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/api/vim.c

@@ -1482,11 +1482,11 @@ Array nvim_list_uis(void)
 
 /// Gets the immediate children of process `pid`.
 ///
-/// @return Array of child process ids. Empty array if process not found.
+/// @return Array of child process ids, empty if process not found.
 Array nvim_get_proc_children(Integer pid, Error *err)
   FUNC_API_SINCE(4)
 {
-  Array proc_array = ARRAY_DICT_INIT;
+  Array rvobj = ARRAY_DICT_INIT;
   int *proc_list = NULL;
 
   if (pid <= 0 || pid > INT_MAX) {
@@ -1498,6 +1498,7 @@ Array nvim_get_proc_children(Integer pid, Error *err)
   int rv = os_proc_children((int)pid, &proc_list, &proc_count);
   if (rv != 0) {
     // syscall failed (possibly because of kernel options), try shelling out.
+    DLOG("fallback to vim._os_proc_children()");
     Array a = ARRAY_DICT_INIT;
     ADD(a, INTEGER_OBJ(pid));
     String s = cstr_to_string("return vim._os_proc_children(select(1, ...))");
@@ -1505,7 +1506,7 @@ Array nvim_get_proc_children(Integer pid, Error *err)
     api_free_string(s);
     api_free_array(a);
     if (o.type == kObjectTypeArray) {
-      proc_array = o.data.array;
+      rvobj = o.data.array;
     } else if (!ERROR_SET(err)) {
       api_set_error(err, kErrorTypeException,
                     "Failed to get process children. pid=%" PRId64 " error=%d",
@@ -1515,10 +1516,49 @@ Array nvim_get_proc_children(Integer pid, Error *err)
   }
 
   for (size_t i = 0; i < proc_count; i++) {
-    ADD(proc_array, INTEGER_OBJ(proc_list[i]));
+    ADD(rvobj, INTEGER_OBJ(proc_list[i]));
   }
 
 end:
   xfree(proc_list);
-  return proc_array;
+  return rvobj;
+}
+
+/// Gets info describing process `pid`.
+///
+/// @return Map of process properties, or NIL if process not found.
+Object nvim_get_proc(Integer pid, Error *err)
+  FUNC_API_SINCE(4)
+{
+  Object rvobj = OBJECT_INIT;
+  rvobj.data.dictionary = (Dictionary)ARRAY_DICT_INIT;
+  rvobj.type = kObjectTypeDictionary;
+
+  if (pid <= 0 || pid > INT_MAX) {
+    api_set_error(err, kErrorTypeException, "Invalid pid: %" PRId64, pid);
+    return NIL;
+  }
+#ifdef WIN32
+  rvobj.data.dictionary = os_proc_info((int)pid);
+  if (rvobj.data.dictionary.size == 0) {  // Process not found.
+    return NIL;
+  }
+#else
+  // Cross-platform process info APIs are miserable, so use `ps` instead.
+  Array a = ARRAY_DICT_INIT;
+  ADD(a, INTEGER_OBJ(pid));
+  String s = cstr_to_string("return vim._os_proc_info(select(1, ...))");
+  Object o = nvim_execute_lua(s, a, err);
+  api_free_string(s);
+  api_free_array(a);
+  if (o.type == kObjectTypeArray && o.data.array.size == 0) {
+    return NIL;  // Process not found.
+  } else if (o.type == kObjectTypeDictionary) {
+    rvobj.data.dictionary = o.data.dictionary;
+  } else if (!ERROR_SET(err)) {
+    api_set_error(err, kErrorTypeException,
+                  "Failed to get process info. pid=%" PRId64, pid);
+  }
+#endif
+  return rvobj;
 }