[PATCH 2/3] Improve vRun error reporting

[Pedro Alves <pedro@palves.net>]

After the previous commit, if starting the inferior process with "run"
(vRun packet) fails, GDBserver reports an error using the "E." verbose
error packet.  On the GDB side, however, GDB doesn't yet do anything
with verbose error strings when handling vRun errors.  This commit
fixes that.

This makes remote debugging output the same as native output, when
possible, another small step in the "local/remote parity" project.

E.g., before, against GNU/Linux GDBserver:

  (gdb) run
  Starting program: .../gdb.base/run-fail-twice/run-fail-twice.nox
  Running ".../gdb.base/run-fail-twice/run-fail-twice.nox" on the remote target failed

After, against GNU/Linux GDBserver (same as native):

  (gdb) run
  Starting program: .../gdb.base/run-fail-twice/run-fail-twice.nox
  During startup program exited with code 126.

Change-Id: Ib386f267522603f554b52a885b15229c9639e870
---
 gdb/remote.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/gdb/remote.c b/gdb/remote.c
index b58dbd4cb66..dca5add260a 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -2400,6 +2400,19 @@ add_packet_config_cmd (const unsigned int which_packet, const char *name,
     }
 }
 
+/* Strings that starts with "E.", are verbose error messages, like
+   "E.ERROR_MESSAGE".  If BUF is such an error message, return a
+   pointer to message after the period.  Otherwise return NULL.  */
+
+static const char *
+verbose_error_message (const char *buf)
+{
+  if (buf[0] == 'E' && buf[1] == '.')
+    return buf + 2;
+  else
+    return nullptr;
+}
+
 static enum packet_result
 packet_check_result (const char *buf)
 {
@@ -2415,7 +2428,7 @@ packet_check_result (const char *buf)
 
       /* Always treat "E." as an error.  This will be used for
 	 more verbose error messages, such as E.memtypes.  */
-      if (buf[0] == 'E' && buf[1] == '.')
+      if (verbose_error_message (buf) != nullptr)
 	return PACKET_ERROR;
 
       /* The packet may or may not be OK.  Just assume it is.  */
@@ -10502,7 +10515,13 @@ remote_target::extended_remote_run (const std::string &args)
     case PACKET_UNKNOWN:
       return -1;
     case PACKET_ERROR:
-      if (remote_exec_file[0] == '\0')
+      /* If we have a verbose error message, print just that.  This
+	 makes remote debugging output the same as native output, when
+	 possible.  */
+      if (const char *msg = verbose_error_message (rs->buf.data ());
+	  msg != nullptr)
+	error (("%s"), msg);
+      else if (remote_exec_file[0] == '\0')
 	error (_("Running the default executable on the remote target failed; "
 		 "try \"set remote exec-file\"?"));
       else
-- 
2.43.0