cmd/go: revamp and simplify logic in PrepareForCoverageBuild

Change the 'PrepareForCoverageBuild' helper function to provide more
sensible defaults in the case where Go packages are listed on the
command line (e.g. cases such as "go run -cover mumble.go").

With the old implementation, if module mode was enabled, we would only
instrument packages in the main module, meaning that if you did
something like this:

  $ ls go.mod
  go.mod
  $ GOCOVERDATA=/tmp/cov go run -cover testdata/somefile.go
  $

no coverage profiles would be generated at all. This is due to the
fact that the pseudo-package created by the Go command internally when
building "somefile.go" is not considered part of the main module.

This patch moves the default to "packages explicitly mentioned on the
command line, plus packages in the main module", which will make more
sense to users passing specific packages and *.go files on the command
line. Examples:

  // Here cmd/compile is part the Go standard library + commands
  // (which we exclude from instrumentation by default), but since
  // 'cmd/compile' is mentioned on the command line, we will instrument
  // just that single package (not any of its deps).
  $ cd $GOROOT/src
  $ go build -o gc.exe -cover cmd/compile
  $ GOCOVERDATA=/tmp/cov ./gc.exe ...
  ...
  $

  // Here we're running a Go file named on the command line, where
  // the pseudo-package for the command line is not part of the
  // main module, but it makes sense to instrument it anyhow.
  $ cd ~/go/k8s.io/kubernetes
  $ GOCOVERDATA=/tmp/cov go run -cover test/typecheck/testdata/bad/bad.go
  ...
  $

This patch also simplifies the logic and improves flow/comments in
in the helper function PrepareForCoverageBuild.

Change-Id: Id8fc8571157dac8c09e44cc73baa05aeba1640ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/445918
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>

0 files changed, 0 insertions, 0 deletions