support: Implement TEST_COMPARE_STRING

diff --git a/support/Makefile b/support/Makefile
index 4d307035b3..2b663fbbfa 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -63,6 +63,7 @@  libsupport-routines = \
   support_shared_allocate \
   support_test_compare_blob \
   support_test_compare_failure \
+  support_test_compare_string \
   support_write_file_string \
   support_test_main \
   support_test_verify_impl \
@@ -198,6 +199,7 @@  tests = \
   tst-support_record_failure \
   tst-test_compare \
   tst-test_compare_blob \
+  tst-test_compare_string \
   tst-xreadlink \
 
 ifeq ($(run-built-tests),yes)
diff --git a/support/check.h b/support/check.h
index b3a4645e92..e6765289f2 100644
--- a/support/check.h
+++ b/support/check.h
@@ -163,6 +163,19 @@  void support_test_compare_blob (const void *left,
                                 const char *right_exp,
                                 const char *right_len_exp);
 
+/* Compare the strings LEFT and RIGHT and report a test failure if
+   they are different.  Also report failure if one of the arguments is
+   a null pointer and the other is not.  The strings should be
+   reasonably short because on mismatch, both are printed.  */
+#define TEST_COMPARE_STRING(left, right)                         \
+  (support_test_compare_string (left, right, __FILE__, __LINE__, \
+                                #left, #right))
+
+void support_test_compare_string (const char *left, const char *right,
+                                  const char *file, int line,
+                                  const char *left_expr,
+                                  const char *right_expr);
+
 /* Internal function called by the test driver.  */
 int support_report_failure (int status)
   __attribute__ ((weak, warn_unused_result));
diff --git a/support/support_test_compare_string.c b/support/support_test_compare_string.c
new file mode 100644
index 0000000000..9958aaeec1
--- /dev/null
+++ b/support/support_test_compare_string.c
@@ -0,0 +1,91 @@ 
+/* Check two strings for equality.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/xmemstream.h>
+
+static void
+report_length (const char *what, const char *str, size_t length)
+{
+  if (str == NULL)
+    printf ("  %s string: NULL\n", what);
+  else
+    printf ("  %s string: %zu bytes\n", what, length);
+}
+
+static void
+report_string (const char *what, const unsigned char *blob,
+               size_t length, const char *expr)
+{
+  if (length > 0)
+    {
+      printf ("  %s (evaluated from %s):\n", what, expr);
+      char *quoted = support_quote_blob (blob, length);
+      printf ("      \"%s\"\n", quoted);
+      free (quoted);
+
+      fputs ("     ", stdout);
+      for (size_t i = 0; i < length; ++i)
+        printf (" %02X", blob[i]);
+      putc ('\n', stdout);
+    }
+}
+
+static size_t
+string_length_or_zero (const char *str)
+{
+  if (str == NULL)
+    return 0;
+  else
+    return strlen (str);
+}
+
+void
+support_test_compare_string (const char *left, const char *right,
+                             const char *file, int line,
+                             const char *left_expr, const char *right_expr)
+{
+  /* Two null pointers are accepted.  */
+  if (left == NULL && right == NULL)
+    return;
+
+  size_t left_length = string_length_or_zero (left);
+  size_t right_length = string_length_or_zero (right);
+
+  if (left_length != right_length || left == NULL || right == NULL
+      || memcmp (left, right, left_length) != 0)
+    {
+      support_record_failure ();
+      printf ("%s:%d: error: blob comparison failed\n", file, line);
+      if (left_length == right_length && right != NULL && left != NULL)
+        printf ("  string length: %lu bytes\n", left_length);
+      else
+        {
+          report_length ("left", left, left_length);
+          report_length ("right", right, right_length);
+        }
+      report_string ("left", (const unsigned char *) left,
+                     left_length, left_expr);
+      report_string ("right", (const unsigned char *) right,
+                     right_length, right_expr);
+    }
+}
diff --git a/support/tst-test_compare_string.c b/support/tst-test_compare_string.c
new file mode 100644
index 0000000000..2a4b258587
--- /dev/null
+++ b/support/tst-test_compare_string.c
@@ -0,0 +1,107 @@ 
+/* Basic test for the TEST_COMPARE_STRING macro.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <string.h>
+#include <support/check.h>
+#include <support/capture_subprocess.h>
+
+static void
+subprocess (void *closure)
+{
+  /* These tests should fail.  They were chosen to cover differences
+     in length (with the same contents), single-bit mismatches, and
+     mismatching null pointers.  */
+  TEST_COMPARE_STRING ("", NULL);             /* Line 29.  */
+  TEST_COMPARE_STRING ("X", "");              /* Line 30.  */
+  TEST_COMPARE_STRING (NULL, "X");            /* Line 31.  */
+  TEST_COMPARE_STRING ("abcd", "abcD");       /* Line 32.  */
+  TEST_COMPARE_STRING ("abcd", NULL);         /* Line 33.  */
+  TEST_COMPARE_STRING (NULL, "abcd");         /* Line 34.  */
+}
+
+/* Same contents, different addresses.  */
+char buffer_abc_1[] = "abc";
+char buffer_abc_2[] = "abc";
+
+static int
+do_test (void)
+{
+  /* This should succeed.  Even if the pointers and array contents are
+     different, zero-length inputs are not different.  */
+  TEST_COMPARE_STRING (NULL, NULL);
+  TEST_COMPARE_STRING ("", "");
+  TEST_COMPARE_STRING (buffer_abc_1, buffer_abc_2);
+  TEST_COMPARE_STRING (buffer_abc_1, "abc");
+
+  struct support_capture_subprocess proc = support_capture_subprocess
+    (&subprocess, NULL);
+
+  /* Discard the reported error.  */
+  support_record_failure_reset ();
+
+  puts ("info: *** subprocess output starts ***");
+  fputs (proc.out.buffer, stdout);
+  puts ("info: *** subprocess output ends ***");
+
+  TEST_VERIFY
+    (strcmp (proc.out.buffer,
+"tst-test_compare_string.c:29: error: blob comparison failed\n"
+"  left string: 0 bytes\n"
+"  right string: NULL\n"
+"tst-test_compare_string.c:30: error: blob comparison failed\n"
+"  left string: 1 bytes\n"
+"  right string: 0 bytes\n"
+"  left (evaluated from \"X\"):\n"
+"      \"X\"\n"
+"      58\n"
+"tst-test_compare_string.c:31: error: blob comparison failed\n"
+"  left string: NULL\n"
+"  right string: 1 bytes\n"
+"  right (evaluated from \"X\"):\n"
+"      \"X\"\n"
+"      58\n"
+"tst-test_compare_string.c:32: error: blob comparison failed\n"
+"  string length: 4 bytes\n"
+"  left (evaluated from \"abcd\"):\n"
+"      \"abcd\"\n"
+"      61 62 63 64\n"
+"  right (evaluated from \"abcD\"):\n"
+"      \"abcD\"\n"
+"      61 62 63 44\n"
+"tst-test_compare_string.c:33: error: blob comparison failed\n"
+"  left string: 4 bytes\n"
+"  right string: NULL\n"
+"  left (evaluated from \"abcd\"):\n"
+"      \"abcd\"\n"
+"      61 62 63 64\n"
+"tst-test_compare_string.c:34: error: blob comparison failed\n"
+"  left string: NULL\n"
+"  right string: 4 bytes\n"
+"  right (evaluated from \"abcd\"):\n"
+"      \"abcd\"\n"
+"      61 62 63 64\n"
+             ) == 0);
+
+  /* Check that there is no output on standard error.  */
+  support_capture_subprocess_check (&proc, "TEST_COMPARE_STRING",
+                                    0, sc_allow_stdout);
+
+  return 0;
+}
+
+#include <support/test-driver.c>