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>.

7 files changed, 45 insertions, 51 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 928dbab..49b91e1 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 6875596..c7c4553 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 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.  */
diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index d54e56c..b5bb162 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 15d6c5b..134a97e 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 197e7ff..2b97118 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 de56213..fd6c30b 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