go-errcheck checker doesn't support multi-element $GOPATH

[bdarnell]

The GOPATH environment variable can be a colon-separated list of paths, which makes the mapping from filename to go package non-trivial (furthermore, in one of my projects where this is an issue, one GOPATH element is a prefix of another (github.com/cockroachdb/cockroach). Ideally the longest match should be used instead of the first match, although I haven't run into a case where this matters). flycheck-go-package-name needs to be augmented to support this; here is my attempt:

(defun flycheck-go-package-name (&optional file-name gopath)
  "Determine the package name for FILE-NAME and GOPATH.

FILE-NAME defaults to `buffer-file-name'.  GOPATH defaults to
$GOPATH.

Return the package name for FILE-NAME, or nil if FILE-NAME is not
part of any package or if GOPATH is nil."
  (-when-let* ((gopath (or gopath (getenv "GOPATH")))
               (file-name (or file-name (buffer-file-name))))
    (--min-by (> (length it) (length other))
     (-non-nil
      (--map
       (let ((gosrc (file-name-as-directory (expand-file-name "src/" it)))
             (file-name (expand-file-name file-name)))
         (when (string-prefix-p gosrc file-name)
           ;; The file is part of a package, so determine the package name, as
           ;; relative file name to the GO source directory
           (directory-file-name            ; Remove trailing /
            (file-relative-name (file-name-directory file-name) gosrc))))
       (split-string gopath ":"))))))

Alternately, we could just call out to the go tool: go list $DIR prints the package name defined by the given directory. The error-filter also needs to be updated to reassemble the path based on the prefix that was chosen by flycheck-go-package-name.

See also kisielk/errcheck#45, which would hopefully make all of this obsolete by making errcheck work more like the other go tools.

[swsnr]

@bdarnell Feel free to open a pull request. I won't change the corresponding code by myself, though: The current implementation is just a best guess, which tries to handle the simple case well. I don't know enough about Go—nothing actually—to properly handle $GOPATH.

[swsnr]

Closing as wontfix, because I'll not fix this myself. I do not use Go, and don't know anything about it.

That said, pull requests and patches are welcome, and I'll be happy to review and merge anything that addresses this issue.