Add support for concurrent processing of pages.

setup.py
    Add new _apply_pages.py to wheels/installs.
src/__init__.py
    New top-level apply_pages(). Also convenience fn get_text() which uses
    apply_pages().
    Unlike Python's `multiprocessing` module, we also support passing keyword
    args to functions in worker process.
src/_apply_pages.py
    New, contains implementation of apply_pages().
tests/test_pylint.py
    Avoid pylint failure by disabling `R0801: Similar lines in 2 files`.
tests/test_textextract.py
    Test get_text() and show timings.
src/fitz___init__.py
tests/conftest.py
    Use functions to manipulate _g_log_items so that things work
    even when using `fitz` alias.

Timings for 8-core MacOS-arm64 and PDF spec:
    method='multiprocessing' : 3.3x.
    method='fork': 3.6x.

setup.py

@@ -606,21 +606,26 @@ def add( ret, from_, to_):
 
     if path_so_leaf_b:
         # Add rebased implementation files.
-        add( ret_p, f'{g_root}/src/fitz___init__.py', 'fitz/__init__.py')   # For `fitz` module alias.
-        add( ret_p, f'{g_root}/src/fitz_table.py', 'fitz/table.py')         # For `fitz` module alias.
-        add( ret_p, f'{g_root}/src/fitz_utils.py', 'fitz/utils.py')         # For `fitz` module alias.
         to_dir = 'pymupdf/'
         add( ret_p, f'{g_root}/src/__init__.py', to_dir)
         add( ret_p, f'{g_root}/src/__main__.py', to_dir)
         add( ret_p, f'{g_root}/src/pymupdf.py', to_dir)
         add( ret_p, f'{g_root}/src/table.py', to_dir)
         add( ret_p, f'{g_root}/src/utils.py', to_dir)
+        add( ret_p, f'{g_root}/src/_apply_pages.py', to_dir)
         add( ret_p, f'{g_root}/src/build/extra.py', to_dir)
         add( ret_p, f'{g_root}/src/build/{path_so_leaf_b}', to_dir)
 
+        # Add support for `fitz` backwards compatibility.
+        add( ret_p, f'{g_root}/src/fitz___init__.py', 'fitz/__init__.py')
+        add( ret_p, f'{g_root}/src/fitz_table.py', 'fitz/table.py')
+        add( ret_p, f'{g_root}/src/fitz_utils.py', 'fitz/utils.py')
+
         if mupdf_local:
+            # Add MuPDF Python API.
             add( ret_p, f'{mupdf_build_dir}/mupdf.py', to_dir)
 
+            # Add MuPDF shared libraries.
             if windows:
                 wp = pipcl.wdev.WindowsPython()
                 add( ret_p, f'{mupdf_build_dir}/_mupdf.pyd', to_dir)

src/__init__.py

@@ -20,6 +20,7 @@
 import string
 import sys
 import tarfile
+import time
 import typing
 import warnings
 import weakref
@@ -61,8 +62,19 @@ def _set_stream(name, default):
 _g_out_log = _set_stream('PYMUPDF_LOG', sys.stdout)
 _g_out_message = _set_stream('PYMUPDF_MESSAGE', sys.stdout)
 
-# Set to list() if we are in test suite.
-_g_log_items = None
+_g_log_items = list()
+_g_log_items_active = False
+
+def _log_items():
+    return _g_log_items
+
+def _log_items_active(active):
+    global _g_log_items_active
+    _g_log_items_active = active
+
+def _log_items_clear():
+    del _g_log_items[:]
+
 
 def log( text='', caller=1):
     '''
@@ -73,7 +85,7 @@ def log( text='', caller=1):
     line        = frame_record.lineno
     function    = frame_record.function
     text = f'{filename}:{line}:{function}: {text}'
-    if _g_log_items is not None:
+    if _g_log_items_active:
         _g_log_items.append(text)
     print(text, file=_g_out_log)
     _g_out_log.flush()
@@ -20942,6 +20954,197 @@ def vdist(dir, a, b):
     return mupdf.fz_abs(dx * dir.y + dy * dir.x)
 
 
+def apply_pages(
+        path,
+        pagefn,
+        *,
+        pagefn_args=(),
+        pagefn_kwargs=dict(),
+        initfn=None,
+        initfn_args=(),
+        initfn_kwargs=dict(),
+        pages=None,
+        method='single',
+        concurrency=None,
+        _stats=False,
+        ):
+    '''
+    Returns list of results from `pagefn()`, optionally using concurrency for
+    speed.
+
+    Args:
+        path:
+            Path of document.
+        pagefn:
+            Function to call for each page; is passed (page, *pagefn_args,
+            **pagefn_kwargs). Return value is added to list that we return. If
+            `method` is not 'single', must be a top-level function - nested
+            functions don't work with concurrency.
+        pagefn_args
+        pagefn_kwargs:
+            Additional args to pass to `pagefn`. Must be picklable.
+        initfn:
+            If true, called once in each worker process; is passed
+            (*initfn_args, **initfn_kwargs).
+        initfn_args
+        initfn_kwargs:
+            Args to pass to initfn. Must be picklable.
+        pages:
+            List of page numbers to process, or None to include all pages.
+        method:
+            'single'
+                Do not use concurrency.
+            'mp'
+                Operate concurrently using Python's `multiprocessing` module.
+            'fork'
+                 Operate concurrently using custom implementation with
+                 `os.fork()`. Does not work on Windows.
+        concurrency:
+            Number of worker processes to use when operating concurrently. If
+            None, we use the number of available CPUs.
+        _stats:
+            Internal, may change or be removed. If true, we output simple
+            timing diagnostics.
+
+    Note: We require a file path rather than a Document, because Document
+    instances do not work properly after a fork - internal file descriptor
+    offsets are shared between the parent and child processes.
+    '''
+    if _stats:
+        t0 = time.time()
+
+    if method == 'single':
+        if initfn:
+            initfn(*initfn_args, **initfn_kwargs)
+        ret = list()
+        document = Document(path)
+        for page in document:
+            r = pagefn(page, *pagefn_args, **initfn_kwargs)
+            ret.append(r)
+
+    else:
+        # Use concurrency.
+        #
+        from . import _apply_pages
+
+        if pages is None:
+            if _stats:
+                t = time.time()
+            with Document(path) as document:
+                num_pages = len(document)
+                pages = list(range(num_pages))
+            if _stats:
+                t = time.time() - t
+                log(f'{t:.2f}s: count pages.')
+
+        if _stats:
+            t = time.time()
+
+        if method == 'mp':
+            ret = _apply_pages._multiprocessing(
+                    path,
+                    pages,
+                    pagefn,
+                    pagefn_args,
+                    pagefn_kwargs,
+                    initfn,
+                    initfn_args,
+                    initfn_kwargs,
+                    concurrency,
+                    _stats,
+                    )
+
+        elif method == 'fork':
+            ret = _apply_pages._fork(
+                    path,
+                    pages,
+                    pagefn,
+                    pagefn_args,
+                    pagefn_kwargs,
+                    initfn,
+                    initfn_args,
+                    initfn_kwargs,
+                    concurrency,
+                    _stats,
+                    )
+
+        else:
+            assert 0, f'Unrecognised {method=}.'
+
+        if _stats:
+            t = time.time() - t
+            log(f'{t:.2f}s: work.')
+
+    if _stats:
+        t = time.time() - t0
+        log(f'{t:.2f}s: total.')
+    return ret
+
+
+def get_text(
+        path,
+        *,
+        pages=None,
+        method='single',
+        concurrency=None,
+
+        option='text',
+        clip=None,
+        flags=None,
+        textpage=None,
+        sort=False,
+        delimiters=None,
+
+        _stats=False,
+        ):
+    '''
+    Returns list of results from `Page.get_text()`, optionally using
+    concurrency for speed.
+
+    Args:
+        path:
+            Path of document.
+        pages:
+            List of page numbers to process, or None to include all pages.
+        method:
+            'single'
+                Do not use concurrency.
+            'mp'
+                Operate concurrently using Python's `multiprocessing` module.
+            'fork'
+                 Operate concurrently using custom implementation with
+                 `os.fork`. Does not work on Windows.
+        concurrency:
+            Number of worker processes to use when operating concurrently. If
+            None, we use the number of available CPUs.
+        option
+        clip
+        flags
+        textpage
+        sort
+        delimiters:
+            Passed to internal calls to `Page.get_text()`.
+    '''
+    args_dict = dict(
+            option=option,
+            clip=clip,
+            flags=flags,
+            textpage=textpage,
+            sort=sort,
+            delimiters=delimiters,
+            )
+
+    return apply_pages(
+            path,
+            Page.get_text,
+            pagefn_kwargs=args_dict,
+            pages=pages,
+            method=method,
+            concurrency=concurrency,
+            _stats=_stats,
+            )
+
+
 class TOOLS:
     '''
     We use @staticmethod to avoid the need to create an instance of this class.