Make NewCustomCommitWriteCloser preserve io.Seeker interface

[bduffany]

Fixes a bug that certain bytestream reads are not retryable, e.g. this one:

buildbuddy/server/cache/dirtools/dirtools.go

Line 1049 in e463a99

w := ioutil.NewCustomCommitWriteCloser(f)

[vanja-p]

I like this in general, partially because I've wanted to make CustomCommitWriteCloser keep the ReadFrom method if it's there. On the other hand, if we wanted to keep ReadFrom as well, we would need 4 implementations of customCommitWriteCloser.

Here are 2 alternatives:

1. Since Seek returns an error, could the existing, concrete CustomCommitWriteCloser always have a Seek method, and return an error if the underlying writer doesn't support it? We would have to change cachetools.GetBlob to not do the initial Seek.
2. We could have a ioutil.GetSeeker function, which accepts x any. If x is an io.Seeker it returns that. If not, it does a type match against all the concrete types in ioutil, and returns the underlying reader or writer if it implements io.Seeker? Then cachetools.GetBlob (and UploadFromReaderWithCompression) would call that instead of doing a type check.

[bduffany]

@vanja-p I think we should optimize for reduced complexity of the ioutil package interface, rather than reduced complexity of the ioutil package implementation, because it is such a high-utility interface with lots of callers.

• Alternative 1 adds complexity to callers because they have to remember to check both whether something is an io.Seeker and also check whether a seek returns an ignorable error.
• Alternative 2 adds complexity to callers because they have to remember to call GetSeeker instead of doing a simple cast to io.Seeker.

if we wanted to keep ReadFrom as well, we would need 4 implementations of customCommitWriteCloser.

I prefer having 4 implementations to both of the above alternatives, since the complexity is hidden in the ioutil package.

[vanja-p]

I have a few other comments pending, but SGTM.

[bduffany]

@vanja-p I addressed your concerns in c8926ae