<fd_to_filename.h>: Add type safety and port to Hurd

The new type struct fd_to_filename makes the allocation of the
backing storage explicit.

Hurd uses /dev/fd, not /proc/self/fd.

Co-Authored-By: Paul Eggert <eggert@cs.ucla.edu>

3 files changed, 144 insertions, 1 deletions

diff --git a/misc/Makefile b/misc/Makefile
index e0465980c7..b8fed5783d 100644
--- a/misc/Makefile
+++ b/misc/Makefile
@@ -72,7 +72,7 @@ routines := brk sbrk sstk ioctl \
 	    fgetxattr flistxattr fremovexattr fsetxattr getxattr \
 	    listxattr lgetxattr llistxattr lremovexattr lsetxattr \
 	    removexattr setxattr getauxval ifunc-impl-list makedev \
-	    allocate_once
+	    allocate_once fd_to_filename
 
 generated += tst-error1.mtrace tst-error1-mem.out \
   tst-allocate_once.mtrace tst-allocate_once-mem.out
@@ -97,6 +97,10 @@ endif
 tests-internal := tst-atomic tst-atomic-long tst-allocate_once
 tests-static := tst-empty
 
+# Test for the internal, non-exported __fd_to_filename function.
+tests-internal += tst-fd_to_filename
+tests-static += tst-fd_to_filename
+
 ifeq ($(run-built-tests),yes)
 tests-special += $(objpfx)tst-error1-mem.out \
   $(objpfx)tst-allocate_once-mem.out
diff --git a/misc/fd_to_filename.c b/misc/fd_to_filename.c
new file mode 100644
index 0000000000..03d19194c1
--- /dev/null
+++ b/misc/fd_to_filename.c
@@ -0,0 +1,38 @@
+/* Construct a pathname under /proc/self/fd (or /dev/fd for Hurd).
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <fd_to_filename.h>
+
+#include <assert.h>
+#include <string.h>
+
+char *
+__fd_to_filename (int descriptor, struct fd_to_filename *storage)
+{
+  assert (descriptor >= 0);
+
+  char *p = mempcpy (storage->buffer, FD_TO_FILENAME_PREFIX,
+                     strlen (FD_TO_FILENAME_PREFIX));
+
+  for (int d = descriptor; p++, (d /= 10) != 0; )
+    continue;
+  *p = '\0';
+  for (int d = descriptor; *--p = '0' + d % 10, (d /= 10) != 0; )
+    continue;
+  return storage->buffer;
+}
diff --git a/misc/tst-fd_to_filename.c b/misc/tst-fd_to_filename.c
new file mode 100644
index 0000000000..d16c869cd3
--- /dev/null
+++ b/misc/tst-fd_to_filename.c
@@ -0,0 +1,101 @@
+/* Test for /proc/self/fd (or /dev/fd) pathname construction.
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <fcntl.h>
+#include <fd_to_filename.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/xunistd.h>
+
+/* Run a check on one value.  */
+static void
+check (int value)
+{
+  if (value < 0)
+    /* Negative descriptor values violate the precondition.  */
+    return;
+
+  struct fd_to_filename storage;
+  char *actual = __fd_to_filename (value, &storage);
+  char expected[100];
+  snprintf (expected, sizeof (expected), FD_TO_FILENAME_PREFIX "%d", value);
+  TEST_COMPARE_STRING (actual, expected);
+}
+
+/* Check various ranges constructed around powers.  */
+static void
+check_ranges (int base)
+{
+  unsigned int power = 1;
+  do
+    {
+      for (int factor = 1; factor < base; ++factor)
+        for (int shift = -1000; shift <= 1000; ++shift)
+          check (factor * power + shift);
+    }
+  while (!__builtin_mul_overflow (power, base, &power));
+}
+
+/* Check that it is actually possible to use a the constructed
+   name.  */
+static void
+check_open (void)
+{
+  int pipes[2];
+  xpipe (pipes);
+
+  struct fd_to_filename storage;
+  int read_alias = xopen (__fd_to_filename (pipes[0], &storage), O_RDONLY, 0);
+  int write_alias = xopen (__fd_to_filename (pipes[1], &storage), O_WRONLY, 0);
+
+  /* Ensure that all the descriptor numbers are different.  */
+  TEST_VERIFY (pipes[0] < pipes[1]);
+  TEST_VERIFY (pipes[1] < read_alias);
+  TEST_VERIFY (read_alias < write_alias);
+
+  xwrite (write_alias, "1", 1);
+  char buf[16];
+  TEST_COMPARE_BLOB ("1", 1, buf, read (pipes[0], buf, sizeof (buf)));
+
+  xwrite (pipes[1], "2", 1);
+  TEST_COMPARE_BLOB ("2", 1, buf, read (read_alias, buf, sizeof (buf)));
+
+  xwrite (write_alias, "3", 1);
+  TEST_COMPARE_BLOB ("3", 1, buf, read (read_alias, buf, sizeof (buf)));
+
+  xwrite (pipes[1], "4", 1);
+  TEST_COMPARE_BLOB ("4", 1, buf, read (pipes[0], buf, sizeof (buf)));
+
+  xclose (write_alias);
+  xclose (read_alias);
+  xclose (pipes[1]);
+  xclose (pipes[0]);
+}
+
+static int
+do_test (void)
+{
+  check_ranges (2);
+  check_ranges (10);
+
+  check_open ();
+
+  return 0;
+}
+
+#include <support/test-driver.c>