Summary

This enables the govet shadow analysis pass, implemented in https://github.com/golang/tools/blob/master/go/analysis/passes/shadow/shadow.go and enabled via golangci-lint.

The shadow linter prevents some cases of shadowed variables being used, which especially impacts the re-use of common variable names like err and ok. In this PR I attempted to resolve these issues mostly be just renaming names like err to more specific and unused names like err2 or osErr.

In a few other cases I updated

err := X()
if err != nil {
    return err
}

to

err = X()
if err != nil {
    return err
}

if the check-and-return came right away. I also removed a couple of named error return variables or unnecessary declarations of names earlier in a function that were shadowed later.

The shadow linter strict: false setting is used which comes with the following explanation:

A span stores the minimum range of byte positions in the file in which a given variable (types.Object) is mentioned. It is lexically defined: it spans from the beginning of its first mention to the end of its last mention. A variable is considered shadowed (if strict is off) only if the shadowing variable is declared within the span of the shadowed variable. In other words, if a variable is shadowed but not used after the shadowed variable is declared, it is inconsequential and not worth complaining about. This simple check dramatically reduces the nuisance rate for the shadowing
check, at least until something cleverer comes along.

One wrinkle: A "naked return" is a silent use of a variable that the Span will not capture, but the compilers catch naked returns of shadowed variables so we don't need to.

Cases this gets wrong (TODO):

• If a for loop's continuation statement mentions a variable redeclared in the block, we should complain about it but don't.
• A variable declared inside a function literal can falsely be identified as shadowing a variable in the outer function.

Test Plan

Existing tests should pass — in most cases I did not change the logic at all, but I did make some bigger changes to resolve some of the warnings.