Add support to write_to_textfile for custom tmpdir (#1115)

* Add support to write_to_textfile for custom tmpdir

While the try/except block does prevent most of the temp files from
persisting, if there is a non catchable exception, those temp files
continue to pollute the directory. Optionally set the temp
directory would let us write to something like /tmp, so the target
directory isn't polluted

Signed-off-by: Aaditya Dhruv <[email protected]>

* Modify write_to_textfile to ensure tmpdir is on same filesystem

The tmpdir must be on the same filesystem to ensure an atomic operation
takes place. If this is not enforced, there could be partial writes
which can lead to partial/incorrect metrics being exported

Signed-off-by: Aaditya Dhruv <[email protected]>

---------

Signed-off-by: Aaditya Dhruv <[email protected]>

Author: aadityadhruv

prometheus_client/exposition.py

@@ -446,12 +446,21 @@ def factory(cls, registry: CollectorRegistry) -> type:
         return MyMetricsHandler
 
 
-def write_to_textfile(path: str, registry: CollectorRegistry, escaping: str = openmetrics.ALLOWUTF8) -> None:
+def write_to_textfile(path: str, registry: CollectorRegistry, escaping: str = openmetrics.ALLOWUTF8, tmpdir: Optional[str] = None) -> None:
     """Write metrics to the given path.
 
     This is intended for use with the Node exporter textfile collector.
-    The path must end in .prom for the textfile collector to process it."""
-    tmppath = f'{path}.{os.getpid()}.{threading.current_thread().ident}'
+    The path must end in .prom for the textfile collector to process it.
+
+    An optional tmpdir parameter can be set to determine where the
+    metrics will be temporarily written to. If not set, it will be in
+    the same directory as the .prom file. If provided, the path MUST be
+    on the same filesystem."""
+    if tmpdir is not None:
+        filename = os.path.basename(path)
+        tmppath = f'{os.path.join(tmpdir, filename)}.{os.getpid()}.{threading.current_thread().ident}'
+    else:
+        tmppath = f'{path}.{os.getpid()}.{threading.current_thread().ident}'
     try:
         with open(tmppath, 'wb') as f:
             f.write(generate_latest(registry, escaping))