112 lines
5.1 KiB
Markdown
112 lines
5.1 KiB
Markdown
## Debugger Visualizers
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Many languages and debuggers enable developers to control how a type is
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displayed in a debugger. These are called "debugger visualizations" or "debugger
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views".
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The Windows debuggers (WinDbg\CDB) support defining custom debugger visualizations using
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the `Natvis` framework. To use Natvis, developers write XML documents using the natvis
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schema that describe how debugger types should be displayed with the `.natvis` extension.
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(See: https://docs.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects?view=vs-2019)
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The Natvis files provide patterns which match type names a description of how to display
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those types.
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The Natvis schema can be found either online (See: https://code.visualstudio.com/docs/cpp/natvis#_schema)
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or locally at `<VS Installation Folder>\Xml\Schemas\1033\natvis.xsd`.
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The GNU debugger (GDB) supports defining custom debugger views using Pretty Printers.
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Pretty printers are written as python scripts that describe how a type should be displayed
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when loaded up in GDB/LLDB. (See: https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing.html#Pretty-Printing)
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The pretty printers provide patterns, which match type names, and for matching
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types, describe how to display those types. (For writing a pretty printer, see: https://sourceware.org/gdb/onlinedocs/gdb/Writing-a-Pretty_002dPrinter.html#Writing-a-Pretty_002dPrinter).
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### Embedding Visualizers
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Through the use of the currently unstable `#[debugger_visualizer]` attribute, the `smallvec`
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crate can embed debugger visualizers into the crate metadata.
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Currently the two types of visualizers supported are Natvis and Pretty printers.
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For Natvis files, when linking an executable with a crate that includes Natvis files,
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the MSVC linker will embed the contents of all Natvis files into the generated `PDB`.
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For pretty printers, the compiler will encode the contents of the pretty printer
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in the `.debug_gdb_scripts` section of the `ELF` generated.
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### Testing Visualizers
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The `smallvec` crate supports testing debugger visualizers defined for this crate. The entry point for
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these tests are `tests/debugger_visualizer.rs`. These tests are defined using the `debugger_test` and
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`debugger_test_parser` crates. The `debugger_test` crate is a proc macro crate which defines a
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single proc macro attribute, `#[debugger_test]`. For more detailed information about this crate,
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see https://crates.io/crates/debugger_test. The CI pipeline for the `smallvec` crate has been updated
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to run the debugger visualizer tests to ensure debugger visualizers do not become broken/stale.
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The `#[debugger_test]` proc macro attribute may only be used on test functions and will run the
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function under the debugger specified by the `debugger` meta item.
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This proc macro attribute has 3 required values:
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1. The first required meta item, `debugger`, takes a string value which specifies the debugger to launch.
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2. The second required meta item, `commands`, takes a string of new line (`\n`) separated list of debugger
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commands to run.
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3. The third required meta item, `expected_statements`, takes a string of new line (`\n`) separated list of
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statements that must exist in the debugger output. Pattern matching through regular expressions is also
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supported by using the `pattern:` prefix for each expected statement.
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#### Example:
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```rust
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#[debugger_test(
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debugger = "cdb",
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commands = "command1\ncommand2\ncommand3",
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expected_statements = "statement1\nstatement2\nstatement3")]
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fn test() {
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}
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```
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Using a multiline string is also supported, with a single debugger command/expected statement per line:
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```rust
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#[debugger_test(
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debugger = "cdb",
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commands = "
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command1
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command2
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command3",
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expected_statements = "
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statement1
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pattern:statement[0-9]+
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statement3")]
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fn test() {
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}
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```
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In the example above, the second expected statement uses pattern matching through a regular expression
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by using the `pattern:` prefix.
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#### Testing Locally
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Currently, only Natvis visualizations have been defined for the `smallvec` crate via `debug_metadata/smallvec.natvis`,
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which means the `tests/debugger_visualizer.rs` tests need to be run on Windows using the `*-pc-windows-msvc` targets.
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To run these tests locally, first ensure the debugging tools for Windows are installed or install them following
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the steps listed here, [Debugging Tools for Windows](https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/).
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Once the debugging tools have been installed, the tests can be run in the same manner as they are in the CI
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pipeline.
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#### Note
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When running the debugger visualizer tests, `tests/debugger_visualizer.rs`, they need to be run consecutively
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and not in parallel. This can be achieved by passing the flag `--test-threads=1` to rustc. This is due to
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how the debugger tests are run. Each test marked with the `#[debugger_test]` attribute launches a debugger
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and attaches it to the current test process. If tests are running in parallel, the test will try to attach
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a debugger to the current process which may already have a debugger attached causing the test to fail.
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For example:
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```
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cargo test --test debugger_visualizer --features debugger_visualizer -- --test-threads=1
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```
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