Close #3285 - lint_file is now dynamic

`lint_file` can now be computed dynamically with a callback function,
which can return a deferred result, as per `ale#command#Run`. This
allows linters to dynamically switch between checking files on disk,
or checking code on the fly.

Some tests have been fixed on Windows.

2 files changed, 95 insertions, 26 deletions

diff --git a/autoload/ale/engine.vim b/autoload/ale/engine.vim
index b00ec453..ae0354b8 100644
--- a/autoload/ale/engine.vim
+++ b/autoload/ale/engine.vim
@@ -417,7 +417,7 @@ function! s:RunJob(command, options) abort
     let l:buffer = a:options.buffer
     let l:linter = a:options.linter
     let l:output_stream = a:options.output_stream
-    let l:read_buffer = a:options.read_buffer
+    let l:read_buffer = a:options.read_buffer && !a:options.lint_file
     let l:info = g:ale_buffer_info[l:buffer]
 
     let l:Callback = function('s:HandleExit', [{
@@ -508,10 +508,15 @@ function! s:AddProblemsFromOtherBuffers(buffer, linters) abort
     endif
 endfunction
 
-function! s:RunIfExecutable(buffer, linter, executable) abort
+function! s:RunIfExecutable(buffer, linter, lint_file, executable) abort
     if ale#command#IsDeferred(a:executable)
         let a:executable.result_callback = {
-        \   executable -> s:RunIfExecutable(a:buffer, a:linter, executable)
+        \   executable -> s:RunIfExecutable(
+        \       a:buffer,
+        \       a:linter,
+        \       a:lint_file,
+        \       executable
+        \   )
         \}
 
         return 1
@@ -519,7 +524,7 @@ function! s:RunIfExecutable(buffer, linter, executable) abort
 
     if ale#engine#IsExecutable(a:buffer, a:executable)
         " Use different job types for file or linter jobs.
-        let l:job_type = a:linter.lint_file ? 'file_linter' : 'linter'
+        let l:job_type = a:lint_file ? 'file_linter' : 'linter'
         call setbufvar(a:buffer, 'ale_job_type', l:job_type)
 
         let l:command = ale#linter#GetCommand(a:buffer, a:linter)
@@ -529,6 +534,7 @@ function! s:RunIfExecutable(buffer, linter, executable) abort
         \   'linter': a:linter,
         \   'output_stream': get(a:linter, 'output_stream', 'stdout'),
         \   'read_buffer': a:linter.read_buffer,
+        \   'lint_file': a:lint_file,
         \}
 
         return s:RunJob(l:command, l:options)
@@ -540,33 +546,62 @@ endfunction
 " Run a linter for a buffer.
 "
 " Returns 1 if the linter was successfully run.
-function! s:RunLinter(buffer, linter) abort
+function! s:RunLinter(buffer, linter, lint_file) abort
     if !empty(a:linter.lsp)
         return ale#lsp_linter#CheckWithLSP(a:buffer, a:linter)
     else
         let l:executable = ale#linter#GetExecutable(a:buffer, a:linter)
 
-        return s:RunIfExecutable(a:buffer, a:linter, l:executable)
+        return s:RunIfExecutable(a:buffer, a:linter, a:lint_file, l:executable)
     endif
 
     return 0
 endfunction
 
-function! ale#engine#RunLinters(buffer, linters, should_lint_file) abort
-    " Initialise the buffer information if needed.
-    let l:new_buffer = ale#engine#InitBufferInfo(a:buffer)
-    call s:StopCurrentJobs(a:buffer, a:should_lint_file)
-    call s:RemoveProblemsForDisabledLinters(a:buffer, a:linters)
+function! s:GetLintFileValues(slots, Callback) abort
+    let l:deferred_list = []
+    let l:new_slots = []
 
-    " We can only clear the results if we aren't checking the buffer.
-    let l:can_clear_results = !ale#engine#IsCheckingBuffer(a:buffer)
+    for [l:lint_file, l:linter] in a:slots
+        while ale#command#IsDeferred(l:lint_file) && has_key(l:lint_file, 'value')
+            " If we've already computed the return value, use it.
+            let l:lint_file = l:lint_file.value
+        endwhile
 
-    silent doautocmd <nomodeline> User ALELintPre
+        if ale#command#IsDeferred(l:lint_file)
+            " If we are going to return the result later, wait for it.
+            call add(l:deferred_list, l:lint_file)
+        else
+            " If we have the value now, coerce it to 0 or 1.
+            let l:lint_file = l:lint_file is 1
+        endif
 
-    for l:linter in a:linters
+        call add(l:new_slots, [l:lint_file, l:linter])
+    endfor
+
+    if !empty(l:deferred_list)
+        for l:deferred in l:deferred_list
+            let l:deferred.result_callback =
+            \   {-> s:GetLintFileValues(l:new_slots, a:Callback)}
+        endfor
+    else
+        call a:Callback(l:new_slots)
+    endif
+endfunction
+
+function! s:RunLinters(
+\   buffer,
+\   slots,
+\   should_lint_file,
+\   new_buffer,
+\   can_clear_results
+\) abort
+    let l:can_clear_results = a:can_clear_results
+
+    for [l:lint_file, l:linter] in a:slots
         " Only run lint_file linters if we should.
-        if !l:linter.lint_file || a:should_lint_file
-            if s:RunLinter(a:buffer, l:linter)
+        if !l:lint_file || a:should_lint_file
+            if s:RunLinter(a:buffer, l:linter, l:lint_file)
                 " If a single linter ran, we shouldn't clear everything.
                 let l:can_clear_results = 0
             endif
@@ -581,11 +616,49 @@ function! ale#engine#RunLinters(buffer, linters, should_lint_file) abort
     " disabled, or ALE itself is disabled.
     if l:can_clear_results
         call ale#engine#SetResults(a:buffer, [])
-    elseif l:new_buffer
-        call s:AddProblemsFromOtherBuffers(a:buffer, a:linters)
+    elseif a:new_buffer
+        call s:AddProblemsFromOtherBuffers(
+        \   a:buffer,
+        \   map(copy(a:slots), 'v:val[1]')
+        \)
     endif
 endfunction
 
+function! ale#engine#RunLinters(buffer, linters, should_lint_file) abort
+    " Initialise the buffer information if needed.
+    let l:new_buffer = ale#engine#InitBufferInfo(a:buffer)
+    call s:StopCurrentJobs(a:buffer, a:should_lint_file)
+    call s:RemoveProblemsForDisabledLinters(a:buffer, a:linters)
+
+    " We can only clear the results if we aren't checking the buffer.
+    let l:can_clear_results = !ale#engine#IsCheckingBuffer(a:buffer)
+
+    silent doautocmd <nomodeline> User ALELintPre
+
+    " Handle `lint_file` callbacks first.
+    let l:linter_slots = []
+
+    for l:linter in a:linters
+        let l:LintFile = l:linter.lint_file
+
+        if type(l:LintFile) is v:t_func
+            let l:LintFile = l:LintFile(a:buffer)
+        endif
+
+        call add(l:linter_slots, [l:LintFile, l:linter])
+    endfor
+
+    call s:GetLintFileValues(l:linter_slots, {
+    \   new_slots -> s:RunLinters(
+    \       a:buffer,
+    \       new_slots,
+    \       a:should_lint_file,
+    \       l:new_buffer,
+    \       l:can_clear_results,
+    \   )
+    \})
+endfunction
+
 " Clean up a buffer.
 "
 " This function will stop all current jobs for the buffer,
diff --git a/autoload/ale/linter.vim b/autoload/ale/linter.vim
index 8d89cd5c..b483fc19 100644
--- a/autoload/ale/linter.vim
+++ b/autoload/ale/linter.vim
@@ -211,21 +211,17 @@ function! ale#linter#PreProcess(filetype, linter) abort
     " file on disk.
     let l:obj.lint_file = get(a:linter, 'lint_file', 0)
 
-    if !s:IsBoolean(l:obj.lint_file)
-        throw '`lint_file` must be `0` or `1`'
+    if !s:IsBoolean(l:obj.lint_file) && type(l:obj.lint_file) isnot v:t_func
+        throw '`lint_file` must be `0`, `1`, or a Function'
     endif
 
     " An option indicating that the buffer should be read.
-    let l:obj.read_buffer = get(a:linter, 'read_buffer', !l:obj.lint_file)
+    let l:obj.read_buffer = get(a:linter, 'read_buffer', 1)
 
     if !s:IsBoolean(l:obj.read_buffer)
         throw '`read_buffer` must be `0` or `1`'
     endif
 
-    if l:obj.lint_file && l:obj.read_buffer
-        throw 'Only one of `lint_file` or `read_buffer` can be `1`'
-    endif
-
     let l:obj.aliases = get(a:linter, 'aliases', [])
 
     if type(l:obj.aliases) isnot v:t_list