Remove st_blocks hack from wxFile::Length() under Linux

This reverts commit 41f6f17d01 ("return 0
(meaning the file is not seekable, as the docs now explain) instead of
4KB for the files in sysfs under Linux") as it seems to be wrong to
return a value different from what "ls -l" or "stat" return here and the
original problem was solved in a better way in the previous commit.

See #9965.

Closes #17818.

interface/wx/file.h

@@ -95,11 +95,14 @@ public:
     /**
         Returns the length of the file.
 
-        This method may return ::wxInvalidOffset if the length couldn't be
+        Returns ::wxInvalidOffset if the length couldn't be determined.
-        determined or 0 even for non-empty files if the file is not seekable.
 
-        In general, the only way to determine if the file for which this function
+        Please also note that there is @e no guarantee that reading that many
-        returns 0 is really empty or not is to try reading from it.
+        bytes from the file will always succeed. While this is true for regular
+        files (unless the file size has been changed by another process in
+        between Length() and Read() calls), some special files, such as most
+        files under @c /sys or @c /proc directories under Linux, don't actually
+        contain as much data as their size indicates.
     */
     wxFileOffset Length() const;
 

src/common/file.cpp

@@ -474,21 +474,6 @@ wxFileOffset wxFile::Length() const
 {
     wxASSERT( IsOpened() );
 
-    // we use a special method for Linux systems where files in sysfs (i.e.
-    // those under /sys typically) return length of 4096 bytes even when
-    // they're much smaller -- this is a problem as it results in errors later
-    // when we try reading 4KB from them
-#ifdef __LINUX__
-    struct stat st;
-    if ( fstat(m_fd, &st) == 0 )
-    {
-        // returning 0 for the special files indicates to the caller that they
-        // are not seekable
-        return st.st_blocks ? st.st_size : 0;
-    }
-    //else: failed to stat, try the normal method
-#endif // __LINUX__
-
     wxFileOffset iRc = Tell();
     if ( iRc != wxInvalidOffset ) {
         wxFileOffset iLen = const_cast<wxFile *>(this)->SeekEnd();

tests/file/filetest.cpp

@@ -144,6 +144,10 @@ void FileTestCase::TempFile()
 // Check that GetSize() works correctly for special files.
 TEST_CASE("wxFile::Special", "[file][linux][special-file]")
 {
+    // We can't test /proc/kcore here, unlike in the similar
+    // wxFileName::GetSize() test, as wxFile must be able to open it (at least
+    // for reading) and usually we don't have the permissions to do it.
+
     // This file is not seekable and has 0 size, but can still be read.
     wxFile fileProc("/proc/diskstats");
     CHECK( fileProc.IsOpened() );
@@ -155,6 +159,7 @@ TEST_CASE("wxFile::Special", "[file][linux][special-file]")
     // All files in /sys seem to have size of 4KiB currently, even if they
     // don't have that much data in them.
     wxFile fileSys("/sys/power/state");
+    CHECK( fileSys.Length() == 4096 );
     CHECK( fileSys.IsOpened() );
     CHECK( fileSys.ReadAll(&s) );
     CHECK( !s.empty() );

tests/filename/filenametest.cpp

@@ -1049,3 +1049,18 @@ void FileNameTestCase::TestShortcuts()
 }
 
 #endif // __WINDOWS__
+
+#ifdef __LINUX__
+
+// Check that GetSize() works correctly for special files.
+TEST_CASE("wxFileName::GetSizeSpecial", "[filename][linux][special-file]")
+{
+    wxULongLong size = wxFileName::GetSize("/proc/kcore");
+    INFO( "size of /proc/kcore=" << size );
+    CHECK( size > 0 );
+
+    // All files in /sys seem to have size of 4KiB currently.
+    CHECK( wxFileName::GetSize("/sys/power/state") == 4096 );
+}
+
+#endif // __LINUX__