How can I detect compilation-mode is waiting for input

Question

This often trips me up when building Linux kernels. I'll kick of a compile and after a while I realise it's not completed. What's usually happened is because the default config has changed it's waiting for user input in the compilation window. For example I find the compilation window stalled on the following:

-*- mode: compilation; default-directory: "~/lsrc/kvm/kernel-v8.build/" -*-
Compilation started at Tue Nov 25 10:34:20

cd /home/alex/lsrc/kvm/kernel-v8.build && ARCH=arm64 make -j9
  GEN     ./Makefile
scripts/kconfig/conf --silentoldconfig Kconfig
*
* Restart config...
*
*
* File systems
*
Second extended fs support (EXT2_FS) [Y/n/m/?] y
  Ext2 extended attributes (EXT2_FS_XATTR) [N/y/?] n
  Ext2 execute in place support (EXT2_FS_XIP) [N/y/?] n
Ext3 journalling file system support (EXT3_FS) [Y/n/m/?] y
  Default to 'data=ordered' in ext3 (EXT3_DEFAULTS_TO_ORDERED) [N/y/?] n
  Ext3 extended attributes (EXT3_FS_XATTR) [N/y/?] n
The Extended 4 (ext4) filesystem (EXT4_FS) [Y/n/m/?] y
  Ext4 POSIX Access Control Lists (EXT4_FS_POSIX_ACL) [N/y/?] n
  Ext4 Security Labels (EXT4_FS_SECURITY) [N/y/?] n
  EXT4 debugging support (EXT4_DEBUG) [N/y/?] n
JBD (ext3) debugging support (JBD_DEBUG) [N/y/?] n
JBD2 (ext4) debugging support (JBD2_DEBUG) [N/y/?] n
Reiserfs support (REISERFS_FS) [N/m/y/?] n
JFS filesystem support (JFS_FS) [N/m/y/?] n
XFS filesystem support (XFS_FS) [N/m/y/?] n
GFS2 file system support (GFS2_FS) [N/m/y/?] n
Btrfs filesystem support (BTRFS_FS) [N/m/y/?] n
NILFS2 file system support (NILFS2_FS) [N/m/y/?] n
Dnotify support (DNOTIFY) [Y/n/?] y
Inotify support for userspace (INOTIFY_USER) [Y/n/?] y
Filesystem wide access notification (FANOTIFY) [Y/n/?] y
  fanotify permissions checking (FANOTIFY_ACCESS_PERMISSIONS) [Y/n/?] y
Quota support (QUOTA) [N/y/?] n
Kernel automounter version 4 support (also supports v3) (AUTOFS4_FS) [N/m/y/?] n
FUSE (Filesystem in Userspace) support (FUSE_FS) [Y/n/m/?] y
  Character device in Userspace support (CUSE) [Y/n/m/?] y
Overlay filesystem support (OVERLAY_FS) [N/m/y/?] (NEW) 

It would be useful if I could detect this programatically so I could flag a notification in my mode-line. Any idea how I could detect this?

Answer 1

Emacs cannot detect why a child process is waiting. It might wait because it is calling read() on stdin, but it might just as well just perform an expensive computation while optimizing C code.

However, what you can detect is whether the compilation process has written a prompt. Compilation Mode runs compilation-filter-hook whenever it has inserted output from the compilation process into the compilation buffer. You are free to add your own functions to it.

For instance, you could write a function that searches for a particular prompt in the output, and emits a warning:

(defun my-compilation-mode-warn-about-prompt ()
  (save-excursion
    (let ((re '(rx "[" (one-or-more (any "n" "N" "m" "M" "Y" "y") "/") "?]"
                   (optional " (NEW)") (zero-or-more whitespace) buffer-end)))
      (when (re-search-backward re nil 'no-error)
        (lwarn 'emacs :warning "Compilation process in %s seems stalled!"
               (buffer-name))))))

(add-hook 'compilation-filter-hook
          #'my-compilation-mode-warn-about-prompt)

This will popup a warning buffer whenever the compilation process prints a prompt at the end of the buffer. The prompt is recognized by looking for variations of the [Y/n/?] part. I use rx to write a readable regular expression.

This should handle the case where the compilation process prints a prompt and then waits for input. In this case, the prompt will be the last text inserted into the buffer.

It might cause some false-positives (I didn't try), but since it's only a warning, these should be tolerable.