Add line count check to avoid out of range

[valterc]

IndexOutOfRangeException is thrown in StringLineGroup when calling Iterator.NextChar

markdig/src/Markdig/Helpers/StringLineGroup.cs

Line 276 in 8155a1e

public char NextChar()

The exception occurs when trying to advance to the next line when the StringLineGroup is at capacity (capacity == line_count)

markdig/src/Markdig/Helpers/StringLineGroup.cs

Line 339 in 8155a1e

_currentSlice = _lines.Lines[SliceIndex];

This exception is kind of an edge case and does not happen often because LeafBlock and other parts that use the StringLineGroup provision some extra capacity in advance and when capacity is needed it is increased by a factor of 2, so it's not very common that the needed capacity matches the line count.

[MihaZupan]

Did you hit this edge-case with any real-world input while parsing?

Looks like some tests aren't happy with this change, mind looking at those?

[valterc]

My bad, I didn't really run all the tests locally.

I did hit this case when I was parsing a yaml front matter which contained exactly 8 lines inside of it. Looking through the code, LeafBlock reserves 4 lines up-front:

markdig/src/Markdig/Syntax/LeafBlock.cs

Line 82 in 8155a1e

Lines = new StringLineGroup(4, ProcessInlines);

And then the StringLineGroup increases the capacity to 8, when using the CharIterator this bug comes up.

In my case I was building a subclass of TextReader based on the CharIterator (avoiding allocations) to use in some other frameworks.

I've added a sort of end-to-end test in TestYamlFrontMatterExtension, let me know if that is OK or should I add in the RoundTrip tests. Without the fix, the test fails with exception:

Example markdown that breaks the CharIterator:

---
key1: value1
key2: value2
key3: value3
key4: value4
key5: value5
key6: value6
key7: value7
key8: value8
---

# Content

[coveralls]

Pull Request Test Coverage Report for Build 4734877669

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 48 of 49 (97.96%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 93.323%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Markdig.Tests/TestYamlFrontMatterExtension.cs	15	16	93.75%

Totals
Change from base Build 4279648756:	0.03%
Covered Lines:	25978
Relevant Lines:	27233

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💛 - Coveralls