gdb: Catch exceptions when accessing source cache

The source_cache::get_line_charpos function can currently throw an
exception if the source file is missing, which doesn't match the
expected behaviour documented in the functions header file.  The
documented behaviour is to return false on failure, and this is how
the function appears to be used throughout GDB.

I spotted this in the 'info source' command, currently for a missing
source file you'll see something like this:

  (gdb) info source
  Current source file is /path/to/src/file.c
  Compilation directory is /path/to/build/
  /path/to/src/file.c: No such file or directory.
  (gdb)

After this patch we see this:

  (gdb) info source
  Current source file is /path/to/src/file.c
  Compilation directory is /path/to/build/
  Source language is c.
  Producer is COMPILER VERSION AND FLAGS.
  Compiled with DWARF 2 debugging format.
  Does not include preprocessor macro info.

We don't currently indicate that the source file can't be found, and
maybe that would be something worth adding in the future.

gdb/ChangeLog:

	* source-cache.c (source_cache::get_line_charpos): Catch
	exceptions and return false, this matches the behaviour documented
	in the header file.

gdb/testsuite/ChangeLog:

	* gdb.base/list-missing-source.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2019-09-17  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* source-cache.c (source_cache::get_line_charpos): Catch
+	exceptions and return false, this matches the behaviour documented
+	in the header file.
+
 2019-09-17  Joel Brobecker  <brobecker@adacore.com>
 
 	* ada-tasks.c (info_task): Remove quoting of the task's name.

gdb/source-cache.c

@@ -231,19 +231,26 @@ bool
 source_cache::get_line_charpos (struct symtab *s,
 				const std::vector<off_t> **offsets)
 {
-  std::string fullname = symtab_to_fullname (s);
-
-  auto iter = m_offset_cache.find (fullname);
-  if (iter == m_offset_cache.end ())
+  try
     {
-      ensure (s);
-      iter = m_offset_cache.find (fullname);
-      /* cache_source_text ensured this was entered.  */
-      gdb_assert (iter != m_offset_cache.end ());
-    }
+      std::string fullname = symtab_to_fullname (s);
 
-  *offsets = &iter->second;
-  return true;
+      auto iter = m_offset_cache.find (fullname);
+      if (iter == m_offset_cache.end ())
+	{
+	  ensure (s);
+	  iter = m_offset_cache.find (fullname);
+	  /* cache_source_text ensured this was entered.  */
+	  gdb_assert (iter != m_offset_cache.end ());
+	}
+
+      *offsets = &iter->second;
+      return true;
+    }
+  catch (const gdb_exception_error &e)
+    {
+      return false;
+    }
 }
 
 /* A helper function that extracts the desired source lines from TEXT,

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2019-09-17  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb.base/list-missing-source.exp: New file.
+
 2019-09-14  Tom de Vries  <tdevries@suse.de>
 
 	PR teststuite/24599

gdb/testsuite/gdb.base/list-missing-source.exp

@@ -0,0 +1,63 @@
+# Copyright 2019 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This test checks how GDB handles missing source files around the
+# 'list' and 'info source' commands.
+
+standard_testfile
+
+# Create a source file in the output directory.
+set srcfile [standard_output_file main.c]
+set fd [open "$srcfile" w]
+puts $fd {
+int
+main ()
+{
+  return 0;
+}
+}
+close $fd
+
+# Compile the source file.
+set options "debug"
+if  { [gdb_compile "${srcfile}" "${binfile}" \
+	   executable $options] != "" } {
+    untested "failed to compile"
+    return -1
+}
+
+# Now delete the source file.
+file delete $srcfile
+
+# Now start GDB, run to main and try to list the source.
+clean_restart ${binfile}
+
+if ![runto_main] then {
+    fail "can't run to main"
+    return 0
+}
+
+gdb_test "list" "1\[ \t\]+in\[ \t\]+$srcfile"
+
+gdb_test "info source" \
+    [multi_line \
+	 "info source" \
+	 "Current source file is $srcfile" \
+	 "Compilation directory is \[^\n\r\]+" \
+	 "Source language is c." \
+	 "Producer is \[^\n\r\]+" \
+	 "Compiled with DWARF $decimal debugging format." \
+	 "Does not include preprocessor macro info." ]
+