Restore sprintbuf(), add macro for string literals

Hawciz pointed out that the previous commit modifies the public
interface of printbuf. Per his suggestion, sprintbuf() was restored
and a new pair of macros was added that wraps printbuf_memappend().

Using a wrapper macro instead of modifying sprintbuf() also reduces
function call overhead, bringing total performance gains to
approximately 400%.

printbuf.h

@@ -29,12 +29,13 @@ struct printbuf {
 extern struct printbuf*
 printbuf_new(void);
 
-/* As an optimization, printbuf_memappend_fast is defined as a macro
+/* As an optimization, printbuf_memappend_fast() is defined as a macro
  * that handles copying data if the buffer is large enough; otherwise
- * it invokes printbuf_memappend_real() which performs the heavy
+ * it invokes printbuf_memappend() which performs the heavy
  * lifting of realloc()ing the buffer and copying data.
- * Your code should not use printbuf_memappend directly--use
- * printbuf_memappend_fast instead.
+ *
+ * Your code should not use printbuf_memappend() directly unless it
+ * checks the return code. Use printbuf_memappend_fast() instead.
  */
 extern int
 printbuf_memappend(struct printbuf *p, const char *buf, int size);
@@ -50,6 +51,28 @@ do {                                                         \
 
 #define printbuf_length(p) ((p)->bpos)
 
+/**
+ * Results in a compile error if the argument is not a string literal.
+ */
+#define _printbuf_check_literal(mystr) ("" mystr)
+
+/**
+ * This is an optimization wrapper around printbuf_memappend() that is useful
+ * for appending string literals. Since the size of string constants is known
+ * at compile time, using this macro can avoid a costly strlen() call. This is
+ * especially helpful when a constant string must be appended many times. If
+ * you got here because of a compilation error caused by passing something
+ * other than a string literal, use printbuf_memappend_fast() in conjunction
+ * with strlen().
+ *
+ * See also:
+ *   printbuf_memappend_fast()
+ *   printbuf_memappend()
+ *   sprintbuf()
+ */
+#define printbuf_strappend(pb, str) \
+   printbuf_memappend ((pb), _printbuf_check_literal(str), sizeof(str) - 1)
+
 /**
  * Set len bytes of the buffer to charvalue, starting at offset offset.
  * Similar to calling memset(x, charvalue, len);
@@ -61,8 +84,22 @@ do {                                                         \
 extern int
 printbuf_memset(struct printbuf *pb, int offset, int charvalue, int len);
 
+/**
+ * Formatted print to printbuf.
+ *
+ * This function is the most expensive of the available functions for appending
+ * string data to a printbuf and should be used only where convenience is more
+ * important than speed. Avoid using this function in high performance code or
+ * tight loops; in these scenarios, consider using snprintf() with a static
+ * buffer in conjunction with one of the printbuf_*append() functions.
+ *
+ * See also:
+ *   printbuf_memappend_fast()
+ *   printbuf_memappend()
+ *   printbuf_strappend()
+ */
 extern int
-sprintbuf(struct printbuf *p, const char *msg);
+sprintbuf(struct printbuf *p, const char *msg, ...);
 
 extern void
 printbuf_reset(struct printbuf *p);