Unittest Error: Debugging 'fatal_vfmt' In Chain Moves

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Hey guys, let's dive into a head-scratcher: the dreaded fatal_vfmt error popping up during a unit test. Specifically, we're talking about the unittest/wallet/test/run-chain_moves_duplicate-detect test within the Bitcoin project. This error, as you'll see, can be a bit of a pain to debug, especially when you're not immediately greeted with a stack trace. But don't worry, we'll break it down and figure out how to tackle this.

The Setup and the Scenario

So, what's the deal? You're running make check-units, and bam! The test in question fails. The specific error message, fatal_vfmt called!, is a signal that something went very wrong deep within the code. This usually indicates a formatting error that has been deemed fatal by the system. In this instance, we are working with Bitcoin Core, and you configured it with a specific set of parameters.

Let's take a look at the configuration used:

./configure --prefix=/usr --disable-debugbuild --disable-compat --disable-valgrind \
    --disable-static --disable-coverage --disable-address-sanitizer --disable-ub-sanitize \
    --disable-fuzzing --disable-rust

You'll notice some key flags here, notably the disabling of debug builds and sanitizers. This is a common setup for production builds, which is great for performance, but it can make debugging a lot trickier. Because debug builds include extra information, debugging without them could be much harder, but we can do it! Also, it's worth noting that this particular build uses PostgreSQL exclusively, which may affect the behavior of certain tests. We'll keep this in mind as we proceed.

Now, the core issue is that fatal_vfmt was called. Because the fatal error did not produce a stack trace, our debugging journey is more difficult from the start. We're essentially flying blind, which requires a more systematic approach to identify the root cause.

Understanding the Error

The fatal_vfmt error generally means there's a problem with the formatting of a string, usually involving a variable argument list (like printf). This can happen for many reasons: a mismatched format specifier, an incorrect number of arguments, or a problem within the formatting process itself. Without a stack trace, we have no instant clue of where the error is. That means we have to go digging.

Deep Dive: Troubleshooting Steps

So, where do we start? Here's a breakdown of how to approach this kind of debugging challenge:

1. Reproduce the Error Reliably

Make sure you can reproduce the error consistently. Run the test again and again to confirm that the fatal_vfmt error keeps happening. This is your baseline; if the error is intermittent, it becomes exponentially harder to debug.

2. Examine the Test's Source Code

Go directly to the source code for run-chain_moves_duplicate-detect. Look at how it sets up the test environment, the variables it uses, and the functions it calls. Look at the specific logging and reporting functions used within that test. Any calls to formatting functions like printf or related functions are of particular interest.

3. Strategic Logging

Since we don't have a stack trace, we have to make our own. Add some strategic logging statements to the test code. Use LogPrintf or a similar logging function to print out the values of variables at various points in the test's execution. Start by logging the arguments just before any calls that might be causing the error. This can help pinpoint what arguments are being passed to the problematic formatting function.

4. GDB to the Rescue

If the logging doesn't immediately reveal the problem, it's time to bring in the big guns: the GNU Debugger (GDB). Compile the code with debug symbols enabled (you may need to temporarily enable the debug build, or at least compile the specific test file with debug symbols) and run the test under GDB.

  • Set a breakpoint: Set a breakpoint at the location where you suspect the error is happening (e.g., at the start of a function that uses printf or a similar function). If you can't pinpoint the exact location, set the breakpoint at the beginning of the test function.
  • Inspect variables: When the breakpoint hits, inspect the values of variables to see if anything looks amiss. Use GDB commands like print to see the values and backtrace to see the call stack (even though the original error didn't provide one). The backtrace command can provide a lot of information.
  • Step through the code: Use next (step to the next line), step (step into the next function call), and continue (continue execution until the next breakpoint) to walk through the code and observe what's happening.

5. Review the PostgreSQL Integration

Because your build uses PostgreSQL exclusively, check if the test interacts with the database in any way. If so, there could be a problem related to data formatting or SQL queries that is causing the issue. Check how the test interacts with the database. Check that the queries are formed correctly and that data types are handled as expected. Ensure that the database connection is established correctly.

6. Isolate the Problem

If the test involves multiple steps or components, try to isolate the problem. For example, comment out sections of the test to see if the error still occurs. If it does not, you know the issue is within the commented-out section. This is a classic divide-and-conquer strategy.

7. Double-Check the Format Strings

Carefully review all the format strings (e.g., those used in printf and LogPrintf) within the test. Make sure the format specifiers (like %d, %s, %f, etc.) match the types of the variables you're passing in. A mismatch here is a classic cause of fatal_vfmt errors.

8. Consider Compiler Warnings

Even though you're using a build that disables debug features, pay attention to any compiler warnings. Warnings often indicate potential problems that can lead to runtime errors. Address any warnings before digging deeper into the specific error.

Specific Tips and Considerations

Here are some more tailored tips to help you in your quest:

  • Look for Memory Corruption: While less likely without debug builds and sanitizers, memory corruption can sometimes lead to unexpected behavior and formatting errors. If you suspect memory corruption, consider running a memory checker tool (e.g., valgrind, although you've disabled it in this build) or re-enabling address sanitizers to help detect these issues.

  • Test Environment: Make sure your test environment is set up correctly. This includes the database configuration, any required dependencies, and any other external factors that the test relies on.

  • Version Control: Make sure you're working with the correct version of the code. If you've made any local changes, make sure you understand how they might be affecting the test.

Example: Debugging with GDB

Let's walk through a simplified GDB example. Imagine the test has a function called process_transaction that uses LogPrintf to output information. We might do this:

  1. Compile with Debug Symbols: If the test fails, recompile the specific test or the whole project with debug symbols. This will usually involve adding a flag like -g to the compiler. It's often easier to build with -g enabled from the start during development and testing.

  2. Start GDB: Run gdb ./wallet/test/run-chain_moves_duplicate-detect

  3. Set Breakpoint: In GDB, set a breakpoint at the start of the process_transaction function using break process_transaction.

  4. Run the Test: Run the test inside GDB with run.

  5. Inspect Variables: When the breakpoint hits, use print variable_name to view the variables. Also, you can print the arguments being passed to LogPrintf to check the values and confirm that it matches with what is expected.

  6. Step Through: Use next or step to execute the code line by line and see what happens.

  7. Check the backtrace: You can use the backtrace command to examine the call stack. This might not directly help with the fatal_vfmt error since the error is happening, but it can still help you understand how the test is working.

Conclusion

Debugging a fatal_vfmt error without a stack trace requires patience and a systematic approach. By carefully examining the test code, adding strategic logging, using GDB, and isolating the problem, you can hopefully track down the cause and fix it. Keep in mind the specific details of your setup (PostgreSQL-only build, disabled debug flags) and adjust your debugging strategy accordingly. Good luck, and happy debugging, guys! Remember to reproduce the error, and isolate the source of error!