Make file generation saving atomic (#792)

* Make file generation saving atomic

* Lint

* Python 2 Compat

* Fix

* Handle file not found error

esphome/helpers.py

@@ -1,10 +1,11 @@
 from __future__ import print_function
 
 import codecs
+
 import logging
 import os
 
-from esphome.py_compat import char_to_byte, text_type
+from esphome.py_compat import char_to_byte, text_type, IS_PY2, encode_text
 
 _LOGGER = logging.getLogger(__name__)
 
@@ -79,15 +80,15 @@ def run_system_command(*args):
 
 
 def mkdir_p(path):
-    import errno
-
     try:
         os.makedirs(path)
-    except OSError as exc:
-        if exc.errno == errno.EEXIST and os.path.isdir(path):
+    except OSError as err:
+        import errno
+        if err.errno == errno.EEXIST and os.path.isdir(path):
             pass
         else:
-            raise
+            from esphome.core import EsphomeError
+            raise EsphomeError(u"Error creating directories {}: {}".format(path, err))
 
 
 def is_ip_address(host):
@@ -151,17 +152,6 @@ def is_hassio():
     return get_bool_env('ESPHOME_IS_HASSIO')
 
 
-def copy_file_if_changed(src, dst):
-    src_text = read_file(src)
-    if os.path.isfile(dst):
-        dst_text = read_file(dst)
-    else:
-        dst_text = None
-    if src_text == dst_text:
-        return
-    write_file(dst, src_text)
-
-
 def walk_files(path):
     for root, _, files in os.walk(path):
         for name in files:
@@ -172,28 +162,99 @@ def read_file(path):
     try:
         with codecs.open(path, 'r', encoding='utf-8') as f_handle:
             return f_handle.read()
-    except OSError:
+    except OSError as err:
         from esphome.core import EsphomeError
-        raise EsphomeError(u"Could not read file at {}".format(path))
+        raise EsphomeError(u"Error reading file {}: {}".format(path, err))
+    except UnicodeDecodeError as err:
+        from esphome.core import EsphomeError
+        raise EsphomeError(u"Error reading file {}: {}".format(path, err))
+
+
+def _write_file(path, text):
+    import tempfile
+    directory = os.path.dirname(path)
+    mkdir_p(directory)
+
+    tmp_path = None
+    data = encode_text(text)
+    try:
+        with tempfile.NamedTemporaryFile(mode="wb", dir=directory, delete=False) as f_handle:
+            tmp_path = f_handle.name
+            f_handle.write(data)
+        # Newer tempfile implementations create the file with mode 0o600
+        os.chmod(tmp_path, 0o644)
+        if IS_PY2:
+            if os.path.exists(path):
+                os.remove(path)
+            os.rename(tmp_path, path)
+        else:
+            # If destination exists, will be overwritten
+            os.replace(tmp_path, path)
+    finally:
+        if tmp_path is not None and os.path.exists(tmp_path):
+            try:
+                os.remove(tmp_path)
+            except OSError as err:
+                _LOGGER.error("Write file cleanup failed: %s", err)
 
 
 def write_file(path, text):
     try:
-        mkdir_p(os.path.dirname(path))
-        with codecs.open(path, 'w+', encoding='utf-8') as f_handle:
-            f_handle.write(text)
+        _write_file(path, text)
     except OSError:
         from esphome.core import EsphomeError
         raise EsphomeError(u"Could not write file at {}".format(path))
 
 
-def write_file_if_changed(text, dst):
+def write_file_if_changed(path, text):
     src_content = None
-    if os.path.isfile(dst):
-        src_content = read_file(dst)
+    if os.path.isfile(path):
+        src_content = read_file(path)
     if src_content != text:
-        write_file(dst, text)
+        write_file(path, text)
+
+
+def copy_file_if_changed(src, dst):
+    import shutil
+    if file_compare(src, dst):
+        return
+    mkdir_p(os.path.dirname(dst))
+    try:
+        shutil.copy(src, dst)
+    except OSError as err:
+        from esphome.core import EsphomeError
+        raise EsphomeError(u"Error copying file {} to {}: {}".format(src, dst, err))
 
 
 def list_starts_with(list_, sub):
     return len(sub) <= len(list_) and all(list_[i] == x for i, x in enumerate(sub))
+
+
+def file_compare(path1, path2):
+    """Return True if the files path1 and path2 have the same contents."""
+    import stat
+
+    try:
+        stat1, stat2 = os.stat(path1), os.stat(path2)
+    except OSError:
+        # File doesn't exist or another error -> not equal
+        return False
+
+    if stat.S_IFMT(stat1.st_mode) != stat.S_IFREG or stat.S_IFMT(stat2.st_mode) != stat.S_IFREG:
+        # At least one of them is not a regular file (or does not exist)
+        return False
+    if stat1.st_size != stat2.st_size:
+        # Different sizes
+        return False
+
+    bufsize = 8*1024
+    # Read files in blocks until a mismatch is found
+    with open(path1, 'rb') as fh1, open(path2, 'rb') as fh2:
+        while True:
+            blob1, blob2 = fh1.read(bufsize), fh2.read(bufsize)
+            if blob1 != blob2:
+                # Different content
+                return False
+            if not blob1:
+                # Reached end
+                return True