[rfc] PR 20569, segv in follow_exec

diff --git a/gdb/exceptions.c b/gdb/exceptions.c
index 9a10f66..402a5af 100644
--- a/gdb/exceptions.c
+++ b/gdb/exceptions.c
@@ -256,3 +256,16 @@  catch_errors (catch_errors_ftype *func, void *func_args, char *errstring,
     return 0;
   return val;
 }
+
+/* Returns non-zero if exceptions E1 and E2 are equal.  Returns zero
+   otherwise.  */
+
+int
+exception_print_same (struct gdb_exception e1, struct gdb_exception e2)
+{
+  return (e1.reason == e2.reason
+	  && e1.error == e2.error
+	  && (e1.message == e2.message
+	      || (e1.message && e2.message
+		  && strcmp (e1.message, e2.message) == 0)));
+}
diff --git a/gdb/exceptions.h b/gdb/exceptions.h
index 5711c1a..a2d39d7 100644
--- a/gdb/exceptions.h
+++ b/gdb/exceptions.h
@@ -88,4 +88,7 @@  extern int catch_exceptions_with_msg (struct ui_out *uiout,
 typedef int (catch_errors_ftype) (void *);
 extern int catch_errors (catch_errors_ftype *, void *, char *, return_mask);
 
+/* Compare two exception objects for print equality.  */
+extern int exception_print_same (struct gdb_exception e1,
+				 struct gdb_exception e2);
 #endif
diff --git a/gdb/exec.c b/gdb/exec.c
index d858e99..b418ef3 100644
--- a/gdb/exec.c
+++ b/gdb/exec.c
@@ -139,39 +139,23 @@  exec_file_clear (int from_tty)
 /* Returns non-zero if exceptions E1 and E2 are equal.  Returns zero
    otherwise.  */
 
-static int
-exception_print_same (struct gdb_exception e1, struct gdb_exception e2)
-{
-  const char *msg1 = e1.message;
-  const char *msg2 = e2.message;
-
-  if (msg1 == NULL)
-    msg1 = "";
-  if (msg2 == NULL)
-    msg2 = "";
-
-  return (e1.reason == e2.reason
-	  && e1.error == e2.error
-	  && strcmp (e1.message, e2.message) == 0);
-}
-
 /* See gdbcore.h.  */
 
 void
 exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
 {
-  char *exec_file, *full_exec_path = NULL;
+  char *exec_file_target, *exec_file_host = NULL;
   struct cleanup *old_chain;
   struct gdb_exception prev_err = exception_none;
 
   /* Do nothing if we already have an executable filename.  */
-  exec_file = (char *) get_exec_file (0);
-  if (exec_file != NULL)
+  exec_file_target = (char *) get_exec_file (0);
+  if (exec_file_target != NULL)
     return;
 
   /* Try to determine a filename from the process itself.  */
-  exec_file = target_pid_to_exec_file (pid);
-  if (exec_file == NULL)
+  exec_file_target = target_pid_to_exec_file (pid);
+  if (exec_file_target == NULL)
     {
       warning (_("No executable has been specified and target does not "
 		 "support\n"
@@ -180,28 +164,8 @@  exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
       return;
     }
 
-  /* If gdb_sysroot is not empty and the discovered filename
-     is absolute then prefix the filename with gdb_sysroot.  */
-  if (*gdb_sysroot != '\0' && IS_ABSOLUTE_PATH (exec_file))
-    {
-      full_exec_path = exec_file_find (exec_file, NULL);
-      if (full_exec_path == NULL)
-	return;
-    }
-  else
-    {
-      /* It's possible we don't have a full path, but rather just a
-	 filename.  Some targets, such as HP-UX, don't provide the
-	 full path, sigh.
-
-	 Attempt to qualify the filename against the source path.
-	 (If that fails, we'll just fall back on the original
-	 filename.  Not much more we can do...)  */
-      if (!source_full_path_of (exec_file, &full_exec_path))
-	full_exec_path = xstrdup (exec_file);
-    }
-
-  old_chain = make_cleanup (xfree, full_exec_path);
+  exec_file_host = exec_file_find (exec_file_target, NULL);
+  old_chain = make_cleanup (xfree, exec_file_target);
   make_cleanup (free_current_contents, &prev_err.message);
 
   /* exec_file_attach and symbol_file_add_main may throw an error if the file
@@ -217,7 +181,9 @@  exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
      errors/exceptions in the following code.  */
   TRY
     {
-      exec_file_attach (full_exec_path, from_tty);
+      /* We must do this step even if exec_file_host is NULL, so that
+	 exec_file_attach will clear state.  */
+      exec_file_attach (exec_file_host, from_tty);
     }
   CATCH (err, RETURN_MASK_ERROR)
     {
@@ -231,19 +197,22 @@  exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
     }
   END_CATCH
 
-  TRY
-    {
-      if (defer_bp_reset)
-	current_inferior ()->symfile_flags |= SYMFILE_DEFER_BP_RESET;
-      symbol_file_add_main (full_exec_path, from_tty);
-    }
-  CATCH (err, RETURN_MASK_ERROR)
+  if (exec_file_host)
     {
-      if (!exception_print_same (prev_err, err))
-	warning ("%s", err.message);
-    }
-  END_CATCH
-  current_inferior ()->symfile_flags &= ~SYMFILE_DEFER_BP_RESET;
+      TRY
+	{
+	  if (defer_bp_reset)
+	    current_inferior ()->symfile_flags |= SYMFILE_DEFER_BP_RESET;
+	  symbol_file_add_main (exec_file_host, from_tty);
+	}
+      CATCH (err, RETURN_MASK_ERROR)
+	{
+	  if (!exception_print_same (prev_err, err))
+	    warning ("%s", err.message);
+	}
+      END_CATCH
+    current_inferior ()->symfile_flags &= ~SYMFILE_DEFER_BP_RESET;
+  }
 
   do_cleanups (old_chain);
 }
diff --git a/gdb/infrun.c b/gdb/infrun.c
index 2636a19..a365a6b 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1077,15 +1077,18 @@  show_follow_exec_mode_string (struct ui_file *file, int from_tty,
   fprintf_filtered (file, _("Follow exec mode is \"%s\".\n"),  value);
 }
 
-/* EXECD_PATHNAME is assumed to be non-NULL.  */
+/* EXEC_FILE_TARGET is assumed to be non-NULL.  */
 
 static void
-follow_exec (ptid_t ptid, char *execd_pathname)
+follow_exec (ptid_t ptid, char *exec_file_target)
 {
   struct thread_info *th, *tmp;
   struct inferior *inf = current_inferior ();
   int pid = ptid_get_pid (ptid);
   ptid_t process_ptid;
+  char *exec_file_host = NULL;
+  struct cleanup *old_chain;
+  struct gdb_exception prev_err = exception_none;
 
   /* This is an exec event that we actually wish to pay attention to.
      Refresh our symbol table to the newly exec'd program, remove any
@@ -1155,7 +1158,7 @@  follow_exec (ptid_t ptid, char *execd_pathname)
   process_ptid = pid_to_ptid (pid);
   printf_unfiltered (_("%s is executing new program: %s\n"),
 		     target_pid_to_str (process_ptid),
-		     execd_pathname);
+		     exec_file_target);
 
   /* We've followed the inferior through an exec.  Therefore, the
      inferior has essentially been killed & reborn.  */
@@ -1164,14 +1167,18 @@  follow_exec (ptid_t ptid, char *execd_pathname)
 
   breakpoint_init_inferior (inf_execd);
 
-  if (*gdb_sysroot != '\0')
-    {
-      char *name = exec_file_find (execd_pathname, NULL);
+  exec_file_host = exec_file_find (exec_file_target, NULL);
+  old_chain = make_cleanup (xfree, exec_file_host);
+  make_cleanup (free_current_contents, &prev_err.message);
 
-      execd_pathname = (char *) alloca (strlen (name) + 1);
-      strcpy (execd_pathname, name);
-      xfree (name);
-    }
+  /* If we were unable to map the executable target pathname onto a host
+     pathname, tell the user that.  Otherwise GDB's subsequent behavior
+     is confusing.  Maybe it would even be better to stop at this point
+     so that the user can specify a file manually before continuing.  */
+  if (!exec_file_host)
+    warning (_("Could not load symbols for executable %s.\n"
+	       "Do you need \"set sysroot\"?"),
+	     exec_file_target);
 
   /* Reset the shared library package.  This ensures that we get a
      shlib event when the child reaches "_start", at which point the
@@ -1193,7 +1200,7 @@  follow_exec (ptid_t ptid, char *execd_pathname)
 
       inf = add_inferior_with_spaces ();
       inf->pid = pid;
-      target_follow_exec (inf, execd_pathname);
+      target_follow_exec (inf, exec_file_target);
 
       set_current_inferior (inf);
       set_current_program_space (inf->pspace);
@@ -1212,22 +1219,62 @@  follow_exec (ptid_t ptid, char *execd_pathname)
 
   gdb_assert (current_program_space == inf->pspace);
 
-  /* That a.out is now the one to use.  */
-  exec_file_attach (execd_pathname, 0);
+  /* exec_file_attach and symbol_file_add_main may throw an error if the file
+     cannot be opened either locally or remotely.
+
+     This happens for example, when the file is first found in the local
+     sysroot (above), and then disappears (a TOCTOU race), or when it doesn't
+     exist in the target filesystem, or when the file does exist, but
+     is not readable.
+
+     Even without a symbol file, the remote-based debugging session should
+     continue normally instead of ending abruptly.  Hence we catch thrown
+     errors/exceptions in the following code.  */
+  TRY
+    {
+      /* We must do this step even if exec_file_host is NULL, so that
+	 exec_file_attach will clear state.  */
+      exec_file_attach (exec_file_host, 0);
+    }
+  CATCH (err, RETURN_MASK_ERROR)
+    {
+      if (err.message != NULL)
+	warning ("%s", err.message);
+
+      prev_err = err;
+
+      /* Save message so it doesn't get trashed by the catch below.  */
+      prev_err.message = xstrdup (err.message);
+    }
+  END_CATCH
+
+  if (exec_file_host)
+    {
+      TRY
+	{
+	  symbol_file_add (exec_file_host,
+			   (inf->symfile_flags
+			    | SYMFILE_MAINLINE | SYMFILE_DEFER_BP_RESET),
+			   NULL, 0);
+	  
+	  if ((inf->symfile_flags & SYMFILE_NO_READ) == 0)
+	    set_initial_language ();
+	}
+      CATCH (err, RETURN_MASK_ERROR)
+	{
+	  if (!exception_print_same (prev_err, err))
+	    warning ("%s", err.message);
+	}
+      END_CATCH
+    }
+
+  do_cleanups (old_chain);
 
   /* SYMFILE_DEFER_BP_RESET is used as the proper displacement for PIE
      (Position Independent Executable) main symbol file will get applied by
      solib_create_inferior_hook below.  breakpoint_re_set would fail to insert
      the breakpoints with the zero displacement.  */
 
-  symbol_file_add (execd_pathname,
-		   (inf->symfile_flags
-		    | SYMFILE_MAINLINE | SYMFILE_DEFER_BP_RESET),
-		   NULL, 0);
-
-  if ((inf->symfile_flags & SYMFILE_NO_READ) == 0)
-    set_initial_language ();
-
   /* If the target can specify a description, read it.  Must do this
      after flipping to the new executable (because the target supplied
      description must be compatible with the executable's
diff --git a/gdb/solib.c b/gdb/solib.c
index 2235505..cfcbd5d 100644
--- a/gdb/solib.c
+++ b/gdb/solib.c
@@ -388,13 +388,17 @@  solib_find_1 (char *in_pathname, int *fd, int is_solib)
 char *
 exec_file_find (char *in_pathname, int *fd)
 {
-  char *result = solib_find_1 (in_pathname, fd, 0);
+  char *result;
+  const char *fskind = effective_target_file_system_kind ();
+
+  if (!in_pathname)
+    return NULL;
 
-  if (result == NULL)
+  if (*gdb_sysroot != '\0' && IS_TARGET_ABSOLUTE_PATH (fskind, in_pathname))
     {
-      const char *fskind = effective_target_file_system_kind ();
+      result = solib_find_1 (in_pathname, fd, 0);
 
-      if (fskind == file_system_kind_dos_based)
+      if (result == NULL && fskind == file_system_kind_dos_based)
 	{
 	  char *new_pathname;
 
@@ -405,6 +409,21 @@  exec_file_find (char *in_pathname, int *fd)
 	  result = solib_find_1 (new_pathname, fd, 0);
 	}
     }
+  else
+    {
+      /* It's possible we don't have a full path, but rather just a
+	 filename.  Some targets, such as HP-UX, don't provide the
+	 full path, sigh.
+
+	 Attempt to qualify the filename against the source path.
+	 (If that fails, we'll just fall back on the original
+	 filename.  Not much more we can do...)  */
+
+      if (!source_full_path_of (in_pathname, &result))
+	result = xstrdup (in_pathname);
+      if (fd != NULL)
+	*fd = -1;
+    }
 
   return result;
 }