bigfile/virtmem: Don't forget to release fileh->writeout_inprogress on storeblk error

Commit fb4bfb32 (bigfile/virtmem: Do storeblk() with virtmem lock
released) added bug-protection to fileh_dirty_writeout() so that it could
not be called twice at the same time or in parallel with other functions
which modify pages.

However it missed the code path when storeblk() call returned with error
and whole writeout was thus erroring out, but with fileh->writeout_inprogress
still left set to 1 incorrectly.

This was leading to things like

    bigfile/virtmem.c:419: fileh_dirty_discard: Assertion `!(fileh->writeout_inprogress)' failed.

and crashes.

Fix it.

bigfile/tests/test_basic.py

@@ -17,6 +17,7 @@
 # See COPYING file for full licensing terms.
 # TODO description
 from wendelin.bigfile import BigFile, WRITEOUT_STORE, WRITEOUT_MARKSTORED
+from wendelin.lib.mem import bzero
 import struct
 import weakref
 import sys
@@ -343,3 +344,35 @@ def test_gc_from_sighandler():
     assert f3.marker_list == []
     m3[0]
     assert f3.marker_list == [2]
+
+
+# test that there is no crash after first store error on writeout
+class StoreError(Exception):
+    pass
+
+class StoreErrBigFile(BigFile):
+
+    def __new__(cls, blksize):
+        return BigFile.__new__(cls, blksize)
+
+    # load: just return all zero
+    def loadblk(self, blk, buf):
+        bzero(buf)
+
+    # store: always error
+    def storeblk(self, blk, buf):
+        raise StoreError("store blk #%d: error " % blk)
+
+def test_writeout_err_notfatal():
+    f   = StoreErrBigFile(PS)
+    fh  = f.fileh_open()
+    vma = fh.mmap(0, 1)
+
+    m = memoryview(vma)
+    m[0] = bord_py3(b'A')   # load + set dirty
+
+    # call writeout 2 times as previously upon error it was leaving internal
+    # flag set that writeout was still in progress and second call to writeout
+    # was leading to BUG and thus process crash.
+    raises(StoreError, "fh.dirty_writeout(WRITEOUT_STORE)")
+    raises(StoreError, "fh.dirty_writeout(WRITEOUT_STORE)")

bigfile/virtmem.c

@@ -397,9 +397,9 @@ int fileh_dirty_writeout(BigFileH *fileh, enum WriteoutFlags flags)
     if (flags & WRITEOUT_MARKSTORED)
         BUG_ON(!list_empty(&fileh->dirty_pages));
 
+out:
     fileh->writeout_inprogress = 0;
 
-out:
     virt_unlock();
     sigsegv_restore(&save_sigset);
     return err;