[Bug]: NULL Pointer Dereference in cypher_execute()

[dirtyfilthy]

Pre-submission Checklist

• I have searched existing issues to ensure this bug hasn't been reported
• I have read the documentation
• I am using a supported platform (Linux x86_64)

Version

main #001122d

Platform

Ubuntu 20.04

Bug Description

Problem: The cypher_execute() SQL function segfaults when called before a graph virtual table is created. The function accesses the global pGraph pointer without null validation.

Root Cause: The global pGraph variable (defined in src/graph.c:27) is initialized to NULL and only set when CREATE VIRTUAL TABLE graph USING graph() is executed. If cypher_execute() is called before table creation, pGraph is NULL, causing segfaults when passed to functions that dereference it.

Affected Files:

• src/cypher/cypher-executor-sql.c - Function cypherExecuteSqlFunc() at lines 82, 116

The function uses pGraph without checking if it's NULL

Steps to Reproduce

import sqlite3
conn = sqlite3.connect(":memory:")
conn.enable_load_extension(True)
conn.load_extension("libgraph.so")

# This will segfault because no graph virtual table exists yet:
conn.execute("SELECT cypher_execute('MATCH (n) RETURN n')")

Code Sample

see above

Error Message / Stack Trace

Expected Behavior

SQLite should return an error indicating that a graph virtual table needs to be created first, e.g.:

Error: Graph virtual table not initialized. Create a graph table first using: CREATE VIRTUAL TABLE graph USING graph();

Actual Behavior

Process crashes with segmentation fault (SIGSEGV) because pGraph is NULL and is dereferenced without validation.

Environment Details

SQLite version: 3.x (system libsqlite3)
Extension: sqlite-graph (current main/master branch)
OS: Linux (aarch64), likely affects all platforms
Reproduction: Confirmed with GDB showing segfault when pGraph is NULL

Additional Context

No response

Severity

• This is a critical bug (crashes, data loss)
• This is a major bug (significant functionality broken)
• This is a minor bug (minor functionality affected)
• This is a cosmetic issue