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.

9 files changed, 278 insertions, 64 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
diff --git a/doc/ale.txt b/doc/ale.txt
index 14d2abbe..5ec542f9 100644
--- a/doc/ale.txt
+++ b/doc/ale.txt
@@ -3038,8 +3038,8 @@ ALELint                                                               *ALELint*
   Run ALE once for the current buffer. This command can be used to run ALE
   manually, instead of automatically, if desired.
 
-  This command will also run linters where `lint_file` is set to `1`, or in
-  other words linters which check the file instead of the Vim buffer.
+  This command will also run linters where `lint_file` is evaluates to `1`,
+  meaning linters which check the file instead of the Vim buffer.
 
   A plug mapping `<Plug>(ale_lint)` is defined for this command.
 
@@ -3252,9 +3252,9 @@ ale#Queue(delay, [linting_flag, buffer_number])                   *ale#Queue()*
   The linters will always be run in the background. Calling this function
   again from the same buffer
 
-  An optional `linting_flag` argument can be given. If `linting_flag`
-  is `'lint_file'`, then linters where the `lint_file` option is set to `1` will be
-  run. Linters with `lint_file` set to `1` are not run by default.
+  An optional `linting_flag` argument can be given. If `linting_flag` is
+  `'lint_file'`, then linters where the `lint_file` option evaluates to `1`
+  will be run. Otherwise, those linters will not be run.
 
   An optional `buffer_number` argument can be given for specifying the buffer
   to check. The active buffer (`bufnr('')`) will be checked by default.
@@ -3588,24 +3588,30 @@ ale#linter#Define(filetype, linter)                       *ale#linter#Define()*
                          if a command manually reads from a temporary file
                          instead, etc.
 
+                         This option behaves as if it was set to `0` when the
+                         `lint_file` option evaluates to `1`.
+
                                                                 *ale-lint-file*
-  `lint_file`              A |Number| (`0` or `1`) indicating whether a command
-                         should read the file instead of the Vim buffer. This
-                         option can be used for linters which must check the
-                         file on disk, and which cannot check a Vim buffer
-                         instead.
-
-                         Linters set with this option will not be run as a
-                         user types, per |g:ale_lint_on_text_changed|. Linters
-                         will instead be run only when events occur against
-                         the file on disk, including |g:ale_lint_on_enter|
-                         and |g:ale_lint_on_save|. Linters with this option
-                         set to `1` will also be run when linters are run
-                         manually, per |ALELintPost-autocmd|.
-
-                         When this option is set to `1`, `read_buffer` will
-                         be set automatically to `0`. The two options cannot
-                         be used together.
+  `lint_file`              A |Number| (`0` or `1`), or a |Funcref| for a function
+                         accepting a buffer number for computing either `0` or
+                         `1`, indicating whether a command should read the file
+                         instead of the Vim buffer. This option can be used
+                         for linters which must check the file on disk, and
+                         which cannot check a Vim buffer instead.
+
+                         The result can be computed with |ale#command#Run()|.
+
+                         Linters where the eventual value of this option
+                         evaluates to `1` will not be run as a user types, per
+                         |g:ale_lint_on_text_changed|. Linters will instead be
+                         run only when events occur against the file on disk,
+                         including |g:ale_lint_on_enter| and
+                         |g:ale_lint_on_save|. Linters where this option
+                         evaluates to `1` will also be run when the |ALELint|
+                         command is run.
+
+                         When this option is evaluates to `1`, ALE will behave
+                         as if `read_buffer` was set to `0`.
 
                                                               *ale-lsp-linters*
   `lsp`                    A |String| for defining LSP (Language Server Protocol)
diff --git a/supported-tools.md b/supported-tools.md
index 70a992a1..66e46348 100644
--- a/supported-tools.md
+++ b/supported-tools.md
@@ -16,7 +16,7 @@ formatting.
 
 |      Key      |             Definition           |
 | ------------- | -------------------------------- |
-| :floppy_disk: | Only checked when saved to disk  |
+| :floppy_disk: | May only run on files on disk    |
 |   :warning:   | Disabled by default              |
 
 ---
diff --git a/test/smoke_test.vader b/test/smoke_test.vader
index 53e08a8d..0b126cc6 100644
--- a/test/smoke_test.vader
+++ b/test/smoke_test.vader
@@ -1,8 +1,10 @@
 Before:
+  Save g:ale_enabled
   Save g:ale_set_lists_synchronously
   Save g:ale_buffer_info
   Save &shell
 
+  let g:ale_enabled = 1
   let g:ale_buffer_info = {}
   let g:ale_set_lists_synchronously = 1
 
diff --git a/test/test_computed_lint_file_values.vader b/test/test_computed_lint_file_values.vader
new file mode 100644
index 00000000..399e96fe
--- /dev/null
+++ b/test/test_computed_lint_file_values.vader
@@ -0,0 +1,134 @@
+Before:
+  Save g:ale_enabled
+  Save g:ale_run_synchronously
+  Save g:ale_set_lists_synchronously
+  Save g:ale_buffer_info
+
+  let g:ale_enabled = 1
+  let g:ale_buffer_info = {}
+  let g:ale_run_synchronously = 1
+  let g:ale_set_lists_synchronously = 1
+
+  function! TestCallback(buffer, output)
+    " Windows adds extra spaces to the text from echo.
+    return [{
+    \ 'lnum': 2,
+    \ 'col': 3,
+    \ 'text': 'testlinter1',
+    \}]
+  endfunction
+  function! TestCallback2(buffer, output)
+    " Windows adds extra spaces to the text from echo.
+    return [{
+    \ 'lnum': 1,
+    \ 'col': 3,
+    \ 'text': 'testlinter2',
+    \}]
+  endfunction
+  function! TestCallback3(buffer, output)
+    " Windows adds extra spaces to the text from echo.
+    return [{
+    \ 'lnum': 3,
+    \ 'col': 3,
+    \ 'text': 'testlinter3',
+    \}]
+  endfunction
+
+  " These two linters computer their lint_file values after running commands.
+  call ale#linter#Define('foobar', {
+  \ 'name': 'testlinter1',
+  \ 'callback': 'TestCallback',
+  \ 'executable': has('win32') ? 'cmd' : 'echo',
+  \ 'command': has('win32') ? 'echo foo bar' : '/bin/sh -c ''echo foo bar''',
+  \ 'lint_file': {b -> ale#command#Run(b, 'echo', {-> 1})},
+  \})
+  call ale#linter#Define('foobar', {
+  \ 'name': 'testlinter2',
+  \ 'callback': 'TestCallback2',
+  \ 'executable': has('win32') ? 'cmd' : 'echo',
+  \ 'command': has('win32') ? 'echo foo bar' : '/bin/sh -c ''echo foo bar''',
+  \ 'lint_file': {b -> ale#command#Run(b, 'echo', {-> ale#command#Run(b, 'echo', {-> 1})})},
+  \})
+  " This one directly computes the result.
+  call ale#linter#Define('foobar', {
+  \ 'name': 'testlinter3',
+  \ 'callback': 'TestCallback3',
+  \ 'executable': has('win32') ? 'cmd' : 'echo',
+  \ 'command': has('win32') ? 'echo foo bar' : '/bin/sh -c ''echo foo bar''',
+  \ 'lint_file': {b -> 1},
+  \})
+
+  let g:filename = tempname()
+  call writefile([], g:filename)
+  call ale#test#SetFilename(g:filename)
+
+After:
+  delfunction TestCallback
+
+  call ale#engine#Cleanup(bufnr(''))
+  Restore
+  call ale#linter#Reset()
+
+  " Items and markers, etc.
+  call setloclist(0, [])
+  call clearmatches()
+  call ale#sign#Clear()
+
+  if filereadable(g:filename)
+    call delete(g:filename)
+  endif
+
+  unlet g:filename
+
+Given foobar(A file with some lines):
+  foo
+  bar
+  baz
+
+Execute(lint_file results where the result is eventually computed should be run):
+  call ale#Queue(0, 'lint_file')
+  call ale#test#FlushJobs()
+
+  AssertEqual
+  \ [
+  \   {
+  \     'bufnr': bufnr('%'),
+  \     'lnum': 1,
+  \     'vcol': 0,
+  \     'col': 3,
+  \     'text': 'testlinter2',
+  \     'type': 'E',
+  \     'nr': -1,
+  \     'pattern': '',
+  \     'valid': 1,
+  \   },
+  \   {
+  \     'bufnr': bufnr('%'),
+  \     'lnum': 2,
+  \     'vcol': 0,
+  \     'col': 3,
+  \     'text': 'testlinter1',
+  \     'type': 'E',
+  \     'nr': -1,
+  \     'pattern': '',
+  \     'valid': 1,
+  \   },
+  \   {
+  \     'bufnr': bufnr('%'),
+  \     'lnum': 3,
+  \     'vcol': 0,
+  \     'col': 3,
+  \     'text': 'testlinter3',
+  \     'type': 'E',
+  \     'nr': -1,
+  \     'pattern': '',
+  \     'valid': 1,
+  \   },
+  \ ],
+  \ ale#test#GetLoclistWithoutModule()
+
+Execute(Linters where lint_file eventually evaluates to 1 shouldn't be run if we don't want to run them):
+  call ale#Queue(0, '')
+  call ale#test#FlushJobs()
+
+  AssertEqual [], ale#test#GetLoclistWithoutModule()
diff --git a/test/test_deferred_command_string.vader b/test/test_deferred_command_string.vader
index 026be6fe..173b6bb2 100644
--- a/test/test_deferred_command_string.vader
+++ b/test/test_deferred_command_string.vader
@@ -12,7 +12,7 @@ Before:
   call ale#linter#Define('foobar', {
   \ 'name': 'lint_file_linter',
   \ 'callback': 'LintFileCallback',
-  \ 'executable': 'echo',
+  \ 'executable': has('win32') ? 'cmd' : 'echo',
   \ 'command': {b -> ale#command#Run(b, 'echo', {-> ale#command#Run(b, 'echo', {-> 'foo'})})},
   \ 'read_buffer': 0,
   \})
@@ -28,7 +28,7 @@ After:
 
 Given foobar (Some imaginary filetype):
 Execute(It should be possible to compute an executable to check based on the result of commands):
-  AssertLinter 'echo', 'foo'
+  AssertLinter has('win32') ? 'cmd' : 'echo', 'foo'
 
   ALELint
   call ale#test#FlushJobs()
@@ -40,7 +40,7 @@ Execute(It should be possible to compute an executable to check based on the res
 Execute(It handle the deferred command failing):
   let g:ale_emulate_job_failure = 1
 
-  AssertLinter 'echo', 0
+  AssertLinter has('win32') ? 'cmd' : 'echo', 0
 
   ALELint
   call ale#test#FlushJobs()
diff --git a/test/test_linter_defintion_processing.vader b/test/test_linter_defintion_processing.vader
index 2c85299b..d000b158 100644
--- a/test/test_linter_defintion_processing.vader
+++ b/test/test_linter_defintion_processing.vader
@@ -174,7 +174,7 @@ Execute(PreProcess should process the lint_file option correctly):
   \}
 
   AssertThrows call ale#linter#PreProcess('testft', g:linter)
-  AssertEqual '`lint_file` must be `0` or `1`', g:vader_exception
+  AssertEqual '`lint_file` must be `0`, `1`, or a Function', g:vader_exception
 
   let g:linter.lint_file = 0
 
@@ -185,14 +185,17 @@ Execute(PreProcess should process the lint_file option correctly):
   let g:linter.lint_file = 1
 
   AssertEqual 1, ale#linter#PreProcess('testft', g:linter).lint_file
-  " The default for read_buffer should change to 0 when lint_file is 1.
-  AssertEqual 0, ale#linter#PreProcess('testft', g:linter).read_buffer
+  " The default for read_buffer should still be 1
+  AssertEqual 1, ale#linter#PreProcess('testft', g:linter).read_buffer
 
   let g:linter.read_buffer = 1
 
-  " We shouldn't be able to set both options to 1 at the same time.
-  AssertThrows call ale#linter#PreProcess('testft', g:linter)
-  AssertEqual 'Only one of `lint_file` or `read_buffer` can be `1`', g:vader_exception
+  " We should be able to set `read_buffer` and `lint_file` at the same time.
+  AssertEqual 1, ale#linter#PreProcess('testft', g:linter).read_buffer
+
+  let g:linter.lint_file = function('type')
+
+  Assert type(ale#linter#PreProcess('testft', g:linter).lint_file) is v:t_func
 
 Execute(PreProcess should set a default value for lint_file):
   let g:linter = {
diff --git a/test/test_swiftlint_executable_detection.vader b/test/test_swiftlint_executable_detection.vader
index dfd4930b..ac83ff8f 100644
--- a/test/test_swiftlint_executable_detection.vader
+++ b/test/test_swiftlint_executable_detection.vader
@@ -23,22 +23,22 @@ Execute(React Native apps using CocoaPods should take precedence over the defaul
   call ale#test#SetFilename('swiftlint-test-files/react-native/testfile.swift')
 
   AssertEqual
-  \ ale#path#Simplify(g:dir . '/swiftlint-test-files/react-native/ios/Pods/SwiftLint/swiftlint'),
-  \ ale_linters#swift#swiftlint#GetExecutable(bufnr(''))
+  \ tolower(ale#path#Simplify(g:dir . '/swiftlint-test-files/react-native/ios/Pods/SwiftLint/swiftlint')),
+  \ tolower(ale_linters#swift#swiftlint#GetExecutable(bufnr('')))
 
 Execute(CocoaPods installation should take precedence over the default executable):
   call ale#test#SetFilename('swiftlint-test-files/cocoapods/testfile.swift')
 
   AssertEqual
-  \ ale#path#Simplify(g:dir . '/swiftlint-test-files/cocoapods/Pods/SwiftLint/swiftlint'),
-  \ ale_linters#swift#swiftlint#GetExecutable(bufnr(''))
+  \ tolower(ale#path#Simplify(g:dir . '/swiftlint-test-files/cocoapods/Pods/SwiftLint/swiftlint')),
+  \ tolower(ale_linters#swift#swiftlint#GetExecutable(bufnr('')))
 
 Execute(Top level CocoaPods installation should take precedence over React Native installation):
   call ale#test#SetFilename('swiftlint-test-files/cocoapods-and-react-native/testfile.swift')
 
   AssertEqual
-  \ ale#path#Simplify(g:dir . '/swiftlint-test-files/cocoapods-and-react-native/Pods/SwiftLint/swiftlint'),
-  \ ale_linters#swift#swiftlint#GetExecutable(bufnr(''))
+  \ tolower(ale#path#Simplify(g:dir . '/swiftlint-test-files/cocoapods-and-react-native/Pods/SwiftLint/swiftlint')),
+  \ tolower(ale_linters#swift#swiftlint#GetExecutable(bufnr('')))
 
 Execute(use-global should override other versions):
   let g:ale_swift_swiftlint_use_global = 1