,7 +12857,7 @@ tracepoint_create_breakpoints_sal (struct gdbarch *gdbarch,
 
 static std::vector<symtab_and_line>
 tracepoint_decode_location (struct breakpoint *b,
-			    const struct event_location *location,
+			    struct event_location *location,
 			    struct program_space *search_pspace)
 {
   return decode_location_default (b, location, search_pspace);
@@ -12869,7 +12869,7 @@ struct breakpoint_ops tracepoint_breakpoint_ops;
 
 static void
 tracepoint_probe_create_sals_from_location
-  (const struct event_location *location,
+  (struct event_location *location,
    struct linespec_result *canonical,
    enum bptype type_wanted)
 {
@@ -12879,7 +12879,7 @@ tracepoint_probe_create_sals_from_location
 
 static std::vector<symtab_and_line>
 tracepoint_probe_decode_location (struct breakpoint *b,
-				  const struct event_location *location,
+				  struct event_location *location,
 				  struct program_space *search_pspace)
 {
   /* We use the same method for breakpoint on probes.  */
@@ -12960,7 +12960,7 @@ dprintf_after_condition_true (struct bpstats *bs)
    markers (`-m').  */
 
 static void
-strace_marker_create_sals_from_location (const struct event_location *location,
+strace_marker_create_sals_from_location (struct event_location *location,
 					 struct linespec_result *canonical,
 					 enum bptype type_wanted)
 {
@@ -13030,7 +13030,7 @@ strace_marker_create_breakpoints_sal (struct gdbarch *gdbarch,
 
 static std::vector<symtab_and_line>
 strace_marker_decode_location (struct breakpoint *b,
-			       const struct event_location *location,
+			       struct event_location *location,
 			       struct program_space *search_pspace)
 {
   struct tracepoint *tp = (struct tracepoint *) b;
@@ -13713,7 +13713,7 @@ breakpoint_re_set_default (struct breakpoint *b)
    calls parse_breakpoint_sals.  Return 1 for success, zero for failure.  */
 
 static void
-create_sals_from_location_default (const struct event_location *location,
+create_sals_from_location_default (struct event_location *location,
 				   struct linespec_result *canonical,
 				   enum bptype type_wanted)
 {
@@ -13750,7 +13750,7 @@ create_breakpoints_sal_default (struct gdbarch *gdbarch,
 
 static std::vector<symtab_and_line>
 decode_location_default (struct breakpoint *b,
-			 const struct event_location *location,
+			 struct event_location *location,
 			 struct program_space *search_pspace)
 {
   struct linespec_result canonical;