Introduce string_appendf/string_vappendf

string_appendf is like string_printf, but instead of allocating a new
string, it appends to an existing string.  This allows reusing a
std::string's memory buffer across several calls, for example.

gdb/ChangeLog:
2017-10-30  Pedro Alves  <palves@redhat.com>

	* common/common-utils.c (string_appendf, string_vappendf): New
	functions.
	* common/common-utils.h (string_appendf, string_vappendf): New
	declarations.
	* unittests/common-utils-selftests.c (string_appendf_func)
	(test_appendf_func, string_vappendf_wrapper, string_appendf_tests)
	(string_vappendf_tests): New functions.
	(_initialize_common_utils_selftests): Register "string_appendf" and
	"string_vappendf tests".

gdb/ChangeLog

@@ -1,3 +1,15 @@
+2017-10-30  Pedro Alves  <palves@redhat.com>
+
+	* common/common-utils.c (string_appendf, string_vappendf): New
+	functions.
+	* common/common-utils.h (string_appendf, string_vappendf): New
+	declarations.
+	* unittests/common-utils-selftests.c (string_appendf_func)
+	(test_appendf_func, string_vappendf_wrapper, string_appendf_tests)
+	(string_vappendf_tests): New functions.
+	(_initialize_common_utils_selftests): Register "string_appendf" and
+	"string_vappendf tests".
+
 2017-10-30  Pedro Alves  <palves@redhat.com>
 
 	* unittests/common-utils-selftests.c (format_func): New typedef.

gdb/common/common-utils.c

@@ -195,6 +195,40 @@ string_vprintf (const char* fmt, va_list args)
   return str;
 }
 
+
+/* See documentation in common-utils.h.  */
+
+void
+string_appendf (std::string &str, const char *fmt, ...)
+{
+  va_list vp;
+
+  va_start (vp, fmt);
+  string_vappendf (str, fmt, vp);
+  va_end (vp);
+}
+
+
+/* See documentation in common-utils.h.  */
+
+void
+string_vappendf (std::string &str, const char *fmt, va_list args)
+{
+  va_list vp;
+  int grow_size;
+
+  va_copy (vp, args);
+  grow_size = vsnprintf (NULL, 0, fmt, vp);
+  va_end (vp);
+
+  size_t curr_size = str.size ();
+  str.resize (curr_size + grow_size);
+
+  /* C++11 and later guarantee std::string uses contiguous memory and
+     always includes the terminating '\0'.  */
+  vsprintf (&str[curr_size], fmt, args);
+}
+
 char *
 savestring (const char *ptr, size_t len)
 {

gdb/common/common-utils.h

@@ -67,6 +67,15 @@ std::string string_printf (const char* fmt, ...)
 std::string string_vprintf (const char* fmt, va_list args)
   ATTRIBUTE_PRINTF (1, 0);
 
+/* Like string_printf, but appends to DEST instead of returning a new
+   std::string.  */
+void 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)
+  ATTRIBUTE_PRINTF (2, 0);
+
 /* Make a copy of the string at PTR with LEN characters
    (and add a null character at the end in the copy).
    Uses malloc to get the space.  Returns the address of the copy.  */

gdb/unittests/common-utils-selftests.c

@@ -77,6 +77,52 @@ string_vprintf_tests ()
   test_format_func (format);
 }
 
+/* Type of both 'string_appendf' and the 'string_vappendf_wrapper'
+   function below.  Used to run the same tests against both
+   string_appendf and string_vappendf.  */
+typedef void (string_appendf_func) (std::string &str, const char *fmt, ...)
+  ATTRIBUTE_PRINTF (2, 3);
+
+static void
+test_appendf_func (string_appendf_func *func)
+{
+  std::string str;
+
+  func (str, "%s", "");
+  SELF_CHECK (str == "");
+
+  func (str, "%s", "test");
+  SELF_CHECK (str == "test");
+
+  func (str, "%d", 23);
+  SELF_CHECK (str == "test23");
+
+  func (str, "%s %d %s", "foo", 45, "bar");
+  SELF_CHECK (str == "test23foo 45 bar");
+}
+
+static void ATTRIBUTE_PRINTF (2, 3)
+string_vappendf_wrapper (std::string &str, const char *fmt, ...)
+{
+  va_list vp;
+
+  va_start (vp, fmt);
+  string_vappendf (str, fmt, vp);
+  va_end (vp);
+}
+
+static void
+string_appendf_tests ()
+{
+  test_appendf_func (string_appendf);
+}
+
+static void
+string_vappendf_tests ()
+{
+  test_appendf_func (string_vappendf_wrapper);
+}
+
 } /* namespace selftests */
 
 void
@@ -84,4 +130,7 @@ _initialize_common_utils_selftests ()
 {
   selftests::register_test ("string_printf", selftests::string_printf_tests);
   selftests::register_test ("string_vprintf", selftests::string_vprintf_tests);
+  selftests::register_test ("string_appendf", selftests::string_appendf_tests);
+  selftests::register_test ("string_vappendf",
+			    selftests::string_vappendf_tests);
 }