Patch for bug #67839 (mysqli does not handle 4-byte floats correctly)

Before the patch, a value of 9.99 in a FLOAT column came out of mysqli
as 9.9998998641968. This is because it would naively cast a 4-byte float
into PHP's internal 8-byte double.
To fix this, with GCC we use the built-in decimal support to "up-convert"
the 4-byte float to a 8-byte double.
When that is not available, we fall back to converting the float
to a string and then converting the string to a double. This mimics
what MySQL does.

1 files changed, 45 insertions, 4 deletions

diff --git a/ext/mysqlnd/mysqlnd_ps_codec.c b/ext/mysqlnd/mysqlnd_ps_codec.c
index e2640c775b..3f7d31002f 100644
--- a/ext/mysqlnd/mysqlnd_ps_codec.c
+++ b/ext/mysqlnd/mysqlnd_ps_codec.c
@@ -195,12 +195,53 @@ void ps_fetch_float(zval *zv, const MYSQLND_FIELD * const field,
 					unsigned int pack_len, zend_uchar **row,
 					zend_bool as_unicode TSRMLS_DC)
 {
-	float value;
+	float fval;
+	double dval;
 	DBG_ENTER("ps_fetch_float");
-	float4get(value, *row);
-	ZVAL_DOUBLE(zv, value);
+	float4get(fval, *row);
 	(*row)+= 4;
-	DBG_INF_FMT("value=%f", value);
+	DBG_INF_FMT("value=%f", fval);
+
+	/*
+	 * The following is needed to correctly support 4-byte floats.
+	 * Otherwise, a value of 9.99 in a FLOAT column comes out of mysqli
+	 * as 9.9998998641968.
+	 *
+	 * For GCC, we use the built-in decimal support to "up-convert" a
+	 * 4-byte float to a 8-byte double.
+	 * When that is not available, we fall back to converting the float
+	 * to a string and then converting the string to a double. This mimics
+	 * what MySQL does.
+	 */
+#ifdef HAVE_DECIMAL_FP_SUPPORT
+	{
+		typedef float dec32 __attribute__((mode(SD)));
+		dec32 d32val = fval;
+
+		/* The following cast is guaranteed to do the right thing */
+		dval = (double) d32val;
+	}
+#else
+	{
+		char num_buf[2048]; /* Over allocated */
+		char *s;
+
+		/* Convert to string. Ignoring localization, etc.
+		 * Following MySQL's rules. If precision is undefined (NOT_FIXED_DEC i.e. 31)
+		 * or larger than 31, the value is limited to 6 (FLT_DIG).
+		 */
+		s = php_gcvt(fval,
+			     field->decimals >= 31 ? 6 : field->decimals,
+			     '.',
+			     'e',
+			     num_buf);
+
+		/* And now convert back to double */
+		dval = zend_strtod(s, NULL);
+	}
+#endif
+
+	ZVAL_DOUBLE(zv, dval);
 	DBG_VOID_RETURN;
 }
 /* }}} */