Return gdb::optional<std::string> from target_fileio_readlink

This changes to_fileio_readlink and target_fileio_readlink to return a
gdb::optional<std::sring>, and then fixes up the callers and
implementations.  This allows the removal of some cleanups.

Regression tested by the buildbot.

gdb/ChangeLog
2018-03-07  Tom Tromey  <tom@tromey.com>

	* linux-tdep.c (linux_info_proc): Update.
	* target.h (struct target_ops) <to_fileio_readlink>: Return
	optional<string>.
	(target_fileio_readlink): Return optional<string>.
	* remote.c (remote_hostio_readlink): Return optional<string>.
	* inf-child.c (inf_child_fileio_readlink): Return
	optional<string>.
	* target.c (target_fileio_readlink): Return optional<string>.

1 files changed, 3 insertions, 7 deletions

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index cc930c6..1bbad7b 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -4709,27 +4709,23 @@ linux_nat_fileio_open (struct target_ops *self,
 
 /* Implementation of to_fileio_readlink.  */
 
-static char *
+static gdb::optional<std::string>
 linux_nat_fileio_readlink (struct target_ops *self,
 			   struct inferior *inf, const char *filename,
 			   int *target_errno)
 {
   char buf[PATH_MAX];
   int len;
-  char *ret;
 
   len = linux_mntns_readlink (linux_nat_fileio_pid_of (inf),
 			      filename, buf, sizeof (buf));
   if (len < 0)
     {
       *target_errno = host_to_fileio_error (errno);
-      return NULL;
+      return {};
     }
 
-  ret = (char *) xmalloc (len + 1);
-  memcpy (ret, buf, len);
-  ret[len] = '\0';
-  return ret;
+  return std::string (buf, len);
 }
 
 /* Implementation of to_fileio_unlink.  */