`Compile' hang on remote files, not sure if it is a concurrent race issue

[Fan Yang]

Hi all,

I’m look for any help for a hang issue when compiling remote files using 
“compile.el”.

Here is how I re-produce it:
  - emacs -Q
  - C-x C-f "/scp:vm01:pbd/virtio_balloon.c” (I’ve injected some typos into 
this file to make the compilation error).
  - M-x compile
  - The “*compilation*” buffer shows with the following content:
    -*- mode: compilation; default-directory: "/scp:vm01:/home/fan/pbd/" -*-
    Compilation started at Sat May 23 23:18:30
    
    make
    make -C /lib/modules/5.3.7-301.fc31.x86_64/build M=$PWD
    make[1]: Entering directory '/usr/src/kernels/5.3.7-301.fc31.x86_64'
      CC [M]  /home/fan/pbd/virtio_balloon.o
  - Then Emacs hangs with no CPU activity.
  - I start a terminal and run "pkill -SIGUSR2 Emacs”
  - The Emacs becomes alive, with the “*compilation*” buffer displaying the 
full content of error, and a “*Backtrace*”:
    Debugger entered--entering a function:
    * tramp-signal-hook-function(quit nil)
      accept-process-output(#<process *tramp/scp vm01*> nil nil t)
      tramp-accept-process-output(#<process *tramp/scp vm01*>)
      tramp-wait-for-regexp(#<process *tramp/scp vm01*> nil 
"\\(^\\|\0\\)[^#$\n]*///4e71ffd477719118f2d80d0a44acf5e3#\\$\\(\33[[0-9]+n\\)?\15?$")
      tramp-wait-for-output(#<process *tramp/scp vm01*>)
      tramp-send-command((tramp-file-name "scp" nil nil "vm01" nil 
"/usr/src/kernels/5.3.7-301.fc31.x86_64" nil) "\\readlink 
--canonicalize-missing /usr/src/kernels/5.3.7-301.fc31.x86_64 2>/dev/null; echo 
tramp_exit_status $?")
      tramp-send-command-and-check((tramp-file-name "scp" nil nil "vm01" nil 
"/usr/src/kernels/5.3.7-301.fc31.x86_64" nil) "\\readlink 
--canonicalize-missing /usr/src/kernels/5.3.7-301.fc31.x86_64")
      
tramp-sh-handle-file-truename("/scp:vm01:/usr/src/kernels/5.3.7-301.fc31.x86_64")
      apply(tramp-sh-handle-file-truename 
"/scp:vm01:/usr/src/kernels/5.3.7-301.fc31.x86_64")
      tramp-sh-file-name-handler(file-truename 
"/scp:vm01:/usr/src/kernels/5.3.7-301.fc31.x86_64")
      apply(tramp-sh-file-name-handler file-truename 
"/scp:vm01:/usr/src/kernels/5.3.7-301.fc31.x86_64")
      tramp-file-name-handler(file-truename 
"/scp:vm01:/usr/src/kernels/5.3.7-301.fc31.x86_64")
      file-truename("/scp:vm01:/usr/src/kernels/5.3.7-301.fc31.x86_64")
      compilation-get-file-structure(("./include/linux/export.h" . 
"/usr/src/kernels/5.3.7-301.fc31.x86_64") nil)
      compilation-internal-error-properties(("./include/linux/export.h" . 
"/usr/src/kernels/5.3.7-301.fc31.x86_64") 45 nil nil nil 0 nil)
      compilation-error-properties(1 2 nil 3 nil (4 . 5) nil)
      compilation-parse-errors(292 1239)
      compilation--parse-region(292 #<marker at 1316 in *compilation*>)
      compilation--ensure-parse(1316)
      compilation-filter(#<process compilation> "In file included from 
./include/linux/export.h:45,\n                 from 
./include/linux/linkage.h:7,\n                 from 
./include/linux/kernel.h:8,\n                 from ./include/linux/list.h:9,\n  
               from ./include/linux/module.h:9,\n                 from 
/home/fan/pbd/virtio_balloon.c:7:\n/home/fan/pbd/virtio_balloon.c: In function 
'file_vpb':\n./include/linux/kernel.h:990:51: error: dereferencing pointer to 
incomplete type 'struct virtio_balloon'\n  990 |  
BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \\\n      |       
                                            
^~\n./include/linux/compiler.h:330:9: note: in definition of macro 
'__compiletime_assert'\n  330 |   if (!(condition))     \\\n      |         
^~~~~~~~~\n./include/linux/compiler.h:350:2: note: in expansion of macro 
'_compiletime_assert'\n  350 |  _compiletime_assert(condition, msg, 
__compiletime_assert_, __LINE__)\n      |  
^~~~~~~~~~~~~~~~~~~\n./include/linux/build_bug.h:39:37: note: in expansion of 
macro 'compiletime_a")
      read-event(nil t 2)
      sit-for(2)
      execute-extended-command(nil "compile" "compile")
      funcall-interactively(execute-extended-command nil "compile" "compile")
      call-interactively(execute-extended-command nil nil)
      command-execute(execute-extended-command)

This happens only to two of my remote projects, and only when I inject such 
typos that producing many lines of compilation errors and warnings.  Once I run 
into this issue (for a specific remote project, and specific typos), I can 
re-produce again and again.  I tried Emacs on mac (--with-ns) and Emacs on 
Ubuntu 18.04, they both have this issue.

I once wondered whether it is due to a race to the same TRAMP connection since 
the “compilation--parse-region” seems can run concurrently with the “compile” 
process.  I boldly hacked “compilation-filter” to make it deferred until the 
compilation completes:

  (defun compilation-filter (proc string)
    "Process filter for compilation buffers.
  Just inserts the text,
   handles carriage motion (see `comint-inhibit-carriage-motion'),
   and runs `compilation-filter-hook'."
     (when (buffer-live-p (process-buffer proc))
  +    (while (memq proc compilation-in-progress) (sleep-for 1))
       (with-current-buffer (process-buffer proc)
         (let ((inhibit-read-only t)

Then the hang issue disappears.  Though I’m totally not sure if the issue is 
really due to the race.  And I know this is only a temporary workaround.

Do you know can TRAMP handle concurrent access to the same connection 
correctly, especially in this case?  And do you have any idea/advise how to fix 
this hang issue?

Thanks
Fan