pdo_odbc: Don't fetch 256 byte blocks for long columns (#10809)

* pdo_odbc: Don't fetch 256 byte blocks for long columns

Fetching 256 byte blocks can confuse some drivers with conversion
routines. That, and it seems to me the round trips to and from a
database could be a major performance impact.

Instead, we try to fetch all at once, and continue fetching if a
driver somehow has more for us.

This has been tested with a problematic case with the Db2i driver
with stateful MBCS encodings.

See GH-10733 for discussion about this and issues it can resolve.

* change to separate by 256 bytes, when C->fetched_len == SQL_NO_TOTAL

change to separate by 256 bytes, when C->fetched_len == SQL_NO_TOTAL

changed from 256 byte to 2048 byte buf block.

* Make long column buffer size single define

Could be configurable maybe, but best to avoid magic numbers even for a
compile-time constant.

* Use ZendMM page size minus zend_string overhead

Change recommended by Christoph.

Probably a little better performance wise I have to guess.

* [skip ci] Update comment to mention constant

* Update UPGRADING for PDO_ODBC change

mention GH issues in UPGRADING too

* Update NEWS for PDO_ODBC change

---------

Co-authored-by: SakiTakamachi <saki@sakiot.com>

Author: NattyNarwhal

NEWS

@@ -2,6 +2,10 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? ????, PHP 8.5.0alpha2
 
+- PDO_ODBC
+  . Fetch larger block sizes and better handle SQL_NO_TOTAL when calling
+    SQLGetData. (Calvin Buckley, Saki Takamachi)
+
 - Standard:
   . Optimized pack(). (nielsdos, divinity76)
 

UPGRADING

@@ -599,6 +599,13 @@ PHP 8.5 UPGRADE NOTES
   . The `-z` or `--zend-extension` option has been removed as it was
     non-functional. Use `-d zend_extension=<path>` instead.
 
+- PDO_ODBC
+  . The fetch behaviour for larger columns has been changed. Rather than
+    fetching 256 byte blocks, PDO_ODBC will try to fetch a larger block size;
+    currently, this is the page size minus string overhead. Drivers that
+    return SQL_NO_TOTAL in SQLGetData are also better handled as well.
+    This should improve compatibility and performance. See GH-10809, GH-10733.
+
 ========================================
 14. Performance Improvements
 ========================================

ext/pdo_odbc/odbc_stmt.c

@@ -26,6 +26,9 @@
 #include "php_pdo_odbc.h"
 #include "php_pdo_odbc_int.h"
 
+/* Buffer size; bigger columns than this become a "long column" */
+#define LONG_COLUMN_BUFFER_SIZE (ZEND_MM_PAGE_SIZE- ZSTR_MAX_OVERHEAD)
+
 enum pdo_odbc_conv_result {
 	PDO_ODBC_CONV_NOT_REQUIRED,
 	PDO_ODBC_CONV_OK,
@@ -615,7 +618,7 @@ static int odbc_stmt_describe(pdo_stmt_t *stmt, int colno)
 	/* tell ODBC to put it straight into our buffer, but only if it
 	 * isn't "long" data, and only if we haven't already bound a long
 	 * column. */
-	if (colsize < 256 && !S->going_long) {
+	if (colsize < LONG_COLUMN_BUFFER_SIZE && !S->going_long) {
 		S->cols[colno].data = emalloc(colsize+1);
 		S->cols[colno].is_long = 0;
 
@@ -631,7 +634,7 @@ static int odbc_stmt_describe(pdo_stmt_t *stmt, int colno)
 	} else {
 		/* allocate a smaller buffer to keep around for smaller
 		 * "long" columns */
-		S->cols[colno].data = emalloc(256);
+		S->cols[colno].data = emalloc(LONG_COLUMN_BUFFER_SIZE);
 		S->going_long = 1;
 		S->cols[colno].is_long = 1;
 	}
@@ -657,37 +660,57 @@ static int odbc_stmt_get_col(pdo_stmt_t *stmt, int colno, zval *result, enum pdo
 		RETCODE rc;
 
 		/* fetch it into C->data, which is allocated with a length
-		 * of 256 bytes; if there is more to be had, we then allocate
+		 * of the page size minus zend_string overhead (LONG_COLUMN_BUFFER_SIZE);
+		 * if there is more to be had, we then allocate
 		 * bigger buffer for the caller to free */
 
 		rc = SQLGetData(S->stmt, colno+1, C->is_unicode ? SQL_C_BINARY : SQL_C_CHAR, C->data,
- 			256, &C->fetched_len);
+ 			LONG_COLUMN_BUFFER_SIZE, &C->fetched_len);
 		orig_fetched_len = C->fetched_len;
 
-		if (rc == SQL_SUCCESS && C->fetched_len < 256) {
+		if (rc == SQL_SUCCESS && C->fetched_len < LONG_COLUMN_BUFFER_SIZE) {
 			/* all the data fit into our little buffer;
 			 * jump down to the generic bound data case */
 			goto in_data;
 		}
 
 		if (rc == SQL_SUCCESS_WITH_INFO || rc == SQL_SUCCESS) {
-			/* this is a 'long column'
-
-			 read the column in 255 byte blocks until the end of the column is reached, reassembling those blocks
-			 in order into the output buffer; 255 bytes are an optimistic assumption, since the driver may assert
-			 more or less NUL bytes at the end; we cater to that later, if actual length information is available
-
-			 this loop has to work whether or not SQLGetData() provides the total column length.
-			 calling SQLDescribeCol() or other, specifically to get the column length, then doing a single read
-			 for that size would be slower except maybe for extremely long columns.*/
-			char *buf2 = emalloc(256);
-			zend_string *str = zend_string_init(C->data, 256, 0);
-			size_t used = 255; /* not 256; the driver NUL terminated the buffer */
+			/*
+			 * This is a long column.
+			 *
+			 * Try to get as much as we can at once. If the
+			 * driver somehow has more for us, get more. We'll
+			 * assemble it into one big buffer at the end.
+			 *
+			 * N.B. with n and n+1 mentioned in the comments:
+			 * n is the size returned without null terminator.
+			 *
+			 * The extension previously tried getting it in 256
+			 * byte blocks, but this could have created trouble
+			 * with some drivers.
+			 *
+			 * However, depending on the driver, fetched_len may
+			 * not contain the number of bytes and SQL_NO_TOTAL
+			 * may be passed.
+			 * The behavior in this case is the same as before,
+			 * dividing the data into blocks. However, it has been
+			 * changed from 256 byte to LONG_COLUMN_BUFFER_SIZE.
+			 */
+			ssize_t to_fetch_len;
+			if (orig_fetched_len == SQL_NO_TOTAL) {
+				to_fetch_len = C->datalen > (LONG_COLUMN_BUFFER_SIZE - 1) ? (LONG_COLUMN_BUFFER_SIZE - 1) : C->datalen;
+			} else {
+				to_fetch_len = orig_fetched_len;
+			}
+			ssize_t to_fetch_byte = to_fetch_len + 1;
+			char *buf2 = emalloc(to_fetch_byte);
+			zend_string *str = zend_string_init(C->data, to_fetch_byte, 0);
+			size_t used = to_fetch_len;
 
 			do {
 				C->fetched_len = 0;
-				/* read block. 256 bytes => 255 bytes are actually read, the last 1 is NULL */
-				rc = SQLGetData(S->stmt, colno+1, C->is_unicode ? SQL_C_BINARY : SQL_C_CHAR, buf2, 256, &C->fetched_len);
+				/* read block. n + 1 bytes => n bytes are actually read, the last 1 is NULL */
+				rc = SQLGetData(S->stmt, colno+1, C->is_unicode ? SQL_C_BINARY : SQL_C_CHAR, buf2, to_fetch_byte, &C->fetched_len);
 
 				/* adjust `used` in case we have proper length info from the driver */
 				if (orig_fetched_len >= 0 && C->fetched_len >= 0) {
@@ -698,13 +721,13 @@ static int odbc_stmt_get_col(pdo_stmt_t *stmt, int colno, zval *result, enum pdo
 				}
 
 				/* resize output buffer and reassemble block */
-				if (rc==SQL_SUCCESS_WITH_INFO || (rc==SQL_SUCCESS && C->fetched_len > 255)) {
+				if (rc==SQL_SUCCESS_WITH_INFO || (rc==SQL_SUCCESS && C->fetched_len > to_fetch_len)) {
 					/* point 5, in section "Retrieving Data with SQLGetData" in http://msdn.microsoft.com/en-us/library/windows/desktop/ms715441(v=vs.85).aspx
-					 states that if SQL_SUCCESS_WITH_INFO, fetched_len will be > 255 (greater than buf2's size)
-					 (if a driver fails to follow that and wrote less than 255 bytes to buf2, this will AV or read garbage into buf) */
-					str = zend_string_realloc(str, used + 256, 0);
-					memcpy(ZSTR_VAL(str) + used, buf2, 256);
-					used = used + 255;
+					 states that if SQL_SUCCESS_WITH_INFO, fetched_len will be > n (greater than buf2's size)
+					 (if a driver fails to follow that and wrote less than n bytes to buf2, this will AV or read garbage into buf) */
+					str = zend_string_realloc(str, used + to_fetch_byte, 0);
+					memcpy(ZSTR_VAL(str) + used, buf2, to_fetch_byte);
+					used = used + to_fetch_len;
 				} else if (rc==SQL_SUCCESS) {
 					str = zend_string_realloc(str, used + C->fetched_len, 0);
 					memcpy(ZSTR_VAL(str) + used, buf2, C->fetched_len);