Fix printing decimal-point character on printf %#a (#1296)

The C standard indicates that when processing the a conversion specifier
"if the precision is zero *and* the # flag is not specified, no decimal-
point character appears.". This means that __fmt needs to ensure that it
prints the decimal-point character not only when the precision is non-0,
but also when the # flag is specified - cosmopolitan currently does not.

This patch fixes this, along with adding a few tests for this behaviour.

Author: GabrielRavier

libc/stdio/fmt.c

@@ -1497,9 +1497,13 @@ int __fmt(void *fn, void *arg, const char *format, va_list va, int *wrote) {
         i1 = prec1 & 7;
         k = prec1 >> 3;
         __FMT_PUT(alphabet[(fpb.bits[k] >> 4 * i1) & 0xf]);
-        if (prec1 > 0 || prec > 0) {
+
+        // decimal-point character appears if the precision isn't 0
+        // or the # flag is specified
+        if (prec1 > 0 || prec > 0 || (flags & FLAGS_HASH)) {
           __FMT_PUT('.');
         }
+
         while (prec1 > 0) {
           if (--i1 < 0) {
             if (--k < 0)

test/libc/stdio/snprintf_test.c

@@ -217,12 +217,24 @@ TEST(snprintf, testLongDoubleRounding) {
   ASSERT_EQ(0, fesetround(previous_rounding));
 }
 
-void check_a_conversion_specifier_prec_1(const char *result_str, double value) {
+void check_a_conversion_specifier_double(const char *fmt, const char *expected_str, double value) {
   char buf[30] = {0};
-  int i = snprintf(buf, sizeof(buf), "%.1a", value);
+  int i = snprintf(buf, sizeof(buf), fmt, value);
 
-  ASSERT_EQ(strlen(result_str), i);
-  ASSERT_STREQ(result_str, buf);
+  ASSERT_EQ(strlen(expected_str), i);
+  ASSERT_STREQ(expected_str, buf);
+}
+
+void check_a_conversion_specifier_long_double(const char *fmt, const char *expected_str, long double value) {
+  char buf[30] = {0};
+  int i = snprintf(buf, sizeof(buf), fmt, value);
+
+  ASSERT_EQ(strlen(expected_str), i);
+  ASSERT_STREQ(expected_str, buf);
+}
+
+void check_a_conversion_specifier_prec_1(const char *expected_str, double value) {
+  check_a_conversion_specifier_double("%.1a", expected_str, value);
 }
 
 TEST(snprintf, testAConversionSpecifierRounding) {
@@ -247,6 +259,11 @@ TEST(snprintf, testAConversionSpecifier) {
   check_a_conversion_specifier_prec_1("0x1.6p+4", 0x1.68p+4);
   check_a_conversion_specifier_prec_1("0x1.ap+4", 0x1.98p+4);
   check_a_conversion_specifier_prec_1("0x1.ap+4", 0x1.a8p+4);
+
+  check_a_conversion_specifier_double("%#a", "0x0.p+0", 0x0.0p0);
+  check_a_conversion_specifier_double("%#A", "0X0.P+0", 0x0.0p0);
+  check_a_conversion_specifier_long_double("%#La", "0x0.p+0", 0x0.0p0L);
+  check_a_conversion_specifier_long_double("%#LA", "0X0.P+0", 0x0.0p0L);
 }
 
 TEST(snprintf, apostropheFlag) {