Merge branch 'pr1/telemetry-privacy-docs' into pr2/source-at-record-time

integrations/agent-plugin-core/python/telemetry.py

@@ -140,36 +140,49 @@ def _install_salt() -> str:
       keys off that file, so recording an event would silently suppress the
       install event.
 
-    On a read-only or full data directory the fallback is derived from the data
-    directory path: stable for the machine rather than random per call, so the
-    failure mode is a weaker salt and not unbounded cardinality in PostHog.
+    Published atomically, and there is deliberately no derived fallback. Creating
+    the file with O_CREAT|O_EXCL and then writing into it leaves a window where
+    the file exists and is empty, and a concurrent hook that reads it in that
+    window gets nothing. Falling back to a digest of the path would hand that
+    process a salt an attacker can compute, memoized for its whole run, which is
+    the privacy control this function exists to provide silently turning itself
+    off under load. The salt is written to a private temp file first and linked
+    into place, so the name either does not exist or already has the full value.
+
+    Returns "" when it genuinely cannot persist. Callers omit the hash entirely
+    rather than emit an unsalted one.
     """
     global _salt_cache
     if _salt_cache:
         return _salt_cache
 
     path = _salt_path()
+    temporary = path.with_name(f"{path.name}.{os.getpid()}.tmp")
     try:
         path.parent.mkdir(parents=True, exist_ok=True)
-        handle = os.open(path, os.O_CREAT | os.O_EXCL | os.O_WRONLY, 0o600)
+        handle = os.open(temporary, os.O_CREAT | os.O_EXCL | os.O_WRONLY, 0o600)
+        with os.fdopen(handle, "w", encoding="utf-8") as stream:
+            stream.write(uuid.uuid4().hex)
+            stream.flush()
+            os.fsync(stream.fileno())
         try:
-            with os.fdopen(handle, "w", encoding="utf-8") as stream:
-                stream.write(uuid.uuid4().hex)
+            # Atomic claim: fails if another process already published one.
+            # os.link rather than replace, which would clobber theirs.
+            os.link(temporary, path)
+        except FileExistsError:
+            pass
+    except OSError:
+        pass
+    finally:
+        try:
+            temporary.unlink()
         except OSError:
             pass
-    except FileExistsError:
-        pass
-    except OSError:
-        # Cannot persist. Stable-per-machine beats random-per-call.
-        _salt_cache = hashlib.sha256(str(path).encode("utf-8")).hexdigest()
-        return _salt_cache
 
     try:
         _salt_cache = path.read_text(encoding="utf-8").strip()
     except OSError:
         _salt_cache = ""
-    if not _salt_cache:
-        _salt_cache = hashlib.sha256(str(path).encode("utf-8")).hexdigest()
     return _salt_cache
 
 
@@ -182,10 +195,17 @@ def _scoped_digest(value: str, length: int = 16) -> str:
     privacy control without the salt. Salting per install keeps every
     within-account join the analytics actually use and gives up only
     cross-machine joins on the same repository, which nothing computes.
+
+    Returns "" when there is no salt, so record() omits the property. An
+    unsalted digest over this input space is close to plaintext, and emitting one
+    under a name that implies it is hashed is worse than sending nothing.
     """
     if not value:
         return ""
-    return hashlib.sha256(f"{_install_salt()}:{value}".encode("utf-8")).hexdigest()[:length]
+    salt = _install_salt()
+    if not salt:
+        return ""
+    return hashlib.sha256(f"{salt}:{value}".encode("utf-8")).hexdigest()[:length]
 
 
 def _safe_value(value: Any) -> Any:
@@ -281,10 +301,17 @@ def record(
             os=sys.platform,
             python_version=platform.python_version(),
         )
+        # Assigned only when the digest is real. _scoped_digest returns "" when
+        # the salt could not be persisted, and an empty property is worse than an
+        # absent one: it survives the None filter below and reads as a value.
         if repo is not None:
-            properties["repo_hash"] = _scoped_digest(getattr(repo, "identity", ""))
+            repo_hash = _scoped_digest(getattr(repo, "identity", ""))
+            if repo_hash:
+                properties["repo_hash"] = repo_hash
         if session_id:
-            properties["session_hash"] = _scoped_digest(session_id)
+            session_hash = _scoped_digest(session_id)
+            if session_hash:
+                properties["session_hash"] = session_hash
         line = json.dumps(
             {
                 "event": f"{EVENT_PREFIX}.{event}",