Avoid a possible recursive copy when destination (build_dir) is underneath source directory

[andrewdavidmackenzie]

If the user makes a mistake in config mdbook could have started a never-ending copy that only ends when your disk is full. This change avoids that.

It also makes it possible to create a valid (if not recommended) configuration where your build-dir is underneath the src directory. One reason for possibly wishing to do that is when you desire src to be the root directory of a project.

Relates to #1024

[andrewdavidmackenzie]

Any feedback on this PR?

[ehuss]

I'm having trouble figuring out the configuration where this is a problem. If I set build.build-dir to src/foo, I don't see any problems happening. What is a configuration where it causes recursion?

[andrewdavidmackenzie]

When you have the book.toml file in the root of your project.

in book.toml you have src = "."

and the build dir is also under the root somewhere, something like

[build]
build-dir = "target"

So, during the build all the contents of "src" (so all files - except filtered .md files) are copied into "target", including "target" itself - recursing.

in short: when "build-dir" is a subdir of "src".

[ehuss]

When you have the book.toml file in the root of your project.

Ah. I still can't reproduce it, but I suspect it depends on how the OS handles mutating the directory while iterating over it. Seems reasonable to try to guard against it, though.

[andrewdavidmackenzie]

OK, I hope that addresses the two comments on texts in the PR.

[andrewdavidmackenzie]

Thanks. I'll switch to release v0.3.7 now!