gh-116622: Redirect stdout and stderr to system log when embedded in an Android app

Lib/_android_support.py

@@ -0,0 +1,94 @@
+import io
+import sys
+
+
+# The maximum length of a log message in bytes, including the level marker and
+# tag, is defined as LOGGER_ENTRY_MAX_PAYLOAD in
+# platform/system/logging/liblog/include/log/log.h. As of API level 30, messages
+# longer than this will be be truncated by logcat. This limit has already been
+# reduced at least once in the history of Android (from 4076 to 4068 between API
+# level 23 and 26), so leave some headroom.
+MAX_BYTES_PER_WRITE = 4000
+
+# UTF-8 uses a maximum of 4 bytes per character, so limiting text writes to this
+# size ensures that TextIOWrapper can always avoid exceeding MAX_BYTES_PER_WRITE.
+# However, if the actual number of bytes per character is smaller than that,
+# then TextIOWrapper may still join multiple consecutive text writes into binary
+# writes containing a larger number of characters.
+MAX_CHARS_PER_WRITE = MAX_BYTES_PER_WRITE // 4
+
+
+# When embedded in an app on current versions of Android, there's no easy way to
+# monitor the C-level stdout and stderr. The testbed comes with a .c file to
+# redirect them to the system log using a pipe, but that wouldn't be convenient
+# or appropriate for all apps. So we redirect at the Python level instead.
+def init_streams(android_log_write, stdout_prio, stderr_prio):
+    if sys.executable:
+        return  # Not embedded in an app.
+
+    sys.stdout = TextLogStream(
+        android_log_write, stdout_prio, "python.stdout", errors=sys.stdout.errors)
+    sys.stderr = TextLogStream(
+        android_log_write, stderr_prio, "python.stderr", errors=sys.stderr.errors)
+
+
+class TextLogStream(io.TextIOWrapper):
+    def __init__(self, android_log_write, prio, tag, **kwargs):
+        kwargs.setdefault("encoding", "UTF-8")
+        kwargs.setdefault("line_buffering", True)
+        super().__init__(BinaryLogStream(android_log_write, prio, tag), **kwargs)
+        self._CHUNK_SIZE = MAX_BYTES_PER_WRITE
+
+    def __repr__(self):
+        return f"<TextLogStream {self.buffer.tag!r}>"
+
+    def write(self, s):
+        if not isinstance(s, str):
+            raise TypeError(
+                f"write() argument must be str, not {type(s).__name__}")
+
+        # In case `s` is a str subclass that writes itself to stdout or stderr
+        # when we call its methods, convert it to an actual str.
+        s = str.__str__(s)
+
+        # We want to emit one log message per line wherever possible, so split
+        # the string before sending it to the superclass. Note that
+        # "".splitlines() == [], so nothing will be logged for an empty string.
+        for line in s.splitlines(keepends=True):
+            while line:
+                super().write(line[:MAX_CHARS_PER_WRITE])
+                line = line[MAX_CHARS_PER_WRITE:]
+
+        return len(s)
+
+
+class BinaryLogStream(io.RawIOBase):
+    def __init__(self, android_log_write, prio, tag):
+        self.android_log_write = android_log_write
+        self.prio = prio
+        self.tag = tag
+
+    def __repr__(self):
+        return f"<BinaryLogStream {self.tag!r}>"
+
+    def writable(self):
+        return True
+
+    def write(self, b):
+        if type(b) is not bytes:
+            try:
+                b = bytes(memoryview(b))
+            except TypeError:
+                raise TypeError(
+                    f"write() argument must be bytes-like, not {type(b).__name__}"
+                ) from None
+
+        # Writing an empty string to the stream should have no effect.
+        if b:
+            # Encode null bytes using "modified UTF-8" to avoid truncating the
+            # message. This should not affect the return value, as the caller
+            # may be expecting it to match the length of the input.
+            self.android_log_write(self.prio, self.tag,
+                                   b.replace(b"\x00", b"\xc0\x80"))
+
+        return len(b)