QFile::copy(): don't trust the old file metadata cache

Instead, let's just use sendfile(2) in a loop until it returns 0, which
indicates EOF.

[ChangeLog][QtCore][QFile] Fixed a regression in QFile::copy() that
caused the original file not to be copied entirely if it was modified
outside of this QFile object between the last time we checked its size
and the copy() call. Note this is not a prevention against race
conditions.

Task-number: QTBUG-69417
Change-Id: Id59bdd8f1a804b809e22fffd15406c8aa31f4a1e
Reviewed-by: Simon Hausmann <[email protected]>
Reviewed-by: Oswald Buddenhagen <[email protected]>
Reviewed-by: Edward Welbourne <[email protected]>

src/corelib/io/qfilesystemengine_unix.cpp

@@ -1113,7 +1113,6 @@ bool QFileSystemEngine::cloneFile(int srcfd, int dstfd, const QFileSystemMetaDat
     QT_STATBUF statBuffer;
     if (knownData.hasFlags(QFileSystemMetaData::PosixStatFlags) &&
             knownData.isFile()) {
-        statBuffer.st_size = knownData.size();
         statBuffer.st_mode = S_IFREG;
     } else if (knownData.hasFlags(QFileSystemMetaData::PosixStatFlags) &&
                knownData.isDirectory()) {
@@ -1126,29 +1125,23 @@ bool QFileSystemEngine::cloneFile(int srcfd, int dstfd, const QFileSystemMetaDat
     }
 
 #if defined(Q_OS_LINUX)
-    if (statBuffer.st_size == 0) {
-        // empty file? we're done.
-        return true;
-    }
-
     // first, try FICLONE (only works on regular files and only on certain fs)
     if (::ioctl(dstfd, FICLONE, srcfd) == 0)
         return true;
 
     // Second, try sendfile (it can send to some special types too).
     // sendfile(2) is limited in the kernel to 2G - 4k
-    auto sendfileSize = [](QT_OFF_T size) { return size_t(qMin<qint64>(0x7ffff000, size)); };
+    const size_t SendfileSize = 0x7ffff000;
 
-    ssize_t n = ::sendfile(dstfd, srcfd, NULL, sendfileSize(statBuffer.st_size));
+    ssize_t n = ::sendfile(dstfd, srcfd, NULL, SendfileSize);
     if (n == -1) {
         // if we got an error here, give up and try at an upper layer
         return false;
     }
 
-    statBuffer.st_size -= n;
-    while (statBuffer.st_size) {
-        n = ::sendfile(dstfd, srcfd, NULL, sendfileSize(statBuffer.st_size));
-        if (n == 0) {
+    while (n) {
+        n = ::sendfile(dstfd, srcfd, NULL, SendfileSize);
+        if (n == -1) {
             // uh oh, this is probably a real error (like ENOSPC), but we have
             // no way to notify QFile of partial success, so just erase any work
             // done (hopefully we won't get any errors, because there's nothing
@@ -1158,9 +1151,6 @@ bool QFileSystemEngine::cloneFile(int srcfd, int dstfd, const QFileSystemMetaDat
             n = lseek(dstfd, 0, SEEK_SET);
             return false;
         }
-        if (n == 0)
-            return true;
-        statBuffer.st_size -= n;
     }
 
     return true;