Add a tor_asprintf() function, and use it in a couple of places.

asprintf() is a GNU extension that some BSDs have picked up: it does a printf
into a newly allocated chunk of RAM.

Our tor_asprintf() differs from standard asprintf() in that:
  - Like our other malloc functions, it asserts on OOM.
  - It works on windows.
  - It always sets its return-field.

configure.in

@@ -189,7 +189,7 @@ dnl -------------------------------------------------------------------
 dnl Check for functions before libevent, since libevent-1.2 apparently
 dnl exports strlcpy without defining it in a header.
 
-AC_CHECK_FUNCS(gettimeofday ftime socketpair uname inet_aton strptime getrlimit strlcat strlcpy strtoull getaddrinfo localtime_r gmtime_r memmem strtok_r writev readv flock prctl)
+AC_CHECK_FUNCS(gettimeofday ftime socketpair uname inet_aton strptime getrlimit strlcat strlcpy strtoull getaddrinfo localtime_r gmtime_r memmem strtok_r writev readv flock prctl vasprintf)
 
 using_custom_malloc=no
 if test x$enable_openbsd_malloc = xyes ; then

src/common/compat.c

@@ -307,6 +307,92 @@ tor_vsnprintf(char *str, size_t size, const char *format, va_list args)
   return r;
 }
 
+/**
+ * Portable asprintf implementation.  Does a printf() into a newly malloc'd
+ * string.  Sets *<b>strp</b> to this string, and returns its length (not
+ * including the terminating NUL character).
+ *
+ * You can treat this function as if its implementation were something like
+   <pre>
+     char buf[_INFINITY_];
+     tor_snprintf(buf, sizeof(buf), fmt, args);
+     *strp = tor_strdup(buf);
+     return strlen(*strp):
+   </pre>
+ * Where _INFINITY_ is an imaginary constant so big that any string can fit
+ * into it.
+ */
+int
+tor_asprintf(char **strp, const char *fmt, ...)
+{
+  int r;
+  va_list args;
+  va_start(args, fmt);
+  r = tor_vasprintf(strp, fmt, args);
+  va_end(args);
+  if (!*strp || r < 0) {
+    log_err(LD_BUG, "Internal error in asprintf");
+    tor_assert(0);
+  }
+  return r;
+}
+
+/**
+ * Portable vasprintf implementation.  Does a printf() into a newly malloc'd
+ * string.  Differs from regular vasprintf in the same ways that
+ * tor_asprintf() differs from regular asprintf.
+ */
+int
+tor_vasprintf(char **strp, const char *fmt, va_list args)
+{
+#ifdef HAVE_VASPRINTF
+  /* If the platform gives us one, use it. */
+  int r = vasprintf(strp, fmt, args);
+  if (r < 0)
+    *strp = NULL;
+  return r;
+#elif defined(MS_WINDOWS)
+  /* On Windows, _vsnprintf won't tell us the length of the string if it
+   * overflows, so we need to use _vcsprintf to tell how much to allocate */
+  int len, r;
+  char *res;
+  len = _vcsprintf(fmt, args);
+  if (len < 0) {
+    strp = NULL;
+    return -1;
+  }
+  *strp = tor_malloc(len + 1);
+  r = _vsnprintf(*strp, len+1, fmt, args);
+  if (r != len) {
+    tor_free(*strp);
+    return -1;
+  }
+  return len;
+#else
+  /* Everywhere else, we have a decent vsnprintf that tells us how many
+   * characters we need.  We give it a try on a short buffer first, since
+   * it might be nice to avoid the second vsnprintf call.
+   */
+  char buf[128];
+  int len, r;
+  va_list tmp_args;
+  va_copy(tmp_args, args);
+  len = vsnprintf(buf, sizeof(buf), fmt, tmp_args);
+  va_end(tmp_args);
+  if (len < (int)sizeof(buf)) {
+    *strp = tor_strdup(buf);
+    return len;
+  }
+  *strp = tor_malloc(len+1);
+  r = vsnprintf(*strp, len+1, fmt, args);
+  if (r != len) {
+    tor_free(*strp);
+    return -1;
+  }
+  return len;
+#endif
+}
+
 /** Given <b>hlen</b> bytes at <b>haystack</b> and <b>nlen</b> bytes at
  * <b>needle</b>, return a pointer to the first occurrence of the needle
  * within the haystack, or NULL if there is no such occurrence.

src/common/compat.h

@@ -235,6 +235,10 @@ int tor_snprintf(char *str, size_t size, const char *format, ...)
 int tor_vsnprintf(char *str, size_t size, const char *format, va_list args)
   ATTR_NONNULL((1,3));
 
+int tor_asprintf(char **strp, const char *fmt, ...)
+  CHECK_PRINTF(2,3);
+int tor_vasprintf(char **strp, const char *fmt, va_list args);
+
 const void *tor_memmem(const void *haystack, size_t hlen, const void *needle,
                        size_t nlen)  ATTR_PURE ATTR_NONNULL((1,3));
 static const void *tor_memstr(const void *haystack, size_t hlen,

src/or/circuitbuild.c

@@ -1086,21 +1086,21 @@ circuit_list_path_impl(origin_circuit_t *circ, int verbose, int verbose_names)
   crypt_path_t *hop;
   smartlist_t *elements;
   const char *states[] = {"closed", "waiting for keys", "open"};
-  char buf[128];
   char *s;
 
   elements = smartlist_create();
 
   if (verbose) {
     const char *nickname = build_state_get_exit_nickname(circ->build_state);
-    tor_snprintf(buf, sizeof(buf), "%s%s circ (length %d%s%s):",
+    char *cp;
+    tor_asprintf(&cp, "%s%s circ (length %d%s%s):",
                  circ->build_state->is_internal ? "internal" : "exit",
                  circ->build_state->need_uptime ? " (high-uptime)" : "",
                  circ->build_state->desired_path_len,
                  circ->_base.state == CIRCUIT_STATE_OPEN ? "" : ", exit ",
                  circ->_base.state == CIRCUIT_STATE_OPEN ? "" :
                  (nickname?nickname:"*unnamed*"));
-    smartlist_add(elements, tor_strdup(buf));
+    smartlist_add(elements, cp);
   }
 
   hop = circ->cpath;
@@ -3067,21 +3067,21 @@ static void
 log_entry_guards(int severity)
 {
   smartlist_t *elements = smartlist_create();
-  char buf[1024];
   char *s;
 
   SMARTLIST_FOREACH(entry_guards, entry_guard_t *, e,
     {
       const char *msg = NULL;
+      char *cp;
       if (entry_is_live(e, 0, 1, 0, &msg))
-        tor_snprintf(buf, sizeof(buf), "%s (up %s)",
+        tor_asprintf(&cp, "%s (up %s)",
                      e->nickname,
                      e->made_contact ? "made-contact" : "never-contacted");
       else
-        tor_snprintf(buf, sizeof(buf), "%s (%s, %s)",
+        tor_asprintf(&cp, "%s (%s, %s)",
                      e->nickname, msg,
                      e->made_contact ? "made-contact" : "never-contacted");
-      smartlist_add(elements, tor_strdup(buf));
+      smartlist_add(elements, cp);
     });
 
   s = smartlist_join_strings(elements, ",", 0, NULL);

src/test/test_util.c

@@ -1050,6 +1050,51 @@ test_util_find_str_at_start_of_line(void *ptr)
   ;
 }
 
+static void
+test_util_asprintf(void *ptr)
+{
+#define LOREMIPSUM                                              \
+  "Lorem ipsum dolor sit amet, consectetur adipisicing elit"
+  char *cp=NULL, *cp2=NULL;
+  int r;
+  (void)ptr;
+
+  /* empty string. */
+  r = tor_asprintf(&cp, "%s", "");
+  tt_assert(cp);
+  tt_int_op(r, ==, strlen(cp));
+  tt_str_op(cp, ==, "");
+
+  /* Short string with some printing in it. */
+  r = tor_asprintf(&cp2, "First=%d, Second=%d", 101, 202);
+  tt_assert(cp2);
+  tt_int_op(r, ==, strlen(cp2));
+  tt_str_op(cp2, ==, "First=101, Second=202");
+  tt_assert(cp != cp2);
+  tor_free(cp);
+  tor_free(cp2);
+
+  /* Glass-box test: a string exactly 128 characters long. */
+  r = tor_asprintf(&cp, "Lorem1: %sLorem2: %s", LOREMIPSUM, LOREMIPSUM);
+  tt_assert(cp);
+  tt_int_op(r, ==, 128);
+  tt_assert(cp[128] == '\0');
+  tt_str_op(cp, ==,
+            "Lorem1: "LOREMIPSUM"Lorem2: "LOREMIPSUM);
+  tor_free(cp);
+
+  /* String longer than 128 characters */
+  r = tor_asprintf(&cp, "1: %s 2: %s 3: %s",
+                   LOREMIPSUM, LOREMIPSUM, LOREMIPSUM);
+  tt_assert(cp);
+  tt_int_op(r, ==, strlen(cp));
+  tt_str_op(cp, ==, "1: "LOREMIPSUM" 2: "LOREMIPSUM" 3: "LOREMIPSUM);
+
+ done:
+  tor_free(cp);
+  tor_free(cp2);
+}
+
 #define UTIL_LEGACY(name)                                               \
   { #name, legacy_test_helper, 0, &legacy_setup, test_util_ ## name }
 
@@ -1071,6 +1116,7 @@ struct testcase_t util_tests[] = {
   UTIL_LEGACY(sscanf),
   UTIL_LEGACY(strtok),
   UTIL_TEST(find_str_at_start_of_line, 0),
+  UTIL_TEST(asprintf, 0),
   END_OF_TESTCASES
 };