Header-only file without final newline causes panic

[birjj]

What happened?

If you create a file that only consists of a single header, and don't leave a final newline at the end of the file, mark will panic with

panic: runtime error: slice bounds out of range [8:7]

goroutine 1 [running]:
github.com/kovetskiy/mark/metadata.ExtractMeta({0xc0003eb0a8, 0x7, 0x8}, {0xc00001009b, 0x3}, 0x1, 0x0, {0xc00058e960, 0x16}, {0xc00072b7a0, ...}, ...)
	/go/src/github.com/kovetskiy/mark/metadata/metadata.go:192 +0x106d
github.com/kovetskiy/mark/util.processFile({0xc00058e960, 0x16}, 0xc000408b60, 0xc0005cc288, {0x0, 0x0}, {0xc0000400fe, 0x3f}, 0xc00066b997)
	/go/src/github.com/kovetskiy/mark/util/cli.go:119 +0x22c
github.com/kovetskiy/mark/util.RunMark({0xc0005cc288?, 0x1b86658?}, 0xc0005cc288)
	/go/src/github.com/kovetskiy/mark/util/cli.go:87 +0x6c5
github.com/urfave/cli/v3.(*Command).run(0xc0005cc288, {0x1b86658, 0xc0004d5c20}, {0xc0000b00a0, 0x1, 0x1})
	/go/pkg/mod/github.com/urfave/cli/v3@v3.4.1/command_run.go:344 +0x2773
github.com/urfave/cli/v3.(*Command).Run(...)
	/go/pkg/mod/github.com/urfave/cli/v3@v3.4.1/command_run.go:94
main.main()
	github.com/kovetskiy/mark/main.go:37 +0x18a

What did you expect to happen?

mark should not panic

How can we reproduce the behavior you experienced?

Steps to reproduce the behavior:

1. Create a file containing exactly # Blogs. The file must not be saved with a final newline.
2. Attempt to run mark on the file.

Minimal markdown (remember to check that the saved file does not have a final newline):

# Blogs

Further Information (please complete the following information)

• Mark Version (mark --version): v15.0.0 (Docker image)
• Mark Parameters: [e.g. --drop-h1 --title-from-h1]
• Confluence Hosting: [e.g. Cloud, Server or Datacenter]
• Confluence Version: [e.g. v7.13]
• Environment specific Information: Running in GitHub Actions

Logs or other output

panic: runtime error: slice bounds out of range [8:7]

goroutine 1 [running]:
github.com/kovetskiy/mark/metadata.ExtractMeta({0xc0003eb0a8, 0x7, 0x8}, {0xc00001009b, 0x3}, 0x1, 0x0, {0xc00058e960, 0x16}, {0xc00072b7a0, ...}, ...)
	/go/src/github.com/kovetskiy/mark/metadata/metadata.go:192 +0x106d
github.com/kovetskiy/mark/util.processFile({0xc00058e960, 0x16}, 0xc000408b60, 0xc0005cc288, {0x0, 0x0}, {0xc0000400fe, 0x3f}, 0xc00066b997)
	/go/src/github.com/kovetskiy/mark/util/cli.go:119 +0x22c
github.com/kovetskiy/mark/util.RunMark({0xc0005cc288?, 0x1b86658?}, 0xc0005cc288)
	/go/src/github.com/kovetskiy/mark/util/cli.go:87 +0x6c5
github.com/urfave/cli/v3.(*Command).run(0xc0005cc288, {0x1b86658, 0xc0004d5c20}, {0xc0000b00a0, 0x1, 0x1})
	/go/pkg/mod/github.com/urfave/cli/v3@v3.4.1/command_run.go:344 +0x2773
github.com/urfave/cli/v3.(*Command).Run(...)
	/go/pkg/mod/github.com/urfave/cli/v3@v3.4.1/command_run.go:94
main.main()
	github.com/kovetskiy/mark/main.go:37 +0x18a

Additional context

I believe this is caused by

mark/metadata/metadata.go

Line 68 in dd67c43

offset += len(line) + 1

Since mark adds 1 to the length of the line, which is presumably done to increment past the \n that the scanner strips, the final offset on a single-line-no-newline file ends up being 1 beyond the length of the file.

Note that the current implementation also has another issue: if the file uses Windows-style newlines (\r\n) then + 1 does not increment to the beginning of the next line as intended.