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hclsyntax fuzzing utilities

This directory contains helper functions and corpora that can be used to fuzz-test the hclsyntax parsers using Go's native fuzz testing capabilities.

Please see https://go.dev/doc/fuzz/ for more information on fuzzing.

Prerequisites

  • Go 1.18

Running the fuzzer

Each exported function in the hclsyntax package has a corresponding fuzz test. These can be run one at a time via go test:

$ cd fuzz
$ go test -fuzz FuzzParseTemplate
$ go test -fuzz FuzzParseTraversalAbs
$ go test -fuzz FuzzParseExpression
$ go test -fuzz FuzzParseConfig

This command will exit only when a crasher is found (see "Understanding the result" below.)

Seed corpus

The seed corpus for each fuzz test function is stored in the corresponding directory under hclsyntax/fuzz/testdata/fuzz/. For example:

$ ls hclsyntax/fuzz/testdata/fuzz/FuzzParseTemplate
empty.tmpl
escape-dollar.tmpl
escape-newline-tmpl
...

Additional seed inputs can be added to this corpus. Each file must be in the Go 1.18 corpus file format. Files can be converted to this format using the file2fuzz tool. To install it:

$ go install golang.org/x/tools/cmd/file2fuzz@latest
$ file2fuzz -help

Understanding the result

A small number of subdirectories will be created in the work directory.

If you let go-fuzz run for a few minutes (the more minutes the better) it may detect "crashers", which are inputs that caused the parser to panic. These are written to hclsyntax/fuzz/testdata/fuzz/<fuzz test name>/:

$ ls hclsyntax/fuzz/testdata/fuzz/FuzzParseTemplate
582528ddfad69eb57775199a43e0f9fd5c94bba343ce7bb6724d4ebafe311ed4

A good first step to fixing a detected crasher is to copy the failing input into one of the unit tests in the hclsyntax package and see it crash there too. After that, it's easy to re-run the test as you try to fix it.