Speed up whole-file read() calls on small gzip files

CHANGELOG.rst

@@ -9,6 +9,10 @@ Changelog
 
 version 1.10.0-dev
 ------------------
++ Speed up ``read()`` without a size on ``igzip.open`` and ``IGzipFile`` files
+  that decompress to at most 64 KiB, on Linux with glibc. Reading many 3 KiB
+  gzip files by path is nearly 2 times faster. The gain applies in processes
+  that have not yet freed a memory block of 2 MiB or more.
 + Fix ``python -m isal.igzip`` replacing the input file with empty output and
   exiting with status 0 when the output file is the input file, as in
   ``-f -o data data``. It exits with an error now.

src/isal/isal_zlibmodule.c

@@ -2164,6 +2164,14 @@ GzipReader_readall(GzipReader *self, PyObject *Py_UNUSED(ignore))
         return NULL;
     }
     if (written_size < chunk_size) {
+        if (written_size <= chunk_size / 32) {
+            /* Shrinking an mmap()ed chunk keeps it mapped, so copy small
+               results and free the chunk. */
+            PyObject *result = PyBytes_FromStringAndSize(
+                PyBytes_AS_STRING(first_chunk), written_size);
+            Py_DECREF(first_chunk);
+            return result;
+        }
         if (_PyBytes_Resize(&first_chunk, written_size) < 0) {
             return NULL;
         }

tests/test_igzip.py

@@ -91,6 +91,38 @@ def test_igzip_reader_readall():
     assert test.read(-1) == DATA
 
 
+# Buffer size 1024: chunks of 4096 bytes, results up to 128 bytes copied.
+@pytest.mark.parametrize("size", [0, 1, 127, 128, 129, 4095, 4096, 4097,
+                                  3 * 4096 + 1])
+@pytest.mark.parametrize("members", [1, 2])
+def test_igzip_reader_readall_sizes(size, members):
+    data = os.urandom(size)
+    compressed = gzip.compress(data) * members
+    test = igzip._IGzipReader(io.BytesIO(compressed), 1024)
+    result = test.readall()
+    assert type(result) is bytes
+    assert result == data * members
+    assert test.tell() == size * members
+    assert test.readall() == b""
+
+
+@pytest.mark.parametrize("size", [100, 4096])
+def test_igzip_reader_readall_after_read(size):
+    data = os.urandom(size)
+    test = igzip._IGzipReader(io.BytesIO(gzip.compress(data)), 1024)
+    assert test.read(10) == data[:10]
+    assert test.readall() == data[10:]
+    assert test.tell() == size
+
+
+# IGzipFile.read() copies results up to 64 KiB out of its 2 MiB chunk.
+@pytest.mark.parametrize("size", [64 * 1024 - 1, 64 * 1024, 64 * 1024 + 1])
+def test_igzipfile_read_all_sizes(size):
+    data = os.urandom(size)
+    with igzip.IGzipFile(fileobj=io.BytesIO(gzip.compress(data))) as test:
+        assert test.read() == data
+
+
 def test_igzip_reader_read_zero():
     data = io.BytesIO(COMPRESSED_DATA)
     test = igzip._IGzipReader(data)