FileID: implement FileID struct

`FileID` should encapsulate `st_dev` and `st_ino`. It is a new abstraction
used to check if two files are the same. `FileID`s will be embeded inside
other struts like `buf_t` or `ff_visited_T`, where a full `FileInfo` would be
to big.

src/nvim/os/fs.c

@@ -345,8 +345,69 @@ bool os_get_file_info_fd(int file_descriptor, FileInfo *file_info)
 /// Compare the inodes of two FileInfos
 ///
 /// @return `true` if the two FileInfos represent the same file.
-bool os_file_info_id_equal(FileInfo *file_info_1, FileInfo *file_info_2)
+bool os_file_info_id_equal(const FileInfo *file_info_1,
+                           const FileInfo *file_info_2)
 {
   return file_info_1->stat.st_ino == file_info_2->stat.st_ino
          && file_info_1->stat.st_dev == file_info_2->stat.st_dev;
 }
+
+/// Get the `FileID` of a `FileInfo`
+///
+/// @param file_info Pointer to the `FileInfo`
+/// @param[out] file_id Pointer to a `FileID`
+void os_file_info_get_id(const FileInfo *file_info, FileID *file_id)
+{
+  file_id->inode = file_info->stat.st_ino;
+  file_id->device_id = file_info->stat.st_dev;
+}
+
+/// Get the inode of a `FileInfo`
+///
+/// @deprecated Use `FileID` instead, this function is only needed in memline.c
+/// @param file_info Pointer to the `FileInfo`
+/// @return the inode number
+uint64_t os_file_info_get_inode(const FileInfo *file_info)
+{
+  return file_info->stat.st_ino;
+}
+
+/// Get the `FileID` for a given path
+///
+/// @param path Path to the file.
+/// @param[out] file_info Pointer to a `FileID` to fill in.
+/// @return `true` on sucess, `false` for failure.
+bool os_get_file_id(const char *path, FileID *file_id)
+{
+  uv_stat_t statbuf;
+  if (os_stat((char_u *)path, &statbuf) == OK) {
+    file_id->inode = statbuf.st_ino;
+    file_id->device_id = statbuf.st_dev;
+    return true;
+  }
+  return false;
+}
+
+/// Check if two `FileID`s are equal
+///
+/// @param file_id_1 Pointer to first `FileID`
+/// @param file_id_2 Pointer to second `FileID`
+/// @return `true` if the two `FileID`s represent te same file.
+bool os_file_id_equal(const FileID *file_id_1, const FileID *file_id_2)
+{
+  return file_id_1->inode == file_id_2->inode
+         && file_id_1->device_id == file_id_2->device_id;
+}
+
+/// Check if a `FileID` is equal to a `FileInfo`
+///
+/// @param file_id Pointer to a `FileID`
+/// @param file_info Pointer to a `FileInfo`
+/// @return `true` if the `FileID` and the `FileInfo` represent te same file.
+bool os_file_id_equal_file_info(const FileID *file_id,
+                                const FileInfo *file_info)
+{
+  return file_id->inode == file_info->stat.st_ino
+         && file_id->device_id == file_info->stat.st_dev;
+}
+

src/nvim/os/fs_defs.h

@@ -0,0 +1,19 @@
+#ifndef NVIM_OS_FS_DEFS_H
+#define NVIM_OS_FS_DEFS_H
+
+#include <uv.h>
+
+/// Struct which encapsulates stat information.
+typedef struct {
+  uv_stat_t stat;  ///< @private
+} FileInfo;
+
+/// Struct which encapsulates inode/dev_id information.
+typedef struct {
+  uint64_t inode;      ///< @private The inode of the file
+  uint64_t device_id;  ///< @private The id of the device containing the file
+} FileID;
+
+#define FILE_ID_EMPTY (FileID){.inode = 0, .device_id = 0}
+
+#endif  // NVIM_OS_FS_DEFS_H

src/nvim/os/os.h

@@ -2,14 +2,9 @@
 #define NVIM_OS_OS_H
 #include <uv.h>
 
+#include "nvim/os/fs_defs.h"
 #include "nvim/vim.h"
 
-/// Struct which encapsulates stat information.
-typedef struct {
-  // TODO(stefan991): make stat private
-  uv_stat_t stat;
-} FileInfo;
-
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "os/fs.h.generated.h"
 # include "os/mem.h.generated.h"

test/unit/os/fs.moon

@@ -314,6 +314,12 @@ describe 'fs function', ->
       is_file_info_filled = (file_info) ->
         file_info[0].stat.st_ino > 0 and file_info[0].stat.st_dev > 0
 
+      file_id_new = () ->
+        file_info = ffi.new 'FileID[1]'
+        file_info[0].inode = 0
+        file_info[0].device_id = 0
+        file_info
+
       describe 'os_get_file_info', ->
         it 'returns false if given a non-existing file', ->
           file_info = file_info_new!
@@ -392,3 +398,67 @@ describe 'fs function', ->
           assert.is_true (fs.os_get_file_info path_2, file_info_2)
           assert.is_true (fs.os_file_info_id_equal file_info_1, file_info_2)
 
+      describe 'os_file_info_get_id', ->
+        it 'extracts ino/dev from file_info into file_id', ->
+          file_info = file_info_new!
+          file_id = file_id_new!
+          path = 'unit-test-directory/test.file'
+          assert.is_true (fs.os_get_file_info path, file_info)
+          fs.os_file_info_get_id(file_info, file_id)
+          eq file_info[0].stat.st_ino, file_id[0].inode
+          eq file_info[0].stat.st_dev, file_id[0].device_id
+
+      describe 'os_file_info_get_inode', ->
+        it 'returns the inode from file_info', ->
+          file_info = file_info_new!
+          path = 'unit-test-directory/test.file'
+          assert.is_true (fs.os_get_file_info path, file_info)
+          inode = fs.os_file_info_get_inode(file_info)
+          eq file_info[0].stat.st_ino, inode
+
+      describe 'os_get_file_id', ->
+        it 'returns false if given an non-existing file', ->
+          file_id = file_id_new!
+          assert.is_false (fs.os_get_file_id '/non-existent', file_id)
+
+        it 'returns true if given an existing file and fills file_id', ->
+          file_id = file_id_new!
+          path = 'unit-test-directory/test.file'
+          assert.is_true (fs.os_get_file_id path, file_id)
+          assert.is_true 0 < file_id[0].inode
+          assert.is_true 0 < file_id[0].device_id
+
+      describe 'os_file_id_equal', ->
+        it 'returns true if two FileIDs are equal', ->
+          file_id = file_id_new!
+          path = 'unit-test-directory/test.file'
+          assert.is_true (fs.os_get_file_id path, file_id)
+          assert.is_true (fs.os_file_id_equal file_id, file_id)
+
+        it 'returns false if two FileIDs are not equal', ->
+          file_id_1 = file_id_new!
+          file_id_2 = file_id_new!
+          path_1 = 'unit-test-directory/test.file'
+          path_2 = 'unit-test-directory/test_2.file'
+          assert.is_true (fs.os_get_file_id path_1, file_id_1)
+          assert.is_true (fs.os_get_file_id path_2, file_id_2)
+          assert.is_false (fs.os_file_id_equal file_id_1, file_id_2)
+
+      describe 'os_file_id_equal_file_info', ->
+        it 'returns true if file_id and file_info represent the same file', ->
+          file_id = file_id_new!
+          file_info = file_info_new!
+          path = 'unit-test-directory/test.file'
+          assert.is_true (fs.os_get_file_id path, file_id)
+          assert.is_true (fs.os_get_file_info path, file_info)
+          assert.is_true (fs.os_file_id_equal_file_info file_id, file_info)
+
+        it 'returns false if file_id and file_info represent different files',->
+          file_id = file_id_new!
+          file_info = file_info_new!
+          path_1 = 'unit-test-directory/test.file'
+          path_2 = 'unit-test-directory/test_2.file'
+          assert.is_true (fs.os_get_file_id path_1, file_id)
+          assert.is_true (fs.os_get_file_info path_2, file_info)
+          assert.is_false (fs.os_file_id_equal_file_info file_id, file_info)
+