gdb: introduce target_waitkind_str, use it in target_waitstatus::to_string

I would like to print target_waitkind values in debug messages, so I
think that a target_waitkind-to-string function would be useful.  While
at it, use it in target_waitstatus::to_string.  This changes the output
of target_waitstatus::to_string a bit, but I think it is for the better.
The debug messages will show a string matching exactly the
target_waitkind enumerator (minus the TARGET_WAITKIND prefix).

As a convenience, make string_appendf return the same reference to
string it got as a parameter.  This allows doing this:

  return string_appendf (str, "foo");

... keeping the code concise.

Change-Id: I383dffc9c78614e7d0668b1516073905e798eef7

2 files changed, 8 insertions, 4 deletions

diff --git a/gdbsupport/common-utils.cc b/gdbsupport/common-utils.cc
index 42bce36..b591537 100644
--- a/gdbsupport/common-utils.cc
+++ b/gdbsupport/common-utils.cc
@@ -119,7 +119,7 @@ string_vprintf (const char* fmt, va_list args)
 
 /* See documentation in common-utils.h.  */
 
-void
+std::string &
 string_appendf (std::string &str, const char *fmt, ...)
 {
   va_list vp;
@@ -127,12 +127,14 @@ string_appendf (std::string &str, const char *fmt, ...)
   va_start (vp, fmt);
   string_vappendf (str, fmt, vp);
   va_end (vp);
+
+  return str;
 }
 
 
 /* See documentation in common-utils.h.  */
 
-void
+std::string &
 string_vappendf (std::string &str, const char *fmt, va_list args)
 {
   va_list vp;
@@ -148,6 +150,8 @@ string_vappendf (std::string &str, const char *fmt, va_list args)
   /* C++11 and later guarantee std::string uses contiguous memory and
      always includes the terminating '\0'.  */
   vsprintf (&str[curr_size], fmt, args); /* ARI: vsprintf */
+
+  return str;
 }
 
 char *
diff --git a/gdbsupport/common-utils.h b/gdbsupport/common-utils.h
index af07847..98a9dc7 100644
--- a/gdbsupport/common-utils.h
+++ b/gdbsupport/common-utils.h
@@ -54,11 +54,11 @@ std::string string_vprintf (const char* fmt, va_list args)
 
 /* Like string_printf, but appends to DEST instead of returning a new
    std::string.  */
-void string_appendf (std::string &dest, const char* fmt, ...)
+std::string &string_appendf (std::string &dest, const char* fmt, ...)
   ATTRIBUTE_PRINTF (2, 3);
 
 /* Like string_appendf, but takes a va_list.  */
-void string_vappendf (std::string &dest, const char* fmt, va_list args)
+std::string &string_vappendf (std::string &dest, const char* fmt, va_list args)
   ATTRIBUTE_PRINTF (2, 0);
 
 /* Make a copy of the string at PTR with LEN characters