[RFA,v3] Return gdb::optional<std::string> from target_fileio_readlink

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 928dbab31f..49b91e1e15 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,14 @@ 
+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>.
+
 2018-03-07  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* regcache.c (cooked_read_test): Add riscv to the list of
diff --git a/gdb/inf-child.c b/gdb/inf-child.c
index 6875596f33..c7c45530b6 100644
--- a/gdb/inf-child.c
+++ b/gdb/inf-child.c
@@ -333,7 +333,7 @@  inf_child_fileio_unlink (struct target_ops *self,
 
 /* Implementation of to_fileio_readlink.  */
 
-static char *
+static gdb::optional<std::string>
 inf_child_fileio_readlink (struct target_ops *self,
 			   struct inferior *inf, const char *filename,
 			   int *target_errno)
@@ -343,22 +343,18 @@  inf_child_fileio_readlink (struct target_ops *self,
 #if defined (PATH_MAX)
   char buf[PATH_MAX];
   int len;
-  char *ret;
 
   len = readlink (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);
 #else
   *target_errno = FILEIO_ENOSYS;
-  return NULL;
+  return {};
 #endif
 }
 
diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index cc930c6334..1bbad7bacc 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.  */
diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index d54e56ce8d..b5bb16215d 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -764,26 +764,20 @@  linux_info_proc (struct gdbarch *gdbarch, const char *args,
   if (cwd_f)
     {
       xsnprintf (filename, sizeof filename, "/proc/%ld/cwd", pid);
-      data = target_fileio_readlink (NULL, filename, &target_errno);
-      if (data)
-	{
-	  struct cleanup *cleanup = make_cleanup (xfree, data);
-          printf_filtered ("cwd = '%s'\n", data);
-	  do_cleanups (cleanup);
-	}
+      gdb::optional<std::string> contents
+	= target_fileio_readlink (NULL, filename, &target_errno);
+      if (contents.has_value ())
+	printf_filtered ("cwd = '%s'\n", contents->c_str ());
       else
 	warning (_("unable to read link '%s'"), filename);
     }
   if (exe_f)
     {
       xsnprintf (filename, sizeof filename, "/proc/%ld/exe", pid);
-      data = target_fileio_readlink (NULL, filename, &target_errno);
-      if (data)
-	{
-	  struct cleanup *cleanup = make_cleanup (xfree, data);
-          printf_filtered ("exe = '%s'\n", data);
-	  do_cleanups (cleanup);
-	}
+      gdb::optional<std::string> contents
+	= target_fileio_readlink (NULL, filename, &target_errno);
+      if (contents.has_value ())
+	printf_filtered ("exe = '%s'\n", contents->c_str ());
       else
 	warning (_("unable to read link '%s'"), filename);
     }
diff --git a/gdb/remote.c b/gdb/remote.c
index 15d6c5bdbf..134a97eed9 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -11713,7 +11713,7 @@  remote_hostio_unlink (struct target_ops *self,
 
 /* Implementation of to_fileio_readlink.  */
 
-static char *
+static gdb::optional<std::string>
 remote_hostio_readlink (struct target_ops *self,
 			struct inferior *inf, const char *filename,
 			int *remote_errno)
@@ -11724,10 +11724,9 @@  remote_hostio_readlink (struct target_ops *self,
   int left = get_remote_packet_size ();
   int len, attachment_len;
   int read_len;
-  char *ret;
 
   if (remote_hostio_set_filesystem (inf, remote_errno) != 0)
-    return NULL;
+    return {};
 
   remote_buffer_add_string (&p, &left, "vFile:readlink:");
 
@@ -11739,16 +11738,15 @@  remote_hostio_readlink (struct target_ops *self,
 				    &attachment_len);
 
   if (len < 0)
-    return NULL;
+    return {};
 
-  ret = (char *) xmalloc (len + 1);
+  std::string ret (len, '\0');
 
   read_len = remote_unescape_input ((gdb_byte *) attachment, attachment_len,
-				    (gdb_byte *) ret, len);
+				    (gdb_byte *) &ret[0], len);
   if (read_len != len)
     error (_("Readlink returned %d, but %d bytes."), len, read_len);
 
-  ret[len] = '\0';
   return ret;
 }
 
diff --git a/gdb/target.c b/gdb/target.c
index 197e7ff542..2b97118217 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -3059,7 +3059,7 @@  target_fileio_unlink (struct inferior *inf, const char *filename,
 
 /* See target.h.  */
 
-char *
+gdb::optional<std::string>
 target_fileio_readlink (struct inferior *inf, const char *filename,
 			int *target_errno)
 {
@@ -3069,22 +3069,22 @@  target_fileio_readlink (struct inferior *inf, const char *filename,
     {
       if (t->to_fileio_readlink != NULL)
 	{
-	  char *ret = t->to_fileio_readlink (t, inf, filename,
-					     target_errno);
+	  gdb::optional<std::string> ret
+	    = t->to_fileio_readlink (t, inf, filename, target_errno);
 
 	  if (targetdebug)
 	    fprintf_unfiltered (gdb_stdlog,
 				"target_fileio_readlink (%d,%s)"
 				" = %s (%d)\n",
 				inf == NULL ? 0 : inf->num,
-				filename, ret? ret : "(nil)",
-				ret? 0 : *target_errno);
+				filename, ret ? ret->c_str () : "(nil)",
+				ret ? 0 : *target_errno);
 	  return ret;
 	}
     }
 
   *target_errno = FILEIO_ENOSYS;
-  return NULL;
+  return {};
 }
 
 static void
diff --git a/gdb/target.h b/gdb/target.h
index de562139c7..fd6c30b983 100644
--- a/gdb/target.h
+++ b/gdb/target.h
@@ -933,12 +933,12 @@  struct target_ops
     /* Read value of symbolic link FILENAME on the target, in the
        filesystem as seen by INF.  If INF is NULL, use the filesystem
        seen by the debugger (GDB or, for remote targets, the remote
-       stub).  Return a null-terminated string allocated via xmalloc,
-       or NULL if an error occurs (and set *TARGET_ERRNO).  */
-    char *(*to_fileio_readlink) (struct target_ops *,
-				 struct inferior *inf,
-				 const char *filename,
-				 int *target_errno);
+       stub).  Return a string, or an empty optional if an error
+       occurs (and set *TARGET_ERRNO).  */
+    gdb::optional<std::string> (*to_fileio_readlink) (struct target_ops *,
+						      struct inferior *inf,
+						      const char *filename,
+						      int *target_errno);
 
 
     /* Implement the "info proc" command.  */
@@ -2095,9 +2095,8 @@  extern int target_fileio_unlink (struct inferior *inf,
    by the debugger (GDB or, for remote targets, the remote stub).
    Return a null-terminated string allocated via xmalloc, or NULL if
    an error occurs (and set *TARGET_ERRNO).  */
-extern char *target_fileio_readlink (struct inferior *inf,
-				     const char *filename,
-				     int *target_errno);
+extern gdb::optional<std::string> target_fileio_readlink
+    (struct inferior *inf, const char *filename, int *target_errno);
 
 /* Read target file FILENAME, in the filesystem as seen by INF.  If
    INF is NULL, use the filesystem seen by the debugger (GDB or, for