Polish libstdc++ 'dg-final' action 'file-io-diff'

Follow-up to recent commit 515da03a838db05443ebcc4c543a405bed764188
"libstdc++: Add file-io-diff to replace @diff@ markup in I/O tests".

Currently, if a 'dg-final' action 'file-io-diff' passes, we print nothing
(should: 'PASS: [...]'), but if it fails, we just print: 'FAIL: files differ',
for example ('*.log' file):

    [...]
    FAIL: 27_io/basic_ostream/inserters_other/wchar_t/2.cc  -std=gnu++17 (test for excess errors)
    [...]
    UNRESOLVED: 27_io/basic_ostream/inserters_other/wchar_t/2.cc  -std=gnu++17 compilation failed to produce executable
    diff: wostream_inserter_other_in.txt: No such file or directory
    diff: wostream_inserter_other_out.txt: No such file or directory
    FAIL: files differ
    diff: wostream_inserter_other_in.txt: No such file or directory
    diff: wostream_inserter_other_out.txt: No such file or directory

When later the '*.sum' files get sorted, these 'FAIL: files differ' instances
aren't grouped anymore with the other test cases' results, but they appear en
bloc, lexically sorted between ('e[...]' and 's[...]'), for example:

    [...]
    PASS: ext/vstring/types/23767.cc  -std=gnu++17 (test for excess errors)
    FAIL: files differ
    FAIL: files differ
    FAIL: files differ
    PASS: special_functions/01_assoc_laguerre/check_nan.cc  -std=gnu++17 (test for excess errors)
    [...]

Also, we shouldn't emit the actual 'diff' into the '*.sum' file, but just into
the '*.log* file, and there's no need for 'spawn'/'expect', as we're not
matching any specific messages.

	libstdc++-v3/
	* testsuite/lib/libstdc++.exp (file-io-diff): Polish.

1 files changed, 11 insertions, 8 deletions

diff --git a/libstdc++-v3/testsuite/lib/libstdc++.exp b/libstdc++-v3/testsuite/lib/libstdc++.exp
index c11e752..18331c8 100644
--- a/libstdc++-v3/testsuite/lib/libstdc++.exp
+++ b/libstdc++-v3/testsuite/lib/libstdc++.exp
@@ -1671,6 +1671,8 @@ if { [info exists env(GCC_RUNTEST_PARALLELIZE_DIR)] \
 
 }
 
+# Utility functions, invoked via dg-final.
+
 # Compare output file written by test to expected result.
 # With two arguments the comparison is done via 'diff arg1 arg2'.
 # With one argument the comparison is done via 'diff arg1.tst arg1.txt'.
@@ -1682,6 +1684,10 @@ proc file-io-diff { args } {
     if { $nargs > 2 } {
 	error "too many arguments to file-io-diff"
     }
+
+    set testcase [testname-for-summary]
+    set description "$testcase file-io-diff $args"
+
     if { $nargs == 1 } {
 	set file1 [lindex $args 0]
 	set file2 "${file1}.txt"
@@ -1691,14 +1697,11 @@ proc file-io-diff { args } {
 	set file2 [lindex $args 1]
     }
 
-    spawn -noecho diff -u $file1 $file2
-    expect {
-      -re ".+" {
-	set msg "files differ\n"
-	append msg $expect_out(0,string)
-	fail $msg
-	exp_continue
-      }
+    if { [catch { exec diff -u $file1 $file2 } msg] } {
+	fail $description
+	verbose -log "'diff':\n$msg"
+    } else {
+	pass $description
     }
     return
 }