[3.10] gh-79579: Improve DML query detection in sqlite3 (GH-93623) (#93801)

The fix involves using pysqlite_check_remaining_sql(), not only to check
for multiple statements, but now also to strip leading comments and
whitespace from SQL statements, so we can improve DML query detection.

pysqlite_check_remaining_sql() is renamed lstrip_sql(), to more
accurately reflect its function, and hardened to handle more SQL comment
corner cases.

(cherry picked from commit 46740073ef)

Misc/NEWS.d/next/Library/2022-06-06-12-58-27.gh-issue-79579.e8rB-M.rst

@@ -0,0 +1,2 @@
+:mod:`sqlite3` now correctly detects DML queries with leading comments.
+Patch by Erlend E. Aasland.

Modules/_sqlite/statement.c

@@ -29,16 +29,7 @@
 #include "util.h"
 
 /* prototypes */
-static int pysqlite_check_remaining_sql(const char* tail);
+static const char *lstrip_sql(const char *sql);
-
-typedef enum {
-    LINECOMMENT_1,
-    IN_LINECOMMENT,
-    COMMENTSTART_1,
-    IN_COMMENT,
-    COMMENTEND_1,
-    NORMAL
-} parse_remaining_sql_state;
 
 typedef enum {
     TYPE_LONG,
@@ -55,7 +46,6 @@ pysqlite_statement_create(pysqlite_Connection *connection, PyObject *sql)
     int rc;
     const char* sql_cstr;
     Py_ssize_t sql_cstr_len;
-    const char* p;
 
     assert(PyUnicode_Check(sql));
 
@@ -87,20 +77,12 @@ pysqlite_statement_create(pysqlite_Connection *connection, PyObject *sql)
 
     /* Determine if the statement is a DML statement.
        SELECT is the only exception. See #9924. */
-    for (p = sql_cstr; *p != 0; p++) {
+    const char *p = lstrip_sql(sql_cstr);
-        switch (*p) {
+    if (p != NULL) {
-            case ' ':
-            case '\r':
-            case '\n':
-            case '\t':
-                continue;
-        }
-
         self->is_dml = (PyOS_strnicmp(p, "insert", 6) == 0)
                     || (PyOS_strnicmp(p, "update", 6) == 0)
                     || (PyOS_strnicmp(p, "delete", 6) == 0)
                     || (PyOS_strnicmp(p, "replace", 7) == 0);
-        break;
     }
 
     Py_BEGIN_ALLOW_THREADS
@@ -118,7 +100,7 @@ pysqlite_statement_create(pysqlite_Connection *connection, PyObject *sql)
         goto error;
     }
 
-    if (rc == SQLITE_OK && pysqlite_check_remaining_sql(tail)) {
+    if (rc == SQLITE_OK && lstrip_sql(tail)) {
         (void)sqlite3_finalize(self->st);
         self->st = NULL;
         PyErr_SetString(pysqlite_Warning,
@@ -431,73 +413,61 @@ stmt_traverse(pysqlite_Statement *self, visitproc visit, void *arg)
 }
 
 /*
- * Checks if there is anything left in an SQL string after SQLite compiled it.
+ * Strip leading whitespace and comments from incoming SQL (null terminated C
- * This is used to check if somebody tried to execute more than one SQL command
+ * string) and return a pointer to the first non-whitespace, non-comment
- * with one execute()/executemany() command, which the DB-API and we don't
+ * character.
- * allow.
  *
- * Returns 1 if there is more left than should be. 0 if ok.
+ * This is used to check if somebody tries to execute more than one SQL query
+ * with one execute()/executemany() command, which the DB-API don't allow.
+ *
+ * It is also used to harden DML query detection.
  */
-static int pysqlite_check_remaining_sql(const char* tail)
+static inline const char *
+lstrip_sql(const char *sql)
 {
-    const char* pos = tail;
+    // This loop is borrowed from the SQLite source code.
-
+    for (const char *pos = sql; *pos; pos++) {
-    parse_remaining_sql_state state = NORMAL;
-
-    for (;;) {
         switch (*pos) {
-            case 0:
-                return 0;
-            case '-':
-                if (state == NORMAL) {
-                    state  = LINECOMMENT_1;
-                } else if (state == LINECOMMENT_1) {
-                    state = IN_LINECOMMENT;
-                }
-                break;
             case ' ':
             case '\t':
-                break;
+            case '\f':
             case '\n':
-            case 13:
+            case '\r':
-                if (state == IN_LINECOMMENT) {
+                // Skip whitespace.
-                    state = NORMAL;
-                }
                 break;
-            case '/':
+            case '-':
-                if (state == NORMAL) {
+                // Skip line comments.
-                    state = COMMENTSTART_1;
+                if (pos[1] == '-') {
-                } else if (state == COMMENTEND_1) {
+                    pos += 2;
-                    state = NORMAL;
+                    while (pos[0] && pos[0] != '\n') {
-                } else if (state == COMMENTSTART_1) {
-                    return 1;
-                }
-                break;
-            case '*':
-                if (state == NORMAL) {
-                    return 1;
-                } else if (state == LINECOMMENT_1) {
-                    return 1;
-                } else if (state == COMMENTSTART_1) {
-                    state = IN_COMMENT;
-                } else if (state == IN_COMMENT) {
-                    state = COMMENTEND_1;
-                }
-                break;
-            default:
-                if (state == COMMENTEND_1) {
-                    state = IN_COMMENT;
-                } else if (state == IN_LINECOMMENT) {
-                } else if (state == IN_COMMENT) {
-                } else {
-                    return 1;
-                }
-        }
-
                         pos++;
                     }
+                    if (pos[0] == '\0') {
+                        return NULL;
+                    }
+                    continue;
+                }
+                return pos;
+            case '/':
+                // Skip C style comments.
+                if (pos[1] == '*') {
+                    pos += 2;
+                    while (pos[0] && (pos[0] != '*' || pos[1] != '/')) {
+                        pos++;
+                    }
+                    if (pos[0] == '\0') {
+                        return NULL;
+                    }
+                    pos++;
+                    continue;
+                }
+                return pos;
+            default:
+                return pos;
+        }
+    }
 
-    return 0;
+    return NULL;
 }
 
 static PyMemberDef stmt_members[] = {