Fixing "Redundant Argument in sprintf" Error in pt-query-digest for MariaDB

• Pipeline Process: Percona Toolkit's pt-query-digest likely uses a multi-step process to analyze database queries. Process 5 refers to a specific stage in this pipeline.
• Redundant Argument in sprintf: The error indicates that the sprintf function, commonly used for formatted string creation in C programming, was called with an extra argument (something it doesn't expect). This typically happens when a format string containing special characters (%) is used within the string itself without proper escaping.

Potential Causes (related to MariaDB):

• Table or Column Names with %: If a MariaDB table or column name contains a percent sign (%), it might be misinterpreted as a format specifier within the sprintf function call in pt-query-digest. To fix this, escape the percent sign using a double percent (%%) within the string passed to sprintf.
• Query Text with %: Similarly, if the query text itself has a percent sign that's not intended for formatting, you'll need to escape it with a double percent.

Troubleshooting Steps:

1. Examine Line 2556: If you have access to the pt-query-digest source code, inspect line 2556 to see how sprintf is being used. Look for strings being passed to sprintf that might contain unescaped percent signs.
2. Review MariaDB Schema: Check your MariaDB table and column names for any that contain percent signs.
3. Inspect Query Text: If possible, analyze the queries being processed by pt-query-digest to identify any percent signs that might be causing the issue.

Resolving the Issue:

• Escape Percent Signs: Once you've identified the problematic string, escape any percent signs that are not intended for formatting by using %%.

Example:

// Incorrect (percent sign interpreted as format specifier)
char query[100];
sprintf(query, "SELECT * FROM my_table WHERE col1 = %s", value);

// Correct (escapes the percent sign)
sprintf(query, "SELECT * FROM my_table WHERE col1 = %%s", value);

Additional Considerations:

• Percona Toolkit documentation or community forums might offer specific solutions or workarounds for pt-query-digest errors related to MariaDB table or column names.
• Consider upgrading pt-query-digest to a newer version, as bug fixes related to sprintf usage might have been addressed.

#include <stdio.h>

int main() {
  char query[100];
  char value[] = "data with%"; // Value contains a percent sign

  // Incorrect (percent sign interpreted as format specifier)
  sprintf(query, "SELECT * FROM my_table WHERE col1 = %s", value);

  printf("Query: %s\n", query); // This might cause an error
  return 0;
}

Correct Usage (Escaping Percent Sign):

#include <stdio.h>

int main() {
  char query[100];
  char value[] = "data with%"; // Value contains a percent sign

  // Correct (escapes the percent sign)
  sprintf(query, "SELECT * FROM my_table WHERE col1 = %%s", value);

  printf("Query: %s\n", query); // This should print correctly
  return 0;
}

In the incorrect example, the percent sign within the value string is misinterpreted as a format specifier because it's not escaped. This leads to the "redundant argument" error. The corrected example demonstrates how to use a double percent (%%) to escape the original percent sign, ensuring it's treated as a literal character within the query string.

• These examples assume you're using C programming, as sprintf is a common function in that language. The concept of escaping format specifiers might hold true for similar functions in other languages as well.
• The specific code used in pt-query-digest might look different, but the principle of escaping percent signs remains the same.

• snprintf: This function is similar to sprintf but takes an additional argument for the maximum buffer size. It can be safer as it prevents buffer overflows if the formatted string exceeds the allocated space. However, it might require adjustments to the code depending on how sprintf is being used.
• C++ String Streams: If you're working with C++, consider using string streams (stringstream or ostringstream) for string formatting. These offer a more robust and type-safe approach compared to sprintf. However, this would require modifying the pt-query-digest code itself, which might not be feasible for everyone.

Upgrading pt-query-digest:

As mentioned earlier, the error might be due to a bug in older versions of pt-query-digest. Upgrading to a newer version might have a fix for the specific sprintf usage that's causing the issue. Check the Percona Toolkit documentation or release notes for details.

Custom String Handling (if applicable):

If you have access to the pt-query-digest source code and understand the context where sprintf is used, you could potentially modify the code to handle string construction differently. This could involve building the query string manually with string concatenation or using alternative formatting methods entirely. However, this approach requires a deep understanding of the code and potential side effects.

Important Considerations:

• These alternative methods have their own trade-offs and might require code modifications. Weigh the complexity and applicability of each approach before proceeding.
• Upgrading pt-query-digest is often the simplest and safest solution if a newer version addresses the bug.
• If you're not comfortable modifying the code, escaping percent signs remains the recommended approach for most cases.