From a00d61d9217c7d7944169a5d4f443c4327e33ddc Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Tue, 8 Apr 2025 20:07:04 +0530
Subject: [PATCH] Add a python wrapper for pread

---
 src/calibre/utils/copy_files_test.py | 13 +++++
 src/calibre/utils/speedup.c          | 75 ++++++++++++++++++++++++++++
 2 files changed, 88 insertions(+)

diff --git a/src/calibre/utils/copy_files_test.py b/src/calibre/utils/copy_files_test.py
index 0100d2d8c1..15900c6556 100644
--- a/src/calibre/utils/copy_files_test.py
+++ b/src/calibre/utils/copy_files_test.py
@@ -65,6 +65,19 @@ class TestCopyFiles(unittest.TestCase):
         contents = set(os.listdir(self.tdir)) - {'base', 'src'}
         self.ae(contents, {'One', 'three'})
 
+    def test_pread_all(self):
+        from calibre_extensions.speedup import pread_all
+        n = os.path.join(self.tdir, 'base')
+        data = os.urandom(1137*1024)
+        with open(n, 'wb') as f:
+            f.write(data)
+        with open(n, 'rb') as f:
+            for n, offset in {
+                0:0, 3:0, 13:13
+            }.items():
+                self.assertEqual(data[offset:offset+n], pread_all(f.fileno(), n, offset))
+            self.assertEqual(data, pread_all(f.fileno(), len(data)))
+
     def test_copying_of_trees(self):
         src, dest = self.s(), self.d()
         copy_tree(src, dest)
diff --git a/src/calibre/utils/speedup.c b/src/calibre/utils/speedup.c
index f6eab24b9a..7249b29953 100644
--- a/src/calibre/utils/speedup.c
+++ b/src/calibre/utils/speedup.c
@@ -28,6 +28,11 @@ typedef unsigned __int8 uint8_t;
 #else
 #include <stdint.h>
 #endif
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <unistd.h>
+#endif
 
 static PyObject*
 barename(PyObject *self, PyObject *tag) {
@@ -705,6 +710,69 @@ deepcopy(PyObject *self, PyObject *o) {
     return ans;
 }
 
+
+static PyObject*
+pread_all(PyObject *self, PyObject *args) {
+    int fd; unsigned long long n, offset = 0;
+    if (!PyArg_ParseTuple(args, "iK|K", &fd, &n, &offset)) return NULL;
+#ifdef _WIN32
+    PyObject *msvcrt = PyImport_ImportModule("msvcrt");
+    if (!msvcrt) return NULL;
+    PyObject *get_osfhandle = PyObject_GetAttrString(msvcrt, "get_osfhandle");
+    Py_CLEAR(msvcrt);
+    if (!get_osfhandle) return NULL;
+    PyObject *ret = PyObject_CallFunctionObjArgs(get_osfhandle, PyTuple_GET_ITEM(args, 0), NULL);
+    Py_CLEAR(get_osfhandle);
+    if (!ret) return NULL;
+    HANDLE file = (HANDLE)PyLong_AsUnsignedLongLong(ret);
+    Py_CLEAR(ret);
+#endif
+    PyObject *output = PyBytes_FromStringAndSize(NULL, n);
+    if (!output || !n) return output;
+    size_t pos = 0;
+    char *buf = PyBytes_AS_STRING(output);
+    int saved_errno = 0;
+    Py_BEGIN_ALLOW_THREADS;
+    while (pos < n) {
+#ifdef _WIN32
+        DWORD nr = 0;
+        OVERLAPPED overlapped;
+        memset(&overlapped, 0, sizeof(OVERLAPPED));
+        overlapped.OffsetHigh = (uint32_t)((offset & 0xFFFFFFFF00000000LL) >> 32);
+        overlapped.Offset = (uint32_t)(offset & 0xFFFFFFFFLL);
+        SetLastError(0);
+        BOOL ok = ReadFile(file, buf + pos, n - pos, &nr, &overlapped);
+        if (!ok) {
+            DWORD err = GetLastError();
+            if (err != ERROR_HANDLE_EOF) saved_errno = err;
+            break;
+        }
+#else
+        ssize_t nr = pread64(fd, buf + pos, n - pos, offset);
+        if (nr < 0) {
+            if (errno == EINTR || errno == EAGAIN || errno == EBUSY) continue;
+            saved_errno = errno;
+            break;
+        } else if (nr == 0) break;
+#endif
+        pos += nr;
+        offset += nr;
+    }
+    Py_END_ALLOW_THREADS
+    if (saved_errno != 0) {
+        Py_CLEAR(output);
+#ifdef _WIN32
+        PyErr_SetFromWindowsErr(saved_errno);
+#else
+        errno = saved_errno;
+        PyErr_SetFromErrno(PyExc_OSError);
+#endif
+        return NULL;
+    }
+    if (pos < n && _PyBytes_Resize(&output, pos) != 0) return NULL;
+    return output;
+}
+
 static PyMethodDef speedup_methods[] = {
     {"deepcopy", deepcopy, METH_O,
         "deepcopy(object)\n\nFast implementation of deepcopy()"
@@ -752,6 +820,13 @@ static PyMethodDef speedup_methods[] = {
 		"set_thread_name(name)\n\nWrapper for pthread_setname_np"
 	},
 
+	{"pread_all", pread_all, METH_VARARGS,
+		"pread_all(fd, n, offset)\n\nRead upto n bytes from the specified fd at offset in a thread safe manner."
+        " If less than n bytes are returned it means there were less than n bytes in the file at offset."
+        " Only works with seekable regular files, not sockets/ttys/etc. Note that on Windows it moves the file pointer"
+        " so cannot be mixed with calls to tell() or ordinary reads."
+	},
+
 	{"get_num_of_significant_chars", get_num_of_significant_chars, METH_O,
 		"get_num_of_significant_chars(elem)\n\nGet the number of chars in specified tag"
 	},