API: nvim_get_proc()

TODO: "exepath" field (win32: QueryFullProcessImageName())

On unix-likes `ps` is used because the platform-specific APIs are
a nightmare.  For reference, below is a (incomplete) attempt:

diff --git a/src/nvim/os/process.c b/src/nvim/os/process.c
index 09769925aca5..99afbbf290c1 100644
--- a/src/nvim/os/process.c
+++ b/src/nvim/os/process.c
@@ -208,3 +210,60 @@ int os_proc_children(int ppid, int **proc_list, size_t *proc_count)
   return 0;
 }

+/// Gets various properties of the process identified by `pid`.
+///
+/// @param pid Process to inspect.
+/// @return Map of process properties, empty on error.
+Dictionary os_proc_info(int pid)
+{
+  Dictionary pinfo = ARRAY_DICT_INIT;
+#ifdef WIN32
+
+#elif defined(__APPLE__)
+  char buf[PROC_PIDPATHINFO_MAXSIZE];
+  if (proc_pidpath(pid, buf, sizeof(buf))) {
+    name = getName(buf);
+    PUT(pinfo, "exepath", STRING_OBJ(cstr_to_string(buf)));
+    return name;
+  } else {
+    ILOG("proc_pidpath() failed for pid: %d", pid);
+  }
+#elif defined(BSD)
+# if defined(__FreeBSD__)
+#  define KP_COMM(o) o.ki_comm
+# else
+#  define KP_COMM(o) o.p_comm
+# endif
+  struct kinfo_proc *proc = kinfo_getproc(pid);
+  if (proc) {
+    PUT(pinfo, "name", cstr_to_string(KP_COMM(proc)));
+    xfree(proc);
+  } else {
+    ILOG("kinfo_getproc() failed for pid: %d", pid);
+  }
+
+#elif defined(__linux__)
+  char fname[256] = { 0 };
+  char buf[MAXPATHL];
+  snprintf(fname, sizeof(fname), "/proc/%d/comm", pid);
+  FILE *fp = fopen(fname, "r");
+  // FileDescriptor *f = file_open_new(&error, fname, kFileReadOnly, 0);
+  // ptrdiff_t file_read(FileDescriptor *const fp, char *const ret_buf,
+  //                     const size_t size)
+  if (fp == NULL) {
+    ILOG("fopen() of /proc/%d/comm failed", pid);
+  } else {
+    size_t n = fread(buf, sizeof(char), sizeof(buf) - 1, fp);
+    if (n == 0) {
+      WLOG("fread() of /proc/%d/comm failed", pid);
+    } else {
+      size_t end = MIN(sizeof(buf) - 1, n);
+      end = (end > 0 && buf[end - 1] == '\n') ? end - 1 : end;
+      buf[end] = '\0';
+      PUT(pinfo, "name", STRING_OBJ(cstr_to_string(buf)));
+    }
+  }
+  fclose(fp);
+#endif
+  return pinfo;
+}

src/nvim/lua/vim.lua

@@ -1,18 +1,54 @@
--- Gets the children of process `ppid` via the shell.
+-- Internal-only until comments in #8107 are addressed.
+-- Returns:
+--    {errcode}, {output}
+local function _system(cmd)
+  local out = vim.api.nvim_call_function('system', { cmd })
+  local err = vim.api.nvim_get_vvar('shell_error')
+  return err, out
+end
+
+-- Gets process info from the `ps` command.
+-- Used by nvim_get_proc() as a fallback.
+local function _os_proc_info(pid)
+  if pid == nil or pid <= 0 or type(pid) ~= 'number' then
+    error('invalid pid')
+  end
+  local cmd = { 'ps', '-p', pid, '-o', 'ucomm=', }
+  local err, name = _system(cmd)
+  if 1 == err and string.gsub(name, '%s*', '') == '' then
+    return {}  -- Process not found.
+  elseif 0 ~= err then
+    local args_str = vim.api.nvim_call_function('string', { cmd })
+    error('command failed: '..args_str)
+  end
+  local _, ppid = _system({ 'ps', '-p', pid, '-o', 'ppid=', })
+  -- Remove trailing whitespace.
+  name = string.gsub(name, '%s+$', '')
+  ppid = string.gsub(ppid, '%s+$', '')
+  ppid = tonumber(ppid) == nil and -1 or tonumber(ppid)
+  return {
+    name = name,
+    pid = pid,
+    ppid = ppid,
+  }
+end
+
+-- Gets process children from the `pgrep` command.
 -- Used by nvim_get_proc_children() as a fallback.
 local function _os_proc_children(ppid)
   if ppid == nil or ppid <= 0 or type(ppid) ~= 'number' then
     error('invalid ppid')
   end
-  local out = vim.api.nvim_call_function('system', { 'pgrep -P '..ppid })
-  local err = vim.api.nvim_get_vvar('shell_error')
-  if 1 == err and out == '' then
+  local cmd = { 'pgrep', '-P', ppid, }
+  local err, rv = _system(cmd)
+  if 1 == err and string.gsub(rv, '%s*', '') == '' then
     return {}  -- Process not found.
   elseif 0 ~= err then
-    error('pgrep failed')
+    local args_str = vim.api.nvim_call_function('string', { cmd })
+    error('command failed: '..args_str)
   end
   local children = {}
-  for s in string.gmatch(out, '%S+') do
+  for s in string.gmatch(rv, '%S+') do
     local i = tonumber(s)
     if i ~= nil then
       table.insert(children, i)
@@ -81,8 +117,12 @@ local function _update_package_paths()
   end
   last_nvim_paths = cur_nvim_paths
 end
---{{{1 Module definition
-return {
+
+local module = {
   _update_package_paths = _update_package_paths,
   _os_proc_children = _os_proc_children,
+  _os_proc_info = _os_proc_info,
+  _system = _system,
 }
+
+return module