ensure filehandles are closed

[docwhat]

See https://www.joeshaw.org/dont-defer-close-on-writable-files/

[docwhat]

I can't reproduce this locally... I think it is a legit error:

=== FAIL: internal/filewatcher TestHasGoFiles/none (0.00s)
    --- FAIL: TestHasGoFiles/none (0.00s)
panic: sync C:\Users\circleci\AppData\Local\Temp\TestHasGoFiles-none-706743447: The handle is invalid. [recovered]
	panic: sync C:\Users\circleci\AppData\Local\Temp\TestHasGoFiles-none-706743447: The handle is invalid.

goroutine 16 [running]:
testing.tRunner.func1.1(0xc62da0, 0xc00005ee40)
	c:/go/src/testing/testing.go:1072 +0x310
testing.tRunner.func1(0xc00016c300)
	c:/go/src/testing/testing.go:1075 +0x43a
panic(0xc62da0, 0xc00005ee40)
	c:/go/src/runtime/panic.go:969 +0x1c7
gotest.tools/gotestsum/internal/filewatcher.hasGoFiles(0xc000012460, 0x42, 0xc00000a800)
	C:/Users/circleci/project/internal/filewatcher/watch.go:167 +0x493
gotest.tools/gotestsum/internal/filewatcher.TestHasGoFiles.func1(0xc00016c300)
	C:/Users/circleci/project/internal/filewatcher/watch_test.go:88 +0x115
testing.tRunner(0xc00016c300, 0xca6a70)
	c:/go/src/testing/testing.go:1123 +0xef
created by testing.(*T).Run
	c:/go/src/testing/testing.go:1168 +0x2b3

=== FAIL: internal/filewatcher TestHasGoFiles (0.00s)

[docwhat]

I'd be okay with disabling at least this lint message due to this reasoning.

internal/filewatcher/watch.go:160:16: Error return value of `fh.Close` is not checked (errcheck)
	defer fh.Close()

[dnephin]

Thank you for tracking down the problem and opening this PR! You are absolutely correct this missing fh.Close() is a problem. golangci-lint has linters for catching missing close on http response bodies, and sql rows, but apparently not for file handlers unfortunately.

That article is a great reminder about how to safely handle writes. Thankfully in this case we are only doing reads (https://golang.org/pkg/os/#Open, "Open opens the named file for reading"). I believe that explains the panic we see from windows. I guess on linux it is not an error to try and sync with a read-only handle, but on windows it is.

Since this is read-only we can remove the fh.Sync(), there isn't anything to sync.

I agree we can also ignore the lint error about the ignored return, we don't care about errors in this case because it is read-only.

I've added some suggested edits which should resolve the build errors. I'll apply those and merge once CI is happy.

[dnephin]

LGTM! CI is happy with those changes!