Use double-NULL byte for empty process environments on Windows

[pyrmont]

While working on Churlish (a wrapper around the curl CLI utility), I noticed intermittent errors while using os/spawn on Windows. After further investigation, I believe the issue is that I was calling os/spawn with an effectively empty¹ environment and that this was causing CreateProcess to occasionally fail due to a bug in os_execute_env.

The issue is that an empty environment block needs to be double-NULL terminated as described in more detail in Microsoft's documentation:

Note that an ANSI environment block is terminated by two zero bytes: one for the last string, one more to terminate the block. A Unicode environment block is terminated by four zero bytes: two for the last string, two more to terminate the block.

I suspect that the error was intermittent because sometimes the memory being accessed happened to have a second NULL byte after the first. This was not guaranteed to be the case.

This commit adds an extra NULL byte if the environment is empty. It also adds a test.

Footnotes

1. I was calling os/spawn with a struct that mapped :in, :out and :err to pipes. I mistakenly thought that this meant I needed to use :pe as the second argument to os/spawn. Because the struct contained no other values, it was empty for the purposes of os_execute_env. ↩

[pyrmont]

For future reference, binaries that are compiled with MinGW can fail on Windows (not WINE) when launched with os/execute (or os/spawn) because the absence of PATH causes system DLLs not to load. As best as I can tell, this is a MinGW-specific issue and results in error 0xC0000135 (exit code -1073741515). The test case checks if running MinGW on Windows and, only if that is the case, permits that exit code.

This is obviously not a great solution but I think it's the most reasonable approach. All the alternatives I could think of (e.g. silently adding PATH to the 'empty' environment when running in MinGW-compiled Janet, always panicking even though there might not be a problem) seemed worse.

[bakpakin]

Looks good, that is a subtle issue. A missing PATH environment variable is a recipe for all sorts of issues, so I wouldn't worry too much about that failure mode