test.vim: more efficient compiling of test files

This change improves the way we compile a Go test. Previously we would
build a test binary to catch any build errors. A callback was then
responsible to delete the test binary after showing build errors (if
there were any).

This could be racy though, because if the tests lasts long and we quit
early, the callback to clean the binary file would never run. So we
would end up with artifacts in our working directory.

To fix the issue we're going to tell to the `go` tool to run a specific,
unique function. We're passing a unique identifier as a function name
(which is a randomly generated). This will cause the go tool to build
the test file and then try to run the test function. Because there is no
such test function, it'll silently quit with zero exit status. As a side
effect it'll compile the test file, so we're able to catch any build
errors (if any)

fixes #907

autoload/go/test.vim

@@ -6,8 +6,9 @@ function! go#test#Test(bang, compile, ...) abort
 
   " don't run the test, only compile it. Useful to capture and fix errors.
   if a:compile
-    let compile_file = "vim-go-test-compile"
-    call extend(args, ["-c", "-o", compile_file])
+    " we're going to tell to run a test function that doesn't exist. This
+    " triggers a build of the test file itself but no tests will run.
+    call extend(args, ["-run", "499EE4A2-5C85-4D35-98FC-7377CD87F263"])
   endif
 
   if a:0
@@ -45,13 +46,10 @@ function! go#test#Test(bang, compile, ...) abort
           \ 'bang': a:bang,
           \ 'winnr': winnr(),
           \ 'dir': getcwd(),
+          \ 'compile_test': a:compile,
           \ 'jobdir': fnameescape(expand("%:p:h")),
           \ }
 
-    if a:compile
-      let job_args['compile_cb'] = function('s:test_compile', [compile_file])
-    endif
-
     call s:test_job(job_args)
     return
   elseif has('nvim')
@@ -62,10 +60,6 @@ function! go#test#Test(bang, compile, ...) abort
       let id = go#jobcontrol#Spawn(a:bang, "test", args)
     endif
 
-    if a:compile
-      call go#jobcontrol#AddHandler(function('s:test_compile_handler'))
-      let s:test_compile_handlers[id] = compile_file
-    endif
     return id
   endif
 
@@ -81,10 +75,6 @@ function! go#test#Test(bang, compile, ...) abort
   let dir = getcwd()
   execute cd fnameescape(expand("%:p:h"))
 
-  if a:compile
-    call delete(compile_file)
-  endif
-
   if go#util#ShellError() != 0
     let errors = s:parse_errors(split(out, '\n'))
     let errors = go#tool#FilterValids(errors)
@@ -150,7 +140,7 @@ function s:test_job(args) abort
         \ 'state': "started",
         \ }
 
-  if has_key(a:args, 'compile_cb')
+  if a:args.compile_test
     let status.state = "compiling"
   endif
 
@@ -171,8 +161,7 @@ function s:test_job(args) abort
           \ 'state': "pass",
           \ }
 
-    if has_key(a:args, 'compile_cb')
-      call a:args.compile_cb(a:job, a:exitval, messages)
+    if a:args.compile_test
       let status.state = "success"
     endif
 
@@ -282,25 +271,5 @@ function! s:parse_errors(lines) abort
   return errors
 endfunction
 
-" test_compile is called when a GoTestCompile call is finished
-function! s:test_compile(test_file, job, exit_status, data) abort
-  call delete(a:test_file)
-endfunction
-
-" -----------------------
-" | Neovim job handlers |
-" -----------------------
-let s:test_compile_handlers = {}
-
-function! s:test_compile_handler(job, exit_status, data) abort
-  if !has_key(s:test_compile_handlers, a:job.id)
-    return
-  endif
-  let l:compile_file = s:test_compile_handlers[a:job.id]
-  call delete(l:compile_file)
-  unlet s:test_compile_handlers[a:job.id]
-endfunction
-
-
 " vim: sw=2 ts=2 et
 "