[llvm-branch-commits] [llvm] [Dexter] Document the structured script model (PR #204365)

[Orlando Cazalet-Hyams via llvm-branch-commits]

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+# Dexter Script Testing
+
+Dexter's script mode can be accessed by using the `--use-script` flag.
+
+Dexter scripts are represented by YAML documents, which contain various "nodes" instructing Dexter how to step through the debuggee program, what information to collect and store from the debugger, and how to evaluate the result. A simple Dexter script looks something like this:
+
+```yaml
+---
+!where {function: foo}:
+    !value arg: 5
+    !type arg: int
+    !and {lines: !range [10, 14]}:
+        !value local: ['a', 'b', 'c']
+!where {function: bar}:
+    !where {function: baz}:
+        !step exactly: [20, 21, 22, 23, 24]
+...
+```
+
+This Dexter test checks that:
+- When the debugger steps into `foo`, the type and value of `arg` is always `(int) 5`.
+- While the debugger is in `foo` and the current line is between 10 and 14 (inclusive), the value of `local` is `'a'`, `'b'`, or `'c'`.
+- While the debugger is in the function `baz`, which was called directly from the function `bar`, the lines that the debugger steps through exactly the lines 20-24 in order.
+
+The Dexter test follows a structure based on the nodes - in the example above, each line starts with a node. Some of the basic types of node are:
+
+- `!where` describes a single stack frame using either a function name or a filename + line range; these will be used by the debugger to set breakpoints. We consider a `!where` node to be "active" when the current stack frame matches the `!where` node. A `!where` node can either appear at the "root" of the script, or it can appear as the child of another `!where`, in which case it will only be active when its parent `!where` matches the frame above it. For example, the `!where {function: baz}` node is only active when the next frame up is `bar`, matching its parent.
+- `!and` is similar to `!where`, but it can only match the same stack frame as its parent `!where` (and cannot appear at the root of the script). For example, the `!and {lines: !range [10, 14]}` node is only active when the current line number is in the range [10-14] *and* the current function is `foo`, because the `!and` is a child of `!where {function: foo}`. `!where` and `!and` nodes are collectively referred to as "state" nodes.
+- `!value` and `!type` are "expect" nodes, meaning they describe testable output from the debugger. These nodes must appear a children of a state node (`!where` or `!and`), and are active whenever their parent is active. The form these nodes take is `!(value|type) <variable-name>: <expected-values>`, and their function is to collect information for `<variable-name>` while the debugger is running and the node is active, and compare that to `<expected-values>` during the evaluation step to produce the final test results.
+- `!step` is another kind of expect node, which tests the line numbers seen while stepping through the program, and its expected value is a list of line numbers that we expect to see (or not see in some cases - see more detailed documentation below).
+- `!range` isn't a "script node" as the others above are, but a "utility node", meaning it is used by other nodes to represent some data. `!range [<start>, <stop>]` represents an inclusive range from `start` to `stop`, and is used by state nodes.
+
+All these nodes are arranged in a nested map structure, where each state node maps to its children. A YAML document containing this structure is embedded in the input test file: the file may either be a YAML file, where the whole file is a single document, or else the first valid YAML document contained in the file which is also a valid Dexter test script will be used. Generally, this requires one line that is just `---` to start the document, and another which is just `...` to end the document.
+
+# Script Nodes
+
+## State Nodes
+
+State nodes are matched against stack frames when the debugger is stopped, and are used to control how Dexter controls the debugger (e.g. what step/continue actions to take after each stop) and determine the scope where other nodes are evaluated. State nodes have child nodes, declare some form of state that can be compared against a particular stack frame to produce either a match or non-match; when they match against the current frame, their children are evaluated. State nodes have the following rules:
+
+- **Root** state nodes are top-level nodes in the script. Each time the debugger stops, Dexter attempts to match each root node to each stack frame, searching from the *root-most* to the *leaf-most* stack frame, and stopping at the first matching frame (if any). Root nodes must always be `!where` nodes.
+- **Nested** state nodes refer to any non-root state nodes. There are two kinds of nested state node possible: `!where` and `!and`. A nested `!where` node can only match the stack frame called from the frame that matches its parent state node. A nested `!and` node can only match the same stack frame as its parent state node.
+- Each state node can match only one frame per-step, e.g. if you have a recursive function `fib` and a root node `!where {function: fib}`, the node will only match the outermost call, not any of the recursive calls. Conversely however, it is possible for a single frame to be matched by different state nodes.
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OCHyams wrote:

outermost -> leafmost/rootmost

https://github.com/llvm/llvm-project/pull/204365