Fix strftime/1 and strflocaltime/1 to allow an empty result (#3670)

These functions failed with "unknown system failure" whenever strftime()
returned 0 for a non-empty format, but 0 also means the format expands to
an empty string, as %p does in a locale without AM/PM.

Prepend a character to the format and trim it from the result, so that 0
can only mean the result is too long. Now 0 | strflocaltime("%p") gives
"" in fr_FR.UTF-8, and a result that is still too long fails with
"result too long" error instead. The size limit is unchanged.

src/builtin.c

@@ -1849,8 +1849,10 @@ static jv f_strftime(jq_state *jq, jv a, jv b) {
   if (!jv2tm(a, &tm, 0))
     return ret_error(b, jv_string("strftime/1 requires parsed datetime inputs"));
 
+  /* Prepend a character so that strftime() returning 0 means the result
+   * did not fit, even if the format expands to nothing; it is trimmed below. */
+  b = jv_string_concat(jv_string(" "), b);
   const char *fmt = jv_string_value(b);
-  int fmt_not_empty = *fmt != '\0';
   size_t max_size = strlen(fmt) + 100;
   char *buf = jv_mem_alloc(max_size);
 #if defined(__APPLE__) || defined(__sun)
@@ -1884,12 +1886,11 @@ static jv f_strftime(jq_state *jq, jv a, jv b) {
   tzset();
 #endif
   jv_free(b);
-  /* POSIX doesn't provide errno values for strftime() failures; weird */
-  if ((n == 0 && fmt_not_empty) || n > max_size) {
+  if (n == 0 || n > max_size) {
     free(buf);
-    return jv_invalid_with_msg(jv_string("strftime/1: unknown system failure"));
+    return jv_invalid_with_msg(jv_string("strftime/1: result too long"));
   }
-  jv ret = jv_string_sized(buf, n);
+  jv ret = jv_string_sized(buf + 1, n - 1);
   free(buf);
   return ret;
 }
@@ -1917,18 +1918,19 @@ static jv f_strflocaltime(jq_state *jq, jv a, jv b) {
   struct tm tm;
   if (!jv2tm(a, &tm, 1))
     return ret_error(b, jv_string("strflocaltime/1 requires parsed datetime inputs"));
+  /* Prepend a character so that strftime() returning 0 means the result
+   * did not fit, even if the format expands to nothing; it is trimmed below. */
+  b = jv_string_concat(jv_string(" "), b);
   const char *fmt = jv_string_value(b);
-  int fmt_not_empty = *fmt != '\0';
   size_t max_size = strlen(fmt) + 100;
   char *buf = jv_mem_alloc(max_size);
   size_t n = strftime(buf, max_size, fmt, &tm);
   jv_free(b);
-  /* POSIX doesn't provide errno values for strftime() failures; weird */
-  if ((n == 0 && fmt_not_empty) || n > max_size) {
+  if (n == 0 || n > max_size) {
     free(buf);
-    return jv_invalid_with_msg(jv_string("strflocaltime/1: unknown system failure"));
+    return jv_invalid_with_msg(jv_string("strflocaltime/1: result too long"));
   }
-  jv ret = jv_string_sized(buf, n);
+  jv ret = jv_string_sized(buf + 1, n - 1);
   free(buf);
   return ret;
 }

tests/jq.test

@@ -1945,6 +1945,22 @@ try ["OK", strflocaltime({})] catch ["KO", .]
 0
 ["KO","strflocaltime/1 requires a string format"]
 
+[strftime("", " %Y ")]
+0
+["", " 1970 "]
+
+try strftime("%c" * 10) catch .
+0
+"strftime/1: result too long"
+
+[strflocaltime("", " %Y ")]
+[2015,2,5,23,51,47,4,63]
+["", " 2015 "]
+
+try strflocaltime("%c" * 10) catch .
+[2015,2,5,23,51,47,4,63]
+"strflocaltime/1: result too long"
+
 [strptime("%Y-%m-%dT%H:%M:%SZ")|(.,mktime)]
 "2015-03-05T23:51:47Z"
 [[2015,2,5,23,51,47,4,63],1425599507]

tests/shtest

@@ -779,6 +779,15 @@ EOF
   fi
 fi
 
+# strflocaltime/1 allows a format that expands to an empty string
+l=fr_FR.UTF-8
+if ! $msys && ! $mingw && [ -z "$(LC_ALL=$l date +%p 2>/dev/null)" ]; then
+  if ! out=$(LC_ALL=$l $VALGRIND $Q $JQ -n '0 | strflocaltime("%p")' 2>&1) || [ "$out" != '""' ]; then
+    echo "strflocaltime(\"%p\") in $l should output an empty string but got: $out"
+    exit 1
+  fi
+fi
+
 # Test syntax error messages of multiline filter (we can't test in jq.test)
 if $VALGRIND $Q $JQ -n '[
   try if .