Custom Collation Function Issues in SQLite: String Length and Null-Termination

Issue Overview: Custom Collation Function Misbehaves with String Length and Null-Termination

When implementing a custom collation function in SQLite, particularly for handling French text with accented characters, two critical issues can arise. The first issue pertains to the incorrect handling of string lengths passed to the collation function. The second issue involves the assumption that the strings passed to the collation function are null-terminated, which is not the case. These issues can lead to unexpected sorting behavior, especially when comparing strings of unequal lengths or when one string is a prefix of the other.

The custom collation function, french_collation_nocase, is designed to sort text by replacing accented characters with their unaccented equivalents. However, the function initially misinterprets the length parameters (nKey1 and nKey2) and assumes that the input strings (pOriginal1 and pOriginal2) are null-terminated. This misunderstanding leads to incorrect comparisons, particularly when one string is a substring of the other, such as "béotien" and "béotiennes".

Possible Causes: Misinterpretation of String Length and Null-Termination Assumptions

The root cause of the first issue lies in the initial misunderstanding of the nKey1 and nKey2 parameters. These parameters represent the length of the strings in bytes, not the number of characters. The original implementation incorrectly assumed that these values were always zero, leading to the use of strlen() to determine the string lengths. However, strlen() relies on null-terminated strings, which brings us to the second issue.

The second issue stems from the assumption that the strings passed to the collation function are null-terminated. SQLite does not guarantee that the strings passed to custom collation functions are null-terminated. Instead, it provides the length of the strings in bytes via nKey1 and nKey2. When the function attempts to use strlen() on these non-null-terminated strings, it reads beyond the intended memory, leading to undefined behavior and incorrect comparisons.

Additionally, the use of size_t for string length calculations can introduce subtle bugs. The strlen() function returns a size_t, which is an unsigned integer type. When subtracting two size_t values, the result can be an exceptionally large positive number if the second string is longer than the first, leading to incorrect sorting behavior.

Troubleshooting Steps, Solutions & Fixes: Correct Handling of String Length and Null-Termination

To resolve these issues, the custom collation function must correctly handle the string lengths provided by SQLite and avoid assumptions about null-termination. Here are the steps to fix the function:

1. Correctly Use nKey1 and nKey2 for String Lengths: The nKey1 and nKey2 parameters should be used directly to determine the length of the strings. This avoids the need for strlen() and ensures that the function works with non-null-terminated strings.

2. Avoid Assumptions About Null-Termination: Since the strings passed to the collation function are not guaranteed to be null-terminated, the function should not rely on strlen(). Instead, it should use the lengths provided by nKey1 and nKey2 to safely access the string data.

3. Handle String Comparisons Properly: When comparing strings of unequal lengths, the function should ensure that the comparison logic correctly handles the case where one string is a prefix of the other. This involves comparing only the relevant portions of the strings and using the provided lengths to avoid reading beyond the intended memory.

Here is the corrected version of the french_collation_nocase function:

static int french_collation_nocase(void *pArg, int nKey1, const void *pOriginal1, int nKey2, const void *pOriginal2) {
    const unsigned char *pOriginalText1 = (const unsigned char*) pOriginal1;
    const unsigned char *pOriginalText2 = (const unsigned char*) pOriginal2;
    
    // Allocate buffers for translated text
    char pTranslatedText1[nKey1 + 1];
    char pTranslatedText2[nKey2 + 1];
    
    // Translate the text, ensuring null-termination
    translitterate(pOriginalText1, pTranslatedText1, nKey1);
    translitterate(pOriginalText2, pTranslatedText2, nKey2);
    
    // Null-terminate the translated strings
    pTranslatedText1[nKey1] = '\0';
    pTranslatedText2[nKey2] = '\0';
    
    // Compare the translated strings
    int diff = 0;
    char *p1 = pTranslatedText1;
    char *p2 = pTranslatedText2;
    
    while (*p1 && *p2) {
        diff = tolower(*p1++) - tolower(*p2++);
        if (diff) {
            return diff;
        }
    }
    
    // If one string is a prefix of the other, the shorter string should come first
    if (!diff) {
        diff = nKey1 - nKey2;
    }
    
    return diff;
}

In this corrected version, the function uses nKey1 and nKey2 to determine the length of the strings and ensures that the translated strings are null-terminated before comparison. This avoids the issues caused by assuming null-termination and incorrect string length handling.

4. Testing and Validation: After implementing the fixes, it is crucial to thoroughly test the collation function with various strings, including edge cases such as strings with accented characters, strings of unequal lengths, and strings where one is a prefix of the other. This ensures that the function behaves as expected in all scenarios.

5. Considerations for size_t: When dealing with string lengths, be mindful of the size_t type. If you need to perform arithmetic operations that could result in negative values, consider casting the result to a signed integer type to avoid unexpected behavior.

By following these steps, the custom collation function can be corrected to handle string lengths and null-termination properly, ensuring accurate and reliable sorting of French text with accented characters in SQLite.