[gdb/testsuite] Make gdb.base/dbx.exp more robust

Test-case gdb.base/dbx.exp overrides:
- the GDBFLAGS variable
- the gdb_file_cmd proc

There's code at the end of the test-case to restore both, but that's not
guaranteed to be executed.

Fix this by:
- using save_vars to restore GDBFLAGS
- using a new proc with_override to restore gdb_file_cmd

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-11  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (with_override): New proc, factored out of ...
	* gdb.base/dbx.exp: ... here.  Use with_override and save_vars.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-06-11  Tom de Vries  <tdevries@suse.de>
+
+	* lib/gdb.exp (with_override): New proc, factored out of ...
+	* gdb.base/dbx.exp: ... here.  Use with_override and save_vars.
+
 2020-06-10  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.ada/ptype_union.exp: Remove PR24713 workaround.

gdb/testsuite/gdb.base/dbx.exp

@@ -147,11 +147,8 @@ proc dbx_reinitialize_dir { subdir } {
 # right sequence of events, allowing gdb_load to do its normal thing? This way
 # remotes and simulators will work, too.
 #
-# [drow 2002-03-30]: We can restore the old gdb_file_cmd afterwards, though.
-set old_gdb_file_cmd_args [info args gdb_file_cmd]
-set old_gdb_file_cmd_body [info body gdb_file_cmd]
 
-proc gdb_file_cmd {arg} {
+proc local_gdb_file_cmd {arg} {
     global loadpath
     global loadfile
     global GDB
@@ -286,24 +283,24 @@ proc test_func { } {
 # Start with a fresh gdb.
 
 gdb_exit
-global GDBFLAGS
-set saved_gdbflags $GDBFLAGS
 
-set GDBFLAGS "$GDBFLAGS --dbx"
-gdb_start
-dbx_reinitialize_dir $srcdir/$subdir
-gdb_load ${binfile}
+with_override gdb_file_cmd local_gdb_file_cmd {
+    save_vars GDBFLAGS {
+	set GDBFLAGS "$GDBFLAGS --dbx"
 
-test_breakpoints
-test_assign
-test_whereis
-gdb_test "file average.c:1" "1\[ \t\]+/. This is a sample program.*"
-test_func
+	gdb_start
+	dbx_reinitialize_dir $srcdir/$subdir
+	gdb_load ${binfile}
 
-#exit and cleanup
-gdb_exit
+	test_breakpoints
+	test_assign
+	test_whereis
+	gdb_test "file average.c:1" "1\[ \t\]+/. This is a sample program.*"
+	test_func
 
-set GDBFLAGS $saved_gdbflags
-eval proc gdb_file_cmd {$old_gdb_file_cmd_args} {$old_gdb_file_cmd_body}
+	#exit and cleanup
+	gdb_exit
+    }
+}
 
 return 0

gdb/testsuite/lib/gdb.exp

@@ -7200,5 +7200,48 @@ proc hex_in_list { val hexlist } {
     return [expr $index != -1]
 }
 
+# Override proc NAME to proc OVERRIDE for the duration of the execution of
+# BODY.
+
+proc with_override { name override body } {
+    # Implementation note: It's possible to implement the override using
+    # rename, like this:
+    #   rename $name save_$name
+    #   rename $override $name
+    #   set code [catch {uplevel 1 $body} result]
+    #   rename $name $override
+    #   rename save_$name $name
+    # but there are two issues here:
+    # - the save_$name might clash with an existing proc
+    # - the override is no longer available under its original name during
+    #   the override
+    # So, we use this more elaborate but cleaner mechanism.
+
+    # Save the old proc.
+    set old_args [info args $name]
+    set old_body [info body $name]
+
+    # Install the override.
+    set new_args [info args $override]
+    set new_body [info body $override]
+    eval proc $name {$new_args} {$new_body}
+
+    # Execute body.
+    set code [catch {uplevel 1 $body} result]
+
+    # Restore old proc.
+    eval proc $name {$old_args} {$old_body}
+
+    # Return as appropriate.
+    if { $code == 1 } {
+        global errorInfo errorCode
+        return -code error -errorinfo $errorInfo -errorcode $errorCode $result
+    } elseif { $code > 1 } {
+        return -code $code $result
+    }
+
+    return $result
+}
+
 # Always load compatibility stuff.
 load_lib future.exp