fix(path): avoid chdir() when resolving path (#28799)

Use uv_fs_realpath() instead.

It seems that uv_fs_realpath() has some problems on non-Linux platforms:
- macOS and other BSDs: this function will fail with UV_ELOOP if more
  than 32 symlinks are found while resolving the given path.  This limit
  is hardcoded and cannot be sidestepped.
- Windows: while this function works in the common case, there are a
  number of corner cases where it doesn't:
  - Paths in ramdisk volumes created by tools which sidestep the Volume
    Manager (such as ImDisk) cannot be resolved.
  - Inconsistent casing when using drive letters.
  - Resolved path bypasses subst'd drives.

Ref: https://docs.libuv.org/en/v1.x/fs.html#c.uv_fs_realpath

I don't know if the old implementation that uses uv_chdir() and uv_cwd()
also suffers from the same problems.
- For the ELOOP case, chdir() seems to have the same limitations.
- On Windows, Vim doesn't use anything like chdir() either. It uses
  _wfullpath(), while libuv uses GetFinalPathNameByHandleW().

5 files changed, 72 insertions, 63 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index b1ee33929c..1fc80e88c4 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -6479,7 +6479,7 @@ static void f_resolve(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   char *v = os_resolve_shortcut(fname);
   if (v == NULL) {
     if (os_is_reparse_point_include(fname)) {
-      v = os_realpath(fname, v);
+      v = os_realpath(fname, NULL, MAXPATHL + 1);
     }
   }
   rettv->vval.v_string = (v == NULL ? xstrdup(fname) : v);
@@ -6631,7 +6631,7 @@ static void f_resolve(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     xfree(buf);
   }
 # else
-  char *v = os_realpath(fname, NULL);
+  char *v = os_realpath(fname, NULL, MAXPATHL + 1);
   rettv->vval.v_string = v == NULL ? xstrdup(fname) : v;
 # endif
 #endif
diff --git a/src/nvim/os/env.c b/src/nvim/os/env.c
index fbc3a243a6..5a79004c41 100644
--- a/src/nvim/os/env.c
+++ b/src/nvim/os/env.c
@@ -474,17 +474,9 @@ void init_homedir(void)
     var = os_homedir();
   }
 
-  if (var != NULL) {
-    // Change to the directory and get the actual path.  This resolves
-    // links.  Don't do it when we can't return.
-    if (os_dirname(os_buf, MAXPATHL) == OK && os_chdir(os_buf) == 0) {
-      if (!os_chdir(var) && os_dirname(IObuff, IOSIZE) == OK) {
-        var = IObuff;
-      }
-      if (os_chdir(os_buf) != 0) {
-        emsg(_(e_prev_dir));
-      }
-    }
+  // Get the actual path.  This resolves links.
+  if (var != NULL && os_realpath(var, IObuff, IOSIZE) != NULL) {
+    var = IObuff;
   }
 
   // Fall back to current working directory if home is not found
diff --git a/src/nvim/os/fs.c b/src/nvim/os/fs.c
index 2eca906d4e..19bdf30311 100644
--- a/src/nvim/os/fs.c
+++ b/src/nvim/os/fs.c
@@ -1320,22 +1320,22 @@ bool os_fileid_equal_fileinfo(const FileID *file_id, const FileInfo *file_info)
 /// Return the canonicalized absolute pathname.
 ///
 /// @param[in] name Filename to be canonicalized.
-/// @param[out] buf Buffer to store the canonicalized values. A minimum length
-//                  of MAXPATHL+1 is required. If it is NULL, memory is
-//                  allocated. In that case, the caller should deallocate this
-//                  buffer.
+/// @param[out] buf Buffer to store the canonicalized values.
+///                 If it is NULL, memory is allocated. In that case, the caller
+///                 should deallocate this buffer.
+/// @param[in] len  The length of the buffer.
 ///
 /// @return pointer to the buf on success, or NULL.
-char *os_realpath(const char *name, char *buf)
+char *os_realpath(const char *name, char *buf, size_t len)
   FUNC_ATTR_NONNULL_ARG(1)
 {
   uv_fs_t request;
   int result = uv_fs_realpath(NULL, &request, name, NULL);
   if (result == kLibuvSuccess) {
     if (buf == NULL) {
-      buf = xmallocz(MAXPATHL);
+      buf = xmalloc(len);
     }
-    xstrlcpy(buf, request.ptr, MAXPATHL + 1);
+    xstrlcpy(buf, request.ptr, len);
   }
   uv_fs_req_cleanup(&request);
   return result == kLibuvSuccess ? buf : NULL;
diff --git a/src/nvim/path.c b/src/nvim/path.c
index f7e8b2b65c..96330e000b 100644
--- a/src/nvim/path.c
+++ b/src/nvim/path.c
@@ -2273,13 +2273,20 @@ bool match_suffix(char *fname)
 /// @return `FAIL` for failure, `OK` for success.
 int path_full_dir_name(char *directory, char *buffer, size_t len)
 {
-  int SUCCESS = 0;
-  int retval = OK;
-
   if (strlen(directory) == 0) {
     return os_dirname(buffer, len);
   }
 
+  if (os_realpath(directory, buffer, len) != NULL) {
+    return OK;
+  }
+
+  // Path does not exist (yet).  For a full path fail, will use the path as-is.
+  if (path_is_absolute(directory)) {
+    return FAIL;
+  }
+  // For a relative path use the current directory and append the file name.
+
   char old_dir[MAXPATHL];
 
   // Get current directory name.
@@ -2287,36 +2294,12 @@ int path_full_dir_name(char *directory, char *buffer, size_t len)
     return FAIL;
   }
 
-  // We have to get back to the current dir at the end, check if that works.
-  if (os_chdir(old_dir) != SUCCESS) {
+  xstrlcpy(buffer, old_dir, len);
+  if (append_path(buffer, directory, len) == FAIL) {
     return FAIL;
   }
 
-  if (os_chdir(directory) != SUCCESS) {
-    // Path does not exist (yet).  For a full path fail,
-    // will use the path as-is.  For a relative path use
-    // the current directory and append the file name.
-    if (path_is_absolute(directory)) {
-      // Do not return immediately since we may be in the wrong directory.
-      retval = FAIL;
-    } else {
-      xstrlcpy(buffer, old_dir, len);
-      if (append_path(buffer, directory, len) == FAIL) {
-        retval = FAIL;
-      }
-    }
-  } else if (os_dirname(buffer, len) == FAIL) {
-    // Do not return immediately since we are in the wrong directory.
-    retval = FAIL;
-  }
-
-  if (os_chdir(old_dir) != SUCCESS) {
-    // That shouldn't happen, since we've tested if it works.
-    retval = FAIL;
-    emsg(_(e_prev_dir));
-  }
-
-  return retval;
+  return OK;
 }
 
 // Append to_append to path with a slash in between.
diff --git a/test/unit/path_spec.lua b/test/unit/path_spec.lua
index 44919368bf..6f6a80f44e 100644
--- a/test/unit/path_spec.lua
+++ b/test/unit/path_spec.lua
@@ -22,11 +22,16 @@ local buffer = nil
 
 describe('path.c', function()
   describe('path_full_dir_name', function()
+    local old_dir
+
     setup(function()
+      old_dir = uv.cwd()
       mkdir('unit-test-directory')
+      uv.fs_symlink(old_dir .. '/unit-test-directory', 'unit-test-symlink')
     end)
 
     teardown(function()
+      uv.fs_unlink('unit-test-symlink')
       uv.fs_rmdir('unit-test-directory')
     end)
 
@@ -37,35 +42,64 @@ describe('path.c', function()
 
     before_each(function()
       -- Create empty string buffer which will contain the resulting path.
-      length = string.len(uv.cwd()) + 22
+      length = string.len(old_dir) + 22
       buffer = cstr(length, '')
     end)
 
+    after_each(function()
+      uv.chdir(old_dir)
+    end)
+
     itp('returns the absolute directory name of a given relative one', function()
-      local result = path_full_dir_name('..', buffer, length)
-      eq(OK, result)
-      local old_dir = uv.cwd()
+      eq(OK, path_full_dir_name('..', buffer, length))
       uv.chdir('..')
       local expected = uv.cwd()
       uv.chdir(old_dir)
-      eq(expected, (ffi.string(buffer)))
+      eq(expected, ffi.string(buffer))
     end)
 
     itp('returns the current directory name if the given string is empty', function()
-      eq(OK, (path_full_dir_name('', buffer, length)))
-      eq(uv.cwd(), (ffi.string(buffer)))
+      eq(OK, path_full_dir_name('', buffer, length))
+      eq(old_dir, ffi.string(buffer))
+    end)
+
+    local function test_full_dir_absolute()
+      itp('works with a normal absolute dir', function()
+        eq(OK, path_full_dir_name(old_dir .. '/unit-test-directory', buffer, length))
+        eq(old_dir .. '/unit-test-directory', ffi.string(buffer))
+      end)
+
+      itp('works with a symlinked absolute dir', function()
+        eq(OK, path_full_dir_name(old_dir .. '/unit-test-symlink', buffer, length))
+        eq(old_dir .. '/unit-test-directory', ffi.string(buffer))
+      end)
+    end
+
+    test_full_dir_absolute()
+
+    describe('when cwd does not exist #28786', function()
+      before_each(function()
+        mkdir('dir-to-remove')
+        uv.chdir('dir-to-remove')
+        uv.fs_rmdir(old_dir .. '/dir-to-remove')
+      end)
+
+      test_full_dir_absolute()
     end)
 
     itp('works with a normal relative dir', function()
-      local result = path_full_dir_name('unit-test-directory', buffer, length)
-      eq(uv.cwd() .. '/unit-test-directory', (ffi.string(buffer)))
-      eq(OK, result)
+      eq(OK, path_full_dir_name('unit-test-directory', buffer, length))
+      eq(old_dir .. '/unit-test-directory', ffi.string(buffer))
+    end)
+
+    itp('works with a symlinked relative dir', function()
+      eq(OK, path_full_dir_name('unit-test-symlink', buffer, length))
+      eq(old_dir .. '/unit-test-directory', ffi.string(buffer))
     end)
 
     itp('works with a non-existing relative dir', function()
-      local result = path_full_dir_name('does-not-exist', buffer, length)
-      eq(uv.cwd() .. '/does-not-exist', (ffi.string(buffer)))
-      eq(OK, result)
+      eq(OK, path_full_dir_name('does-not-exist', buffer, length))
+      eq(old_dir .. '/does-not-exist', ffi.string(buffer))
     end)
 
     itp('fails with a non-existing absolute dir', function()